intercomparison results

Transcription

1 WATER RESOURCES RESEARCH, VOL. 34, NO. 2, PAGES , FEBRUARY 1998 Adjustment f daily precipitatin data at 10 climate statins in Alaska: Applicatin f Wrld Meterlgical Organizatin intercomparisn results Daqing Yang, Barry E. Gdisn, and Shig shida Climate Research Branch, Atmspheric Envirnment Service, Dwnsview, Ontari, Canada Carl S. Bensn Gephysical nstitute, University f Alaska, Fairbanks Abstract. A methdlgy fr adjusting the daily precipitatin measured by the U.S. Natinal Weather Service (NWS) 8-inch standard gauge fr wind-induced undercatch, wetting lss, and trace amunt f precipitatin is prvided. The applicatin f the prpsed adjustment prcedures was made at 10 NWS climate statins in Alaska fr 1982 and The results shw the fllwing: (1) Daily adjustment fr wind-induced undercatch, wetting lss, and trace amunt f precipitatin increases the gauge-measured annual precipitatin by mm fr the 2 years (abut % f the gaugemeasured yearly ttal) at the 10 statins in Alaska; (2) cmpared t wetting lss and trace amunt f precipitatin, wind-induced undercatch is the surce f greatest errr, althugh wetting lss and trace amunt f precipitatin are als significant systematic errrs in the nrthern Alaska regins f lw precipitatin; (3) in the similar climate cnditin, the NWS 8-inch standard gauges with an Alter wind shield have a much lwer adjustment fr wind-induced undercatch than the unshielded gauges at nearby statins, and the unshielded gauges placed n the rf f the weather ffice building have a higher adjustment fr wind-induced errrs than thse gauges munted n the grund; (4) mnthly adjustment factrs (adjusted/measured precipitatin) differ by statin, and at an individual statin by type f precipitatin; (5) cnsiderable intra-annual variatin f the magnitude f the adjustments has been fund in Alaska wing t the fluctuatin f wind speed, air temperature, and frequency f snwfall. Using the cnstant crrectin factrs (derived at a single intercmparisn site) t the archived mnthly precipitatin recrds prduces significant undercrrectin f the wind-induced errrs at high wind sites and vercrrecting f the errrs at lw wind sites. T avid the undercrrectin r vercrrectin f the wind-induced errrs, a cnstant crrectin factr shuld nt be applied t gauge-measured snw data. Daily adjustments fr systematic errrs need t be applied t the archived precipitatin data. t is expected that the adjustments will have an impact n climate mnitring. 1. ntrductin Hnassen-Bauer and Frland, 1994; Grisman and Easterling, 1994; Grisman and Legates, 1994; Grisman et al., 1991]. Systematic errrs (biases) caused by wind-induced undercatch, wetting, and evapratin lsses in precipitatin measurement have lng been recgnized as affecting all types f precipitatin gauges [Gdisn et al., 1981; Sevruk, 1982; Tabler et al., 1990]. The need t crrect these systematic errrs, especially fr slid precipitatin measurement, has nw been mre widely acknwledged, as the magnitude f the errrs and their variatin amng gauges became knwn and their ptential effects n reginal, natinal, and glbal climatlgical, hydr- The Wrld Meterlgical Organizatin (WMO) initiated the Slid Precipitatin Measurement ntercmparisn in 1985 [WMO/Cmmissin fr nstruments and Methds f Observatins (CMO), 1985]. The gal f the prject was t assess natinal methds f measuring slid precipitatin against methds whse accuracy and reliability were knwn, including past and current prcedures, autmatic systems, and new methds f bservatin [Gdisn et al., 1989]. The intercmparisn was designed t (1) determine wind-induced errrs in lgical, and climate change studies were recgnized [Gdisn natinal methds f measuring slid precipitatin, including and Yang, 1995; Desbis and Desalmand, 1995; Legates, 1995; wetting and evapratin lsses; (2) derive standard methds fr crrecting slid precipitatin measurements; and (3) intrduce a reference methd f slid precipitatin measurement fr general use t calibrate any type f precipitatin gauge On leave frm Lanzhu nstitute f Glacilgy and Gecrylgy, [Gdisn et al., 1994]. Lanzhu, China. The reference methd fr snwfall measurement was ex- Cpyright 1998 by the American Gephysical Unin. Paper number 97WR /98/97WR $09. O0 tremely critical in this intercmparisn. After reviewing all pssible practical methds (bush shield, duble-fence shield, frest clearing, snw bard, dual-gauge system) f measuring 241

2 242 YANG ET AL.: ADJUSTMENT OF DALY PRECPTATON DATA 12.0 m 3.5 m Tretyakv Gauge and Wind Shield 1.5 ll 2.0 m Figure 1. Crss sectin f the WMO duble-fence intercmparisn reference (DFR). "true" snwfall in a range f climatic cnditins, the WMO rganizing cmmittee fr the intercmparisn designated the ctagnal vertical duble fence, surrunding a shielded Tretyakv gauge, as the intercmparisn reference (DFR) (Figure 1) [Gdisn et al., 1981; Glubev, 1985a, b; Gdisn et al., 1989]. The DFR was perated at 19 statins in 10 cuntries arund the wrld during the study. Pit gauge was recmmended by WMO as the reference fr rainfall measurement [Sevruk and Hamn, 1984]. Althugh it was nt an bjective f this WMO prject t study rainfall measurement, intercmparisn results at Jkiinen in Finland demnstrated a very data cllected during the rainfall perid f 1966 t 1969 at the Valdai Hydrlgical Research Statin in Russia. Effrts t crrect gauge measured precipitatin have been reprted. A study f the land-based water balance f the Earth during the 1960s t 1970s in the frmer USSR resulted in the adjustment f the gauge-measured precipitatin n the glbal scale [UNESCO, 1974]. Grisman et al. [1991] reviewed the frmer USSR experience n precipitatin measurements and systematic errr crrectin as the results frm the early Russian studies have been widely used. Legates and DeLiberty [1993] and Grisman and Easterling [1994] reprted n adjustgd agreement between rainfall measured by the DFR and ments cnducted fr climate statins acrss the cntinental the pit gauge in a number f different seasns [Yang et al., 1995a]. Hence it was reasnable t accept the DFR as a reference fr rainfall measurement in this study, since mst f the sites did nt have a pit gauge r did nt perate it in winter. The U.S. standard 8-inch nnrecrding gauge has been used thrughut the life f the U.S. Natinal Weather Service (NWS) as the fficial precipitatin measuring instrument at climatlgical statins [U.S. Department f Cmmerce, 1963]. Tday this gauge is still widely used at 7500 lcatins in the United States. Grisman and Legates [1994] reviewed the precipitatin crrectin in the United States, and recently, Grisman et al. [1996] reprted the phase 3 adjustments. Adjustments have als been cnducted fr Switzerland [Sevruk et al., 1993] and fr Nrthwest Territries, Canada [Metcalfe et al., 1994], and ther study areas [Metcalfe and Gdisn, 1993; E. L. Peck, unpublished data, 1991]. The adjustments were based n a variety f studies with the Tretyakv gauge, pit gauge, snw bards, and snw cver data. United States [Glubev et al., 1992] and at abut 1340 statins Generally tw techniques were develped and used in the in ther cuntries such as the Bahamas, Bangladesh, Saudi Arabia, Thailand, and Philippines [Sevruk and Klemm, 1989]. Relatively few f the NWS 8-inch standard gauges in the U.S. netwrk are presently equipped with the Alter wind shield analysis f mnthly data. The first methd [Grisman et al., 1991; Grisman and Legates, 1994; E. L. Peck, unpublished data, 1991] simply used a cnstant adjustment-factr, btained frm an intercmparisn experiment at ther sites, fr each (NWS standard wind shield), althugh it has been dcumented mnth r seasn. The results f this adjustment did nt have that an Alter shield can increase the catch f slid precipitatin by tens f percent and rainfall by several percent [Larkin, 1947; Larsn and Peck, 1974]. Since 1940 the number f Altershielded gauges at U.S. Weather Bureau statins has been reduced frm abut 500 t less than 200 [Karl et al., 1993a, b]. The cmbinatin f precipitatin recrds frm shielded gauges any impact n climate change studies, such as trend analysis, since the mnthly and annual ttals were changed nly by the cnstant factrs. The ther technique [Legates and DeLiberty, 1993] required mnthly climate data t estimate the adjustment factrs. The "crrected" estimates were btained using site-specific infrmatin including wind speed, air temperawith thse frm unshielded gauges results in inhmgeneus ture, gauge height, and sheltering. Wind speed during precipprecipitatin time series and leads t incrrect spatial interpretatins. Thus use f such data fr climatlgical and hydrlgical studies culd be misleading. Many studies n the perfrmance f the NWS 8-inch standard gauge have been cnducted since the 1940s [Larkin, 1947; itatin was estimated frm mean mnthly wind speed using a prcedure utlined by Sevruk [1982]. The WMO Slid Precipitatin Measurement ntercmparisn prvided the pprtunity t develp and evaluate the imprved adjustment prcedures n a daily r 6-hurly time- Black, 1954; Larsn and Peck, 1974; Glubev et al., 1992]. Frm scale fr a number f precipitatin gauges cmmnly used 1972 t 1976 the NW S 8-inch standard gauge was tested in the arund the wrld [Gdisn and Yang, 1995]. This paper presents a methdlgy f adjusting the daily precipitatin measured by the NWS 8-inch standard gauge fr the biases f wind-induced errr, wetting lss, and trace amunt f precipitatin. The methdlgy has been established by analyses f nternatinal Rainfall Cmparisn f Natinal Precipitatin Gauges with a Reference Pit Gauge [Sevruk and Hamn, 1984]. Bensn [1982] cmpared the snwfall measurements f this gauge against the Wyming-shielded gauges and snw survey data n the arctic slpes f nrthern Alaska. Recently, Glubev et al. [1992] reprted sme results f intercmparisn the cmbined internatinal data cllected at several WMO intercmparisn sites fr many years. Applicatin f the pr-

3 YANG ET AL.: ADJUSTMENT OF DALY PRECPTATON DATA 243 psed methdlgy was made at 10 NWS climate statins in Alaska fr 2 example years in rder t identify the magnitude f the systematic errrs and t demnstrate their seasnal and spatial variability in Alaska. Discussins f the crrectin methds and results were made by cmparisn f this wrk t ther studies. t is expected that the adjustment such as demnstrated by this study will significantly imprve the accuracy and hmgeneity f precipitatin data and will have an impact n climate mnitring and hydrlgical mdelling. Mixed Precipitatin RAlterShiel d Ws RUnshielded = Ws Rain n = 75, r 2 = 0.59 n = 59, r 2= 0.37 RAlter Shield: exp ( WS 0'69) n = 64, r 2= 0.18 (3) (4) 2. Results f WMO ntercmparisn n the NWS 8-nch Standard Gauge During the WMO Slid Precipitatin Measurement ntercmparisn, the NWS 8-inch standard gauge was tested at Reynlds Creek Experimental Watershed (43ø12'N, 116ø45'W; 1193 m abve sea level (asl)) in dah [Hansn and Rang, 1990], Sleepers River Research Watershed (44ø29'N, 72ø10'W; 552 m asl)) in Danville, Vermnt [Bales el al., 1987], and Valdai Hydrlgical Research Statin (57ø59'N, 33ø15'E; 194 m asl) in Russia [Glubev el al., 1995]. The intercmpari- sn data cllected at thse sites fr mre than three winter seasns were c%mpiled; they represented a great variety f verall ttals f adjusted precipitatin and the grund true climate, terrain, and expsure. The wetting lsses fr bth the amunt were within 3-6% fr snw and less than 2% fr bth Tretyakv gauge and the NSW 8-inch standard gauge were adjusted when the vlumetric methd was used fr the gauge rain and mixed precipitatin. Fr the unshielded gauge the deviatins were slightly larger, ranging frm 5-10% fr snw, readings. The evapratin lsses were nt adjusted since ex- 3-6% fr the mixed, and 0-2% fr rain [Yang et al., 1995b]. Student's t test was used t the snw data at Valdai in rder periments shwed that they were very small [,4altnen el al., 1993; Glubev el al., 1992]. When wind speed at the gauge height was nt measured, it was reduced frm a higher height. The DFR data were crrected fr wind-induced lsses using the prcedure derived by Yang el al. [1993]. Blwing snw events in the intercmparisn data were carefully identified t investigate the imprvement f the adjustment n the gauge-measured amunts. The results shw a statistically significant (a < 0.05) difference between the gauge-measured and the adjusted snw data and als shw a statistically significant (a < 0.05) agreement f the adjusted gauge measureand eliminated frm further analysis. ments t the estimated "true" snw f the DFR. Similar Studies have shwn that gauge catch depends n bth the results were btained frm the tests cnducted at ther WMO envirnmental factrs and the precipitatin features, such as rainfall rate [Sevruk, 1982] and the shape f snw flakes [Gdisn el al., 1981], and varies in each individual precipitatin event [Yang el al., 1995a, b]. n rder t investigate the sites [Yang et al., 1995b]; therefre the authrs feel that these crrectin equatins wrk well at the intercmparisn statins and that they shuld be used fr testing adjustment f the gauge-measuredaily precipitatin at statins where the NWS dependence f the NWS 8-inch gauge catch n envirnmental 8-inch standard gauge is used. factrs such as air temperature, wind speed, and wind directin, statistical analysis was applied t thse intercmparisn data 3. Applicatin f WMO ntercmparisn when the DFR measurement was greater than 3.0 mm, since Results t Alaska unrealistically large variatin f the catch rati (measured precipitatin/true) will be intrduced by the small abslute differences between gauges fr small precipitatin events. The results indicate that wind speed is the nly factr with the statistically significant relatinship t the gauge catch and that air temperature des nt have a statistically significant effect n the catch f the Ten climate statins in Alaska using the NWS 8-inch nnrecrding gauge in years 1982 and 1983 were chsen fr this study. These statins spread ver Alaska and represent varius climatic cnditins (Figure 2). The site and instrumental infrmatin (gauge height, shielding infrmatin, anemmeter NWS 8-inch standard gauge when precipitatin is classified int snw, rain, and mixed. The best-fit regressin f daily catch rati (R, in percent) f the NWS 8-inch gauge (with an Alter shield r unshielded) as a functin f the daily wind speed (Ws, m/s) at the gauge height has been derived frm the WMO intercmparisn data fr varius types f precipitatin: Snw R Alter Shield -' exp ( Ws '75) (1) n = 108, r 2= 0.72 Runshielded-- exp ( WS 1'28) (2) n = 55, r 2= 0.77 Runshielded-- exp ( Ws ø'58) (6) n = 64, r 2= 0.27 t is imprtant t mentin that a wide range f wind speed and catch rati has been cvered by the cmbined data set and, thus the adjustment equatins develped frm the data set are mre likely t be used in a great range f envirnment cnditins. The perfrmance f the adjustment equatins fr the gauge was checked independently by using all the intercmparisn data (withut the DFR > 3.0 mm limitatin) at the three WMO intercmparisn statins. The results indicated that fr the Alter-shielded gauge, the differences between the height) is given in Table 1. The daily data f temperature, wind speed, precipitatin, snwfall, snw depth n the grund and the weather cde fr the 2 years were btained frm the U.S. Natinal Climatic Data Center. The mean yearly values f temperature, gaugemeasured precipitatin, and wind speed at the 10 sites are summarized fr 1982 and 1983 in Table 2. Adjustments n the gauge-measured precipitatin shuld be made fr trace precipitatin, wetting lsses, evapratin lss, and wind-induced errrs caused by the wind field defrmatin ver gauge rifice. Since the wind field defrmatin affects the ttal gauge catch including bth the wetting and evapratin lsses, we mdified the general mdel [Sevruk and Hamn, 1984] fr precipitatin adjustment

4 244 YANG ET AL.: ADJUSTMENT OF DALY PRECPTATON DATA Bar w Barter sland Ktzebue Hmer Kd 1,0,?, 0,01 Figure 2. Selected NWS climate statins in Alaska fr this study. Pa = K(Pg + AP w + APe) + AP t (7) Where Pa is the adjusted precipitatin; Pa is the gaugemeasured precipitatin; AP w and AP e are wetting lss and evapratin lss, respectively; AP t is the trace precipitatin, and K is the adjustment cefficient (usually K _> 1) fr windinduced errrs. The methd f determining each f the terms in (7) is given belw Trace Precipitatin Fr the NWS 8-inch standard gauge a measurement f precipitatin f less than inch (0.127 mm) is less than half the distance frm the end f the measuring stick t the first etched line. t is recrded as a trace f precipitatin by entering the letter "T" fr trace [U.S. Natinal Weather Service, 1989]. Officially, all f the trace precipitatin is treated quantitatively as a zer event, which cntributes nthing t the mnthly ttals. Hwever, the day during which trace precipitatin was recrded is cunted as a precipitatin day. Climate data in Alaska indicate that the annual gaugemeasured precipitatin varied greatly, frm 100 mm in the nrthern arctic drainage t 1850 mm in the suth cast. The percent f snw in annual precipitatin decreased frm 60-75% in the nrth t 20% in the suth cast, with the temperature decreasing frm suth t nrth. The ttal number f precipitatin days (e.g., the sum f the days with trace precipitatin and measurable precipitatin reprted) ranged in a relatively narrw margin f at the 10 statins (Table 2). The numbers f trace precipitatin days ranged frm 36 t 103 in 1982 and frm 37 t 136 in 1983 at the ten statins. Gegraphically, mre trace precipitatin days were recrded in the nrthern Alaska regins (e.g., arctic drainage), and trace precipitatin recrdings there can make up 50-70% f the annual ttal f precipitatin days. Fr example, 103 f the 197 precipitatin days were trace days at Barrw in 1982, and the values were 122 ut f 170 at Barter sland. Bensn [1982] summarized 7 years ( ) f trace data fr 14 primary Table 1. Siting and nstrumental nfrmatin at 10 Climate Statins in Alaska Statin Latitude, Lngitude, Elevatin, Wind Sensr Statin dentificatin øn øw m Height, m Gauge Height, m Wind Shield Nte Barrw Barter Ktzebue Bettles Nme Talkeenta Valdez Bethel Hmer Kdiak yes n yes n n n n n n n gauge n rf gauge n rf gauge n rf

5 YANG ET AL.: ADJUSTMENT OF DALY PRECPTATON DATA 245 Table 2. Mean Temperature, Wind Speed, and Gauge-Measured Precipitatin at 10 Climate Statins in Alaska fr 1982 and 1983 Statin Temperature, øc Wind Gauge Percent Percent f Speed, Precipitatin Trace Measured, f Blwing Minimum Maximum m/s Days Days mm Snw Snw Barrw Barter Ktzebue Bettles Nme Talkeenta Valdez Bethel Hmer Kdiak weather statins in Alaska; the wrst case was at Barter sland, where 80% f the winter recrd cnsisted f trace (146 trace days ut f 183 precipitatin days). Seasnally, mre trace precipitatin days were reprted in summer than in winter in nrthern Alaska, and the percent f trace precipitatin days t the ttal number f days f precipitatin is much higher in the relatively dry winter seasn than in wet summer seasn. n sme winter mnths, n measurable precipitatin was recrded in nrthern Alaska except trace amunts f snwfall. The 6-hurly bservatins at Barrw shw that a number f events f trace precipitatin are reprted in a single trace precipitatin day. n 1982 and 1983 the ttal number f 6-hurly trace bservatins at Barrw were 329 and 322, crrespnding t 103 and 93 days with trace. On average, there were 3.5 trace bservatins fr each reprted trace precipitatin day at Barrw. W and Steer [1979] designed a methd f measuring trace rainfall in the high arctic and determined a mean rate f 0.01 mm/hr. Unfrtunately, there were n data during the perid f trace precipitatin in the climate archive since trace precipitatin is nt measurable by an rdinary precipitatin gauge. As mentined befre, a number f trace bservatins were reprted fr each trace day; thus it is nt unreasnable t assume that trace precipitatin culd be a measurable amunt f mm. T be cnservative, trace precipitatin was adjusted n a daily basis in this study; that is, fr any given trace day regardless the number f the trace bservatins reprted, a value f 0.10 mm was assigned and added t the mnthly ttal. n 1982 the yearly adjustments f the trace precipitatin varied frm 3.6 t 12.2 mm at the 10 statins, r abut 9% f the gauge-measured annual precipitatin in the nrthern re- gins and less than 3% in the rest f the study areas. n 1983 the yearly ttal f the adjustments ranged frm 3.7 t 13.6 mm at the 10 statins, which accunted fr 12% f the annual gauge-measured precipitatin in the nrthern regins and fr less than 3% fr the ther regins (Table 3). Seasnal variatin f the trace adjustment shws a similar pattern as the number f days f trace precipitatin. Fr instance, the mnthly ttals change between 0.1 and 1.8 mm in 1982 at Barter sland. On the average, the ttal adjustments at Barter sland accunt fr 5% f the gauge-measured precipitatin ttal fr the warm perid (June t August) and 18% f Table 3. Adjustments f Gauge-Measured Annual Precipitatin at 10 Climate Statins in Alaska fr 1982 and 1983 Temperature, øc Wind Speed, Precipitatin Pa, Statin Minimum Maximum m/s Days mm Adjustments, mm Percent f Pa, Adjustment Snw Wind Wetting Trace Sum mm Factr 1982 Barrw Barter Ktzebue Bettles Nme Talkeenta Valdez Bethel Hmer Kdiak Barrw Barter Ktzebue Bettles Nme Talkeenta Valdez Bethel Hmer Kdiak

6 246 YANG ET AL.: ADJUSTMENT OF DALY PRECPTATON DATA q Annual precipitatin amunt (mm) Figure 3. Number f trace precipitatin days versus gauge-measured annual precipitatin at the 10 climate statins in Alaska fr 1982 and the gauge-measured precipitatin ttal fr the cld perid (September t May) in The amunt f trace recrd is inversely prprtinal t the gauge-measured annual precipitatin fr 1982 and 1983 (Figure 3). Bensn [1982] shwed this relatinship was even strnger ver the 7 years f the perid ( ). Because f this, adjustment fr trace precipitatin is impr. tant especially in the nrthern Alaska regins f lw precipitatin. After identifying trace precipitatin days, the number f measurable precipitatin days exhibits a larger variatin acrss Alaska (Figure 4). Thus we recmmend t separate trace precipitatin days frm measurable precipitatin days when analyzing precipitatin climate in the nrthern Alaska regins Wetting Lsses Wetting lss refers t the rain r snw water subject t evapratin frm the surface f the inner walls f the precipitatin gauge after a precipitatin event and frm the gauge cntainer after its emptying [WMO/CMO, 1993]. Wetting lsses are gauge specific and vary by precipitatin type and the number f times the gauge is emptied. Accrding t the wetting lss experiments, the average wetting lss f the NWS 8-inch standard gauge was 0.03 mm per bservatin fr rainfall measurement when the gauge is equipped with the funnel and the measuring tube [Glubev et al., 1992]. Hwever, when the gauge is perated withut the funnel and the measuring tube in the cld seasn, snw is melted in the larger cylinder, and then pured int the measuring tube fr measurement, a higher wetting lss, f 0.15 mm per bservatin fr snw and mixed precipitatin, was reprted [Sevruk, 1982]. Wetting lss was calculated n a daily basis in this study. Fr each precipitatin day an averaged wetting lss was determined accrding t the type f precipitatin and added t the daily recrd. This is the minimum adjustment since generally mre than ne bservatin was made in a precipitatin day. 250, [] measurable [] trace Barrw Barter Ktzebue Betties Nme Talkeenta Valdez Bethel Hmer Kdiak Figure 4. Number f precipitatin days in 1982 at the 10 climate statins in Alaska.

7 YANG ET AL.: ADJUSTMENT OF DALY PRECPTATON DATA 247 ß [] :j 10 n Gauge-measured annual preciptatin (mm) Figure 5. Annual adjustment fr wetting lss versus gauge-measured annual precipitatin at the 10 climate statins in Alaska fr 1982 and Fr instance, bservatins were made every 6 hurs in a reprted precipitatin day at Barrw in 1982 and N adjustment fr wetting lss was applied t trace precipitatin events. prehensive assessment f evapratin lsses indicated that average daily lsses varied by gauge type and time f the year. Fr the Tretyakv gauge at Jkiinen, in Finland, evapratin lsses in summer f mm/d and winter f n 1982 the yearly ttals f the wetting lss adjustment were between 7.2 and 23.3 mm at the 10 statins acrss Alaska, r 5-9% f the gauge-measured annual precipitatin in the nrthern regin and 1-3% fr the ther regins. n 1983 the annual ttals f the adjustment varied frm 7.4 t 25.9 mm at mm/d were measured [Aaltnen et al., 1993]. An evapratin experiment at Valdai n the NWS 8-inch standard gauge fund that the lss fr this gauge fr rainfall measurement was s small, mm/hr, that it culd be neglected [Glubev et al., 1992]. the 10 statins, r equivalent 5-14% f the gauge-measured deally, evapratin lss shuld be adjusted. Hwever, beyearly precipitatin in the nrthern regin and 1-3% fr the ther areas in Alaska (Table 3). cause f its strng dependence n weather and its daily variatin and seasnal change, which can be very site dependent, it At each individual climate statin the annual amunt f the is nt apprpriate t estimate the daily evapratin lss at wetting lss adjustment is different between 1982 and 1983 (Table 3), and the percentage f the annual adjustment the gauge-measured yearly ttals is different in the 2 years as well. climate statins by using the average amunt btained frm ther experimental sites. Therefre evapratin lss was nt adjusted in this study. Generally, there is a clear wetting lss increase with increasing gauge-measured annual precipitatin in Alaska (Figure 5), 3.4. Wind-nduced Errrs since mre precipitatin generally requires mre bservatins, T apply the adjustment fr wind-induced errrs t the and mre bservatins lead t higher wetting lss. gauge-measured precipitatin data, wind speed at gauge height t is imprtant t realize that in sme cases, wetting lsses is required. When wind speed is nt measured at the height f cntribute significantly t the undermeasurement f precipitathe gauge, it must be estimated frm measurements at higher tin. Fr instance, wetting lss was calculated t be 15-20% f heights. The fllwing lgarithmic wind prfile was used t measured winter precipitatin at sme Canadian synptic stareduce the wind speed frm the standard height (10 m) t the tins [Metcalfe and Gdisn, 1993]. Our study indicates a height f the gauge rifice at the 10 statins: wetting lss adjustment f up t 10-14% f the gaugemeasured annual precipitatin in nrthern Alaska, and it als shws the difference f the wetting lsses between the warm U(h) = U(H)[ln (h/z)/ln (H/z)] (8) seasn (June t August) and the cld seasn (September t May). n the warm seasn, because f the lw mean wetting lss fr each bservatin f rainfall, the ttal adjustment at Barrw, fr instance, was calculated t be % f the gauge-measured precipitatin. Hwever, during the cld perid the wetting lss was estimated t be abut 21% f the gauge-measured precipitatin wing t the much higher mean wetting lss per bservatin f snwfall. where U(h) is the estimated daily wind speed (m/s) at the gauge rifice, U(H) is the measured daily wind speed at 10 m (m/s), h and H are the heights (m) f the gauge and f the anemmeter, and Z is the rughness parameter (m). Accrding t Sevruk [1982] and Glubev et al. [1992], Z = 0.01 m fr a winter snw surface and Z = 0.03 m fr shrt grass in the summer are apprpriate average rughness parameters fr mst sites. n this study Z = 0.01 m was used fr the cld 3.3. Evapratin Lss perid in Alaska, frm September t May, and Z = 0.03 m was assigned t the warm perid, frm June t August. Evapratin lsses are defined as the undermeasurement caused by water lss by evapratin befre bservatin. Cm- t is well knwn that fr a given wind speed, gauge undercatch fr snw is much higher than that fr rain, because snw

8 248 YANG ET AL.: ADJUSTMENT OF DALY PRECPTATON DATA has larger surface area per unit mass. Therefre classifying the type f precipitatin is necessary in rder t apply the best wind-lss adjustment. n this study, type f precipitatin was classified int snw, mixed, and rain by checking bth the weather cde and the new snw depth recrds. Fr the snw categry, blwing snw events were reprted n sme f precipitatin days in Alaska. Climate data shw that blwing snw ccurred quite ften n precipitatin days in winter mnths, particularly in the nrthern regins f Alaska, such as at Barrw, Barter sland, and Ktzebue. The ttal amunt f precipitatin with blwing snw reprted accunted fr 15-25% f the gauge-measured yearly ttals in these regins (Table 3). Blwing snw cnditins n precipitatin days is a special case when adjusting the gauge-measured precipitatin data. t is pssible that under certain cnditins, any gauge can catch sme blwing snw. Since wind speeds are generally greater during blwing snw events, a larger adjustment fr "undercatch" culd be applied t a measured ttal already augmented by blwing snw. This prblem wuld be mst severe fr gauges munted clse t the grund, such as the NWS 8-inch standard gauge at the standard height f 1 m, which are efficient in cllecting snw passing ver their rifices. T avid the t the wind shield and gauge height. Precipitatin gauges have been placed n the rf f the weather ffice building at sme f the NWS statins, namely, Nme, Bethel, and Hmer in the Alaska interir basin and west central regin. The annual wind-lss adjustments fr the NWS 8-inch standard gauges n the rf were in the rder f 30-96% f the gauge measured annual ttal precipitatin, cmpared t 10-15% fr the unshielded NWS 8-inch standard gauges at m abve the grund at Valdez and Talkeenta, althugh the percentages f snw in annual precipitatin were clse amng these statins. Mean daily wind speeds at the gauge height during precipitatin days were m/s fr the gauges n the rf versus m/s fr the gauges n the grund. Therefre it is clear that perating precipitatin gauges n the rf f a building intrduce significant undercatch wing t the higher wind at higher height. Jint use f these data results in inhmgeneus precipitatin time series and leads t incrrect spatial interpretatins. n sme cases, wind-induced undercatch was lw fr the unshielded gauges placed at the standard height f 1 m, if the sites were well sheltered with lw wind speeds and lw percentages f snwfall. Fr instance, the yearly ttals f windpssible vercrrectin caused by high wind n precipitatin induced errrs were calculated frm 60 t 250 mm at Bettles, days, an upper value f wind speed has t be determined fr the adjustment. Adjustments at higher wind speed are esti- Talkeenta, and Valdez (Table 3), r abut 10-17% f the gauge-measured annual precipitatin. These are the lwest mated by using this threshld wind speed [WMO/CMO, 1993]. annual adjustments fr wind-induced undercatch in Alaska. n This is imprtant since the regressin equatins that are de- ther cases, higher wind-induced undercatch was fund t be rived frm the ntercmparisn data are nly valid statistically assciated with large amunts f precipitatin in windy climate fr the interval fr which they are develped and shuld nt be used fr extraplatin utside f this range. The threshld wind speed was set up at 6.5 m/s at gauge height fr the cnditins. At Kdiak the annual adjustments fr wind lss were as high as 775 mm fr 1982 and 564 mm fr 1983 (Table 3). These are the highest abslute values in Alaska. adjustment equatins in this study, since wind data greater than this threshld were nt cllected at the WMO sites Mnthly and Yearly Ttal Adjustments When daily wind speed at the gauge height was available, the daily catch rati (R, in percent) fr the Alter-shielded r unshielded NWS 8-inch standard gauges was calculated n a daily basis using the regressin equatins (1)-(6) fr snw, mixed, and rain. The wind-lss adjustment cefficient (K) was calculated as K = 1/R. The yearly adjustments fr the wind-induced errrs at the 10 statins range frm 67 t 775 mm in 1982 and frm 44 t 564 mm in 1983 (Table 3). Generally, the abslute amunt f the yearly adjustments increased with increasing gauge-measured annual precipitatin frm nrth t suth. The percentage f the yearly adjustment the gauge-measured annual ttal decreased frm % in the nrthern regins t 30-50% in the suthern regins. Spatial variatin f the yearly adjustments fr wind lss in Alaska can be explained by wind speed, percent f snw in annual precipitatin, gauge-measured precipitatin, and gauge installatin (e.g., gauge height and wind shield). n the nrthern arctic drainage (i.e., Barrw, Barter sland, and Ktzebue), percentages f snw in annual precipitatin range frm 60 t 80% in 1982 and Climatlgically, Barter sland is a little windier than the ther tw sites, with the mean wind speed at 9 m height being 6.2 m/s at Barter and m/s at Barrw and Ktzebue (Table 2). Hwever, the gauges at Barrw and Ktzebue were perated at 1.8 and 1.2 m with an Alter shield, while the gauge at Barter sland was unshielded and placed 3.4 m abve the grund (Table 1). Thus the huge difference in the yearly adjustment f the windinduced errrs t the annual gauge measurement between Barrw (53%) and Barter sland (115%) can nly be attributed The mnthly variatin f the adjustments at the 10 statins is given in Figures 6 and 7, which shw the fllwing: (1) n each mnth the abslute mnthly amunt f wind-induced errr was always greater than wetting lss and trace precipitatin at all the sites. (2) The cntributin f trace precipitatin and wetting lss t the ttal adjustments decreased frm nrth t suth; in the mst suthern regin (such as at Kdiak) trace precipitatin was almst negligible. (3) Mnthly precipitatin was increased by 30-50% wing t the adjustments, and it was even dubled fr winter mnths in the nrthern regins. t is imprtant t nte the seasnal variatin f the mnthly adjustment factr (AF, rati f the mnthly adjusted t gaugemeasured precipitatin), that is, the high values fr snw data in the cld seasn, frm September t May, and the lw values fr rain data in the warm seasn, frm June t August, wing t the higher wind lss fr snw than fr rain and t the smaller amunt f abslute precipitatin in the cld seasn than in the warm seasn. t is even mre imprtant t realize the intraannual variatin f the mnthly AF values due t the fluctuatin f wind speed, frequency (r percentage) f snwfall, number f trace precipitatin events, amunt f gaugemeasured precipitatin, and air temperature (Figure 7). Table 3 summarizes the yearly ttal adjustments fr windinduced errr, wetting lss, and trace precipitatin. Fr each statin the individual adjustments are stated and summed. The sum f the adjustments is added t the gauge catch (Pa) t btain the adjusted value (Pa)' The verall annual adjustment factr (AF) is then the rati Pa/Pa. Althugh the sum f the abslute adjustments was highest fr Kdiak, the highest annual adjustment factrs were generally in the arctic statins.

9 YANG ET AL.' ADJUSTMENT OF DALY PRECPTATON DATA 249 3O (a) 1982,-. 20 E = 15-10,, _ _ _ /.//////// Jan Feb Mar Apr May Jun Jul Aug Spt Oct Nv ß measured ß wind lss [] wetting lss [] trace Dec 45 4O (b) O 10 Jan Feb Mar Apr May Jun Jul Aug Spt Oct Nv Dec Figure 6. Mnthly adjustments fr wind-induced errr, wetting lss, and trace amunt f precipitatin at Barrw statin fr (a) 1982 and (b) Fr bth years the maximum was at Barter sland and the minimum was at Valdez. Althugh the annual AF values were nt cnstant frm year t year, their ranking amng the statins remained the same at the 10 climate statins during 1982 and This may indicate sme degree f spatial stability which reflects stability in climate and gauge installatin in Alaska. T demnstrate the imprtance f blwing snw in the wind-lss adjustment in Alaska, the range f the ptential adjusted mnthly precipitatin was calculated fr Barrw. The lwer value f the interval was btained by excluding all f wing t fewer blwing snw events during the cld seasn than in 1982, the abslute difference f the mnthly range was smaller, that is, between 0.4 and 6.1 mm, and the yearly difference was 20.9 mm. This shws that blwing snw events can be very imprtant when cmputing the adjustment fr the effect f wind lss. t is recmmended that all blwing snw events n precipitatin days shuld be identified and that wind data during these events shuld be analyzed when adjusting gauge measurements f snwfall fr wind lss at cld and windy sites. Canadian studies n the winter precipitatin crrectin indicated that at sme nrthern statins trace precipitatin and thse daily snwfall measurements frm wind-lss adjustments wetting lss crrectins were imprtant. Fr instance, at Rewhen blwing snw was reprted n the precipitatin day. The gina, Saskatchewan, the average wetting and trace crrectin upper value was estimated by adjusting all f daily snwfall fr the winters f increased 30% f the measured measurements (including thse when blwing snw was reprted) fr wind lss, using the measured daily wind data (including thse f high values n blwing snw days). snwfall [Metcalfe and Gdisn, 1993]. This study in Alaska shwed that wing t the higher undercatch f snwfall f the NWS 8-inch standard gauge, wind-induced errr was the larg- Fr the cld seasn in 1982 the abslute difference f the est systematic errr, and wetting lss and the trace amunt f mnthly range (upper value minus lwer value) varied frm 1 t 15 mm. The yearly difference was abut 65 mm. n 1983, precipitatin were significant errrs as well, particularly in the nrthern Alaska regins (Figure 8).

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11 YANG ET AL.: ADJUSTMENT OF DALY PRECPTATON DATA 251 i!!!! E... : :.... E.: i., :::::::::::::::::::::::::::::::::::::::::::: i :::::::::::::::::::::::::::::::::::::::::::::::::::::::: i " 8,,:, - 8,g.,.- (rw) u0!l l!d!0e, d!!!ii i :i: ::: :::: 0 :::::::::::::::::::::::::::::::::::: E E,_ :::::::::::::::::::::::::::::::::::::::: 8 ø (rw) u0!l l!d!0jd :i i: :! : : : ::,-] "'. 8 8 (ww)... ullelldle d

12 252 YANG ET AL.' ADJUSTMENT OF DALY PRECPTATON DATA 6OO Kdiak, unshielded gauge at 1.2m O loo Jan Feb Mar Apr May Jun Jul Aug Spt Oct Nv Dec Figure 7. (cntinued) 4. Discussin Different methds f crrecting gauge measurements fr systematic errrs have been develped and applied fr climatlgical and hydrlgical studies [Grisman et al., 1991; Met- calfe and Gdisn, 1993; Sevruk, 1993; Grisman and Legates, 1994; Legates and DeLiberty, 1993; Gdisn and Yang, 1995]. Recently there has been a tendency t apply the cnstant crrectin factrs t gauge-measured mnthly precipitatin fr climatlgical and hydrlgical analyses [Grisman and 120 ' 100 a ßwind lss []wetting lss [] trace amunt l:: 80 a) 60 a) 40 r 20 Barrw Barter Ktzebue Betties Nme Talkeenta Valdez Bethel Hmer Kdiak O 0 Barrw Barter Ktzebue Betties Nme Talkeenta Valdez Bethel Hmer Kdiak Figure 8. Cmpnents (percent) f the annual ttal adjustments at the l0 climate statins in Alaska fr (a) 1982 and (b) 1983.

13 YANG ET AL.: ADJUSTMENT OF DALY PRECPTATON DATA 253 Table 4. Average Catch Ratis and the Crrespnding Crrectin Factrs fr the NWS 8-nch Standard Gauge, Alter Shield and Unshielded, at Valdai and Reynld Creek WMO Sites Catch Rati, % Crrectin Factr Type f Alter Alter Precipitatin shield Unshielded Shield Unshielded Snw Mixed Rain Easterling, 1994; Grisman et al., 1996; E. L. Peck, unpublished data, 1991]. The validity f these cnstant crrectin factrs fr different climate cnditins has nt been investigated. On the basis f analysis f the intercmparisn data at Valdai and Reynlds Creek WMO sites, the average catch ratis and the crrespnding crrectin factrs (CF) fr the unshielded and Alter-shielded NWS 8-inch standard gauge at 1 m abve the grund are given in Table 4. Figure 9 illustrates the yearly ttal adjustments fr wind-induced errrs at the 10 statins using the cnstant CF and daily crrectin prcedure (i.e., equatins (1)-(6)). Cmpared t the results f daily adjustments, vercrrectin ccurred at the lw-wind sites (Talkeenta, Valdez, and Bettles; yearly mean between 2.5 and 3.8 m/s) and undercrrectin ccurred at high-wind statins (Barrw, Barter sland, Ktzebue, Nme, Bethel, and Kdiak; yearly mean frm 3.8 t 6.2 m/s), because a cnstant CF des nt take int accunt f the varying effect f mean wind speed n the mean gauge catch. Yang et al. [1995] fund that the mean gauge catch rati f the Tretyakv gauge decreases with increasing wind speed fr precipitatin days. Thus, t avid the undercrrectin r vercrrectin f the wind-induced errrs, a mean catch rati (e.g., a cnstant crrectin factr) is nt recmmended fr gauge-measured snw data. nstead, the relatin f daily r event gauge catch as a functin f crrespnding daily mean r event mean wind speed shuld be applied t the gauge-measuredaily r event data, as studies shw that gauge catch varies with individual precipitatin event [Gdisn et al., 1981; Yang et al., 1995a, b]. T apply the adjustments, wind speed at gauge height during precipitatin days is required. t can be measured r derived using a mean wind speed reductin prcedure. Sevruk [1982] reprted the relatin between mnthly mean wind speed and the mean wind speed during precipitatin days. The current study shws that mean wind speed n precipitatin days was generally higher than the mnthly mean wind speed, especially in the cld seasn in Alaska. Statistical analysis f the Barrw wind data indicated n significan t crrelatin between the mnthly mean wind speed and the mean wind speed n precipitatin days. The reasn fr the pr crrelatin culd be the lw number f precipitatin days (fewer than 10 days) in mst f the mnths. Nevertheless, fr the purpse f wind-lss adjustments, we recmmend use f the wind data n precipitatin days when they are available. t is imprtant t pint ut that the threshld wind speed (6.5 m/s) fr adjusting wind-induced errr is nt very lw fr the 10 statins in Alaska. Fr instance, Barter sland is the mst windy f the 10 statins. The annual mean daily wind in 1982 was 6.2 m/s at 9 m and 4.1 m/s at gauge height f 3.4 m. There were nly 8 days (ut f 68 precipitatin days) in 1982 when daily wind speed at gauge height was greater than the threshld wind. Therefre it is apprpriate t set up the threshld wind at 6.5 m/s and t apply the adjustment prcedures with limit f the threshld wind t the 10 sites in Alaska. t is interesting t cmpare this wrk t ther studies. Black [1954] cmpared gauge-measured precipitatin data with the water equivalent f snw n the tundra, and he reprted that the U.S. Weather Bureau gauge at Barrw was recrding nly 25-50% f the actual precipitatin. Bensn [1982] cmpared the NWS standard unshielded gauges at Barrw and Barter sland t Wyming shielded gauges at each site fr fur cnsecutive winters ( t ). The Wyming shielded gauge averaged 3.5 times mre than the unshielded gauge at Barrw and 2.6 times mre at Barter sland. n additin t these crrectin factrs fr winter data, a factr f 1.1 was applied t the summer rain data t crrect fr trace amunts, fllwing the ideas f Brwn et al. [1968] and W and Steer, [1979]. Bensn [1982] applied a single value f 3.0 t the lng-term data fr winter snw at bth Barrw (65 mm) and Barter sland (84 mm) and added this t the crrected summer rain data. The results were annual ttals f 248 mm at Barrw and 318 mm at Barter sland. When cmpared t the lng-term gauge measured values (113 and 114 mm), this yields annual adjustment factrs f 2.0 at Barrw and 1.8 at Barter sland. t has been reprted that the Wyming shield des nt vercatch [Sturges, 1984, 1986; Gdisn and Metcalfe, 1982]. Gdisn and Metcalfe [1983] shwed that the Wyming gauge catches less than the Canadian Nipher gauge. S the crrectins cited abve can be regarded as cnservative. Wyming shield was tested at a number f the statins in the WMO intercmparisn. t is interesting and pssible t evaluate the perfrmance f the Wyming fence againsthe WMO reference f the DFR. Bensn's [1982] values fr Barrw and Barter sland cmpare well with the average annual adjustment factrs fr 1982 and 1983, that is, 1.8 fr Barrw and 2.3 fr Barter sland (Table 3). n the present study, wetting lsses, which were nt cnsidered by Bensn, were adjusted befre dealing with errrs intrduced by trace precipitatin and wind. Our results yield annual adjustment factrs f 1.2 fr rain and 1.9 fr snw at Barrw and 1.5 fr rain and 3.3 fr snw at Barter sland. Given the fact that different instruments fr determining the "true precipitatin" were used and that different analysis techniques were applied, the results frm these studies were quite cmpatible fr rain but were different fr snw. t is likely that ur wrk cnducted the minimum adjustment n the gaugemeasured snw data since (1) bth trace precipitatin events and wetting lsses were adjusted n a daily basis instead f fr each bservatin and (2) the snwfall events when blwing snw was reprted at high wind speeds were adjusted at a threshld wind speed. Canada has cnducted preliminary tests in applying adjustment prcedures n its digital archive data. Metcalfe et al. [1994] applied the adjustments fr wind lss, wetting lss, and trace precipitatin fr Canadian Nipher gauge 6-hurly data at selected Canadian climate statins in Nrthwest Territries. The results indicated that actual annual precipitatin can be % greater than the gauge-measured amunt at these statins. Fr instance, at Cape Parry (70ø10'N, 124ø42'W) and nuvik (68ø18'N, 133ø29'W), near the cast f the Beaufrt Sea, the relative increase due t the crrectins fr 1982 and 1983 was 5-7% fr rain and 50-75% fr snw, and the annual ttal precipitatin was increased by 46-50%. Generally, the

14 254 YANG ET AL.: ADJUSTMENT OF DALY PRECPTATON DATA loo Barrw Barter Ktzebue Betties Nme Talkeenta Valdez Bethel Hmer Kdiak b [] daily methd ß cnstant CF 600 E 500 m 400 ' 300 ' Barrw Barter Ktzebue Betties Nme Talkeenta Valdez Bethel Hmer Kdiak Figure 9. Cmparisn f the adjustments fr wind-induced errr using the cnstant crrectin factrs (CF) and the daily methd in Alaska fr (a) 1982 and (b) relative amunts f increase intrduced by the adjustments are cuntries is certainly necessary t evaluate the validity f the mre cmpatible in the cast regins f the Beaufrt Sea fr precipitatin adjustment prcedures fr the plar regins. these tw studies, since bth studies used the same reference instrument (i.e., DFR) fr true snw estimatin. Hwever, 5. Cnclusin cmpared t the adjustments at Barrw and Barter sland (Table 3), the relative increase due t the crrectins at the n this study the adjustment prcedures derived frm the Canadian statins were lwer fr bth snw and rain; this is WMO Slid Precipitatin Measurement ntercmparisn data reasnable because (1) the snw percent in annual precipita- set fr the NWS 8-inch standard gauge have been applied at 10 tin was higher at Barrw and Barter than at thse tw Cana- NWS climate statins in Alaska fr 2 test years. Systematic dian sites (i.e., 67-75% versus 60%) and (2) mre imprtantly, errrs f wind-induced undercatch, wetting lss, and trace the WMO ntercmparisn shws that the NWS 8-inch stan- amunt f precipitatin were adjusted n a daily basis and the dard gauge catches less snw than the Canadian Nipher gauge gauge-measured annual precipitatin was increased signifiat the same wind speeds (Figure 10). cantly by mm (abut % f the gauge-measured Legates and DeLiberty [1993] reprted a general pleward yearly ttal) at the 10 statins. increase f the percent f crrectin in winter seasns ver the Of the biases in precipitatin measurement, wind lss is the cntinental United States due t the increasing snwfall pr- greatest. Wetting lss and trace amunt f precipitatin are prtin. This study in Alaska shws the basic feature f in- als significant errrs in the nrthern Alaska regins f lw creasing percentage f the annual adjustment frm the suth precipitatin. n similar climate cnditins the NWS 8-inch cast t the nrthern arctic drainage. T achieve a better standard gauges with an Alter wind shield have a much lwer understand f the spatial variability f the adjustments, mre adjustment fr wind-induced undercatch than the unshielded climate statins in Alaska, particularly thse in muntain re- gauges at nearby statins. The unshielded gauges placed n the gins, shuld be invlved, and mre years f precipitatin data rf f the weather ffice building have a higher adjustment shuld be adjusted. Further analysis f the adjusted precipita- fr wind lss than thse gauges munted n the grund. Thus tin in Alaska and nrthern Canada and ther circumplar gauge installatin (gauge height and wind shield) is critical t

15 YANG ET AL.: ADJUSTMENT OF DALY PRECPTATON DATA 255 '-' e- Canadian,ø Nipher O:::: _.-a-nws 8" Alter ß 8. nsh _ ' Wind speed at gauge height (m/s) Figure 10. Cmparisn f the catch rati f snw as a functin f wind speed at gauge height fr the Alter-shielded r unshielded NWS 8-inch standard gauge and the Canadian Nipher snw gauge. the efficiency f gauge catch especially fr slid precipitatin adjustment prcedures are recmmended fr testing adjustmeasurement. Seasnally, the adjustment is greater in winter ment f the gauge-measured daily precipitatin in thse cunand smaller in summer wing t the increased effect f wind n tries where natinal meterlgical hydrlgical statin netgauge undercatch f snwfall. wrks perate these gauges fr precipitatin bservatin. t is The mnthly adjustment factrs (adjusted/measured precip- expected that the adjustments, such as thse described in this itatin) in Alaska differed frm statin t statin. At an indi- paper, will prvide reliable unbiased (r at least less biased) vidual statin the mnthly adjustment factrs varied by type f estimates f the true amunt f precipitatin fr the natinal precipitatin and by mnth even fr the same type f precip- bservatinal netwrks and the adjustments will have an imitatin, since these errrs depend nt nly n wind speed but pact n climate mnitring. t is hped that mre effrt will be als n the wetting lss, trace amunt f precipitatin, and the made by the natinal meterlgical services t apply the actual gauge-measured precipitatin amunt. n additin, adjustment prcedures t their archived precipitatin data. t there is a cnsiderable intra-annual variatin f the magnitude is believed that t d s will significantly imprve the accuracy f the adjustments due t the fluctuatin f wind speed, air and hmgeneity f precipitatin data ver large regins in the temperature, and the frequency f snwfall; this has been nrthern hemisphere. clearly demnstrated in this study and was als dcumented by Legates and DeLiberty [1993]. t is clear that the mnthly ad- Acknwledgments. This study wuld nt have been pssible withjustment factrs are nt cnstant. The adjustment f the errrs ut the cperatin f the participating cuntries and the many bshuld nt be cnducted n a mnthly basis, and the mnthly servers at the statins where the WMO ntercmparisn was cnducted fr a number f years. The authrs wuld als like t thank adjustment factrs btained frm ne intercmparisn statin David Legates f the University f Oklahma and tw annymus shuld nt be used fr ther climatic and hydrlgical statins reviewers fr their helpful suggestins. Carl Bensn received supprt withut further detailed analysis n wind and snw climate. frm the U.S. Natinal Science Fundatin (grant DPP ). The adjustments have t be cnducted n a daily basis fr the entire recrding perid fr each individual statin in an bser- References vatinal netwrk. These adjustments require cnsiderable sta- Aaltnen, A., E. Elmaa, A. Tuminen, and P. Valkvuri, Measuretin infrmatin (metadata) and additinal meterlgical inment f precipitatin, in Prceedings f Sympsium n Prec!pitatin frmatin (i.e., wind at gauge height n precipitatin days, and Evapratin, vl. 1, edited by B. Sevruk and M. Lapin, pp. temperature, precipitatin type, and gauge-measured amunt 42-46, Slvak Hydrmeterl. nst., Bratislava, Slvakia, f precipitatin) fr their implementatin. Unfrtunately, the Bates, R. E., T. Pangburn, and H. Greenan, WMO slid precipitatin measurement intercmparisn at Sleepers river research watershed, climate recrds generally d nt prvide infrmatin f the in Prceedings f the 44th Meeting f the Eastern Snw Cnference, gauge type and its installatin. This infrmatin is nt easy t Frederictn, New Brunswick, pp. 1-7, btain even frm the NWS publicatins. But it is essential if Bensn, C. S., Reassessment f winter precipitatin n Alaska's Arctic histrical precipitatin data are t be adjusted. The need fr slpe and measurement n the flux f wind blwn snw, 26 pp., Gephys. nst., Univ. f Ala., Fairbanks, metadata can nt be veremphasized [Gdisn and Yang, 1995]. Black, R. F., Precipitatin Barrw, Alaska, greater than recrded, The WMO Slid Precipitatin Measurement ntercmpari- Es. Trans. AGU, 35(2), , sn has prvided better adjustment prcedures fr a number Brwn, J., S. L. Dingman, and R.. Lewellen, Hydrlgy f a drainage f precipitatin gauges cmmnly used arund the wrld, such basin n the Alaskan castal plain, CRREL Res. Rep., 240, Cld Reg. as the Canadian Nipher snw gauge [Gdisn and Metcalfe, Res. and Eng. Lab., Hanver, N.H., ], the U.S. NWS 8-inch standard gauge [Yang et al., Desbis, M., and F. Desalmand, Past and present precipitatins related t climate changes--methdlgies and perspectives, Bull. 1995b], the Russian Tretyakv gauge [Yang et al., 1995a], and Am. Meterl. Sc., 76(7), , the Hellman gauge [Gunther, 1993; Yang et al., 1994]. These Glubev, V., On the prblem f standard cnditins fr precipitatin

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