Persistent layers of enhanced C} in the lower stratosphere

Transcription

1 Radio Science, Volume 36, Number 1, Pages , January-February, 2001 Persistent layers of enhanced C} in the lower stratosphere from VHF radar observations G. D. Nastrom St. Cloud State University, St. Cloud, Minnesota F. D. Eaton Air Force Research Laboratory, Kirtland Air Force Base, New Mexico Abstract. Seasonal climatologies of persistent layers of enhanced refractive index structure parameter C} were developed for the lower stratosphere from VHF radar observations White Sands Missile Range, New Mexico, for the period January 1991 to September Knowledge of the nature of enhanced refractivity layers is of high interest to the atmospheric sciences, propagation, and remote sensing communities. The layers reported have C enhanced at least 7 db above the background continuously for at least 11 hours and migrate vertically no more than one radar range gate (150 m) over 1 hour. The cumulative frequency of the lengths (11-37 hours) of the 259 persistent layers identified shows that 25% of the layers last over 17 hours. Comparisons of profiles of wind speeds, variances of the wind components, vertical shear of the horizontal wind, Doppler spectral width, temperature, Brunt-Vaisala frequency, and Richardon's number for times with and without persistent layers at 17 km show that wind speed at 5.6 km in addition to spectral width, wind shear, and vertical velocity variances at 17 km are stronger during enhanced layer episodes than during nonlayer periods. Possible sources for the persistent layers are suggested, and the shortcomings of each hypothesis are discussed. Several case studies of radiosonde ascents during persistent layers give no obvious indication of the source of these layers. 1. Introduction stratosphere than the free troposphere from a study In the stable boundary layer, laminated structures of strong temperature gradients known as "sheets" are commonly seen by sodar or radar observations as sinusoidal oscillations and are often interpreted as internal gravity waves. For example, Gossard et al. [1984] showed several hours of such layering in the lower atmosphere by using simultaneous FM-CW within thin atmospheric layers to 25 km. The diurnal evolution of the boundary layer responding to the local heating-cooling cycle can only support the layered structure during stable conditions (generally nocturnal) as seen in the Brunt-Vaisala frequency. radar and sodar observations. Their results can be In the free troposphere, long-lasting layers of enhanced reflectivity seen by mesosphere-stratospheretroposphere (MST) radars are often attributed to moist layers. In the moist atmosphere the backscattered echo power depends strongly on humidity gradients [Tsuda et al., 1988], although density gradients associated with fronts can also give persistent en- compared with several hours of layering, described as internal waves in the ocean, reported by Proni and Apel [1975] by sensing temperature fluctuations with high-frequency acoustics. Eaton et al. [1997] presented a 24 hour period of boundary layer observations with sodar that displayed the daytime development of convection, changes in the capping inversion, thermal plume structures, "neutral" events, and the wave-turbulence interaction at night. Dalaudier et al. [1994] found that temperature gradients within the stable boundary layer more closely resemble the Copyright 2001 by the American Geophysical Union. Paper number 2000RS /01/2000RS hanced reflectivity [Ruester et al., 1998]. The lower stratosphere, without the disturbing diurnal influence of the surface heat flux as in the boundary layer, or moisture as in the troposphere, but with long-term stable conditions, may support layer- ing of the refractivity structure constant, C}, for long periods of time. Highly sensitive balloon-borne instrumentation has provided excellent measurements of the turbulent thermal and velocity fields under

2 138 NASTROM AND EATON: PERSISTENT LAYERS OF ENHANCED various conditions [Barat, 1982; Barat et al., 1984; Bertin et al., 1997]. These in situ measurements have provided the ability to perform several important profile calculations but do not provide much information on long-term structure behavior as can be obtained with ground-based remote sensors. description of the physical surroundings for the radar and a map of the local terrain are given by Eaton et al. [1999a]. The WS radar observes winds using Doppler spectra on three beams: vertical and 15 ø from the zenith in the north-south and east-west planes. The radar In the lower stratosphere, thin layers of enhanced system observes along each beam for 1 min and reflectivity lasting several hours have been observed cycles through a complete profile approximately every in case studies using both UHF and VHF MST radars 3 min. Useful data are obtained on the oblique beams [e.g., Sato and Woodman, 1982; Worthington and from -- 5 to 20 km altitude and on the vertical beam Thomas, 1997]. Studies of density perturbations with from -- 7 to 20 km with a nominal vertical spacing of lidars [Mitchell et al., 1994] suggesthat persistent 150 m. The lower altitude limits are determined by layers are also found into the midstratosphere. These radar electronics systems, and the upper limit is layers are often attributed to inertial-gravity waves determined from signal-to-noise thresholds. The one- [Maekawa et al., 1984; Cornish and Larsen, 1989; way beam width is 2.9 ø, and the power aperture Larsen, 1995] or mountain waves [Hines, 1989, 1995a; product is 108 W m 2 (see Nastrom and Eaton Thomas et al., 1992]. for further details of the radar). Observations from the VHF wind profiler radar at The data set spans the time from January 1991 White Sands Missile Range (WS) also often show through September 1996, with occasional gaps of persistent layers of enhanced CN 2 [Nastrom and varying length. These data were subjected to a rigor- Eaton, 1997a; Nastrom et al., 1997]. While other MST ous quality control procedure that included rejection radars have been used in campaign modes, taking of layers with winds greater than 3 standard deviaobservations for usually only a few hours or a few days tions from the mean within any given hour, rejection per month, the WS radar was used to make continu- of layers with unphysically large vertical wind shears, ous observations over a period of several years. The and rejection of deep layers with winds near zero WS data thus provide a unique opportunity to assess (implying residual ground clutter contamination of the length and frequency of persistent layers of the spectra). Data were also rejected if the CN 2 enhanced C 2. The purpose of this paper is to present measured by the radar above 9 km altitude was the climatology of the frequency of occurrence of outside the calibrated range of the radar. In addition, if more than 40% of the levels from an individual persistent layers of enhanced CN 2 at WS. Our paper is organized as follows. The data and method used are described in section 2. The clima- tology of persistent layers is given in section 3. The relationship of persistent layers to prevailing wind and temperature conditions is presented in section 4. Possible sources of the observed layers are discussed Range (32ø24'N, 106ø21'W, 1220 rn above sea level). Organ Peak (2704 rn mean sea level (msl) and km west of the radar) is the highest peak in the San Andreas and Organ Mountains, which run northsouth west of WS and are characterized by a series of knife-edge ridges. The Sacramento Mountains also run north-south and are km east of WS. Further [1993] beam's observation were rejected for any reason, the entire profile for that beam was discarded. Some of the results given below were obtained using the individual 3 min interval profiles. However, most of this study was made using hourly median profiles since test cases showed that they generally gave the same results as individual values. Only hourly medians based on three or more observations in section 5. Our conclusions and summary comments are given in section 6. were used (typically, there were 16 observations per hour at each level). Hourly medians of horizontal 2. Data and Method winds were computed from the radar's radial winds from the oblique beams under the assumption that The data used in this study were obtained with the the hourly mean vertical velocity is negligible. Verti- 50 MHz radar located at the White Sands Missile cal velocities, taken directly from the vertical beam, generally have very small hourly medians. The backscattered power was calibrated as CN 2 using an internal noise source under the "traditional" assumption that the scattering was from homogeneous, isotropic turbulence within the sampling volume. While some studies have shown that specular reflection effects can extend to zenith angles greater

3 NASTROM AND EATON: PERSISTENT LAYERS OF ENHANCED C 139 than 15 ø [e.g., Tsuda et al., 1997], such effects are believed to be associated with gravity waves of relatively short periods that would have negligible impact on hourly medians. The WS radar data were augmented with the twice daily radiosonde observations from the National Weather Service station at E1 Paso, km south of WS. Estimates of temperature, pressure, and Brunt- Vaisala frequency squared (N2 =!70 In 0/0 z) were interpolated height and time corresponding to each hourly median value from the WS radar. Also, esti- mates of the tropopause height at E1 Paso based on Figure 1. Tropopause heights at E1 Paso (50 km south of the World Meteorological Organization (WMO) def- WS) during 1991 through Dots are individual daily values, the heavy curve is the 5 year mean for each day of inition were interpolated in time for each hourly the year, and the horizontalines show seasonal values. median profile. The yearly march of tropopause Note that the variability is larger in winter and spring heights at E1 Paso (Figure 1) is characterized by two because of occasional summer-like values. regimes: a period of high tropopauses from about June into November (when the tropical jet stream is north of E1 Paso) and a period of low tropopauses variability in Figure 2c is relatively very small at the (when the jet stream is usually south of E1 Paso). The tropopause. variability of the tropopause height is relatively small An example of a persistent layer of enhanced C 2 is during the high-tropopause period. Summaries of the winds, turbulence, and C 2 from given in Figure 3. Figure 3a shows profiles C 2 at 3 min intervals for the 12 hour period from 1700 UT on the WS radar are given by Nastrom and Eaton [1995, June 26, 1991, to 0500 UT on June 27, The 1997b, and references therein]. Since earlier studies abscissapplies for the first profile, and later profiles have shown thathe logarithm of C 2 is approximately are offset by 2 decades per hour. The layered struc- normally distributed, log C 2 is the variable we will use in this study, and for convenience we will refer to [e.g., Gage et al., 1980; Sato and Woodman, 1982]. it hereafter simply as C 2. The seasonal mean profiles Some of the layers of enhanced C 2 in Figure 3a of Ct3 based on the entire period of record are shown appear to propagate slowly upward or downward in Figure 2a. These are very similar to the earlier while some layers are at a fixed height. For example, values based on a shorter period of record given by at km the value of C 2 is enhanced relative to Nastrom and Eaton [1995]. In Figure 2a, note that CN 2 surrounding values on every profile in this example. decreases with altitude through the troposphere and Figure 3b shows all 183 profiles plotted on the same then has a local maximum near km during spring abscissa, and it is clear that the values near 17.5 km and km during summer and autumn. These local are always enhanced. Figure 3c shows the means and maxima result from the increase of C 2 at the tropo- the medians of C 2 for this episode. Note that the pause because of the increased potential refractivity means and medians are nearly identical, as expected related to the increased stability of the stratosphere when the variable is normally distributed. [Gage and Green, 1979]; in fact, when the distance In order to make a climatolog of the occurrence of from the tropopause is used as the vertical coordinate persistent layers of enhanced C the following objec- (Figure 2b), the relative shapes of the profiles are very tive definition was adopted. Each hourly profile of similar from season to season. median values of C 2 above 12 km (the upper 51 levels Most of the variability among seasons at the tropo- of the profile) was fitted in a least squaresense with pause level in Figure 2b is due to changes of density a second-order polynomial. The deviations from the p associated with the changes in height of the tropo- polynomial, excluding the 10 largest values, were pause since a dry atmosphere C 2 cr p2. Figure 2c shows the mean profiles the hourly values of C multiplied by p02/p 2, where P0 is the density in the standard atmosphere at 14 km. The interseasonal E ¾ '--' (I)...' :,...:':... ',. '.)...-;, x.....q r I' Jan' ' A F ' dul oct' dan ture seen in any individual profile is typical of C averaged to define a "background" profile C 2 for each hour. Deviations were then computed relative to the background profile. Figure 4 illustrates our procedure for finding en-

4 140 NASTROM AND EATON: PERSISTENT LAYERS OF ENHANCED CN 2, / YEAR I _ - -- Winter 4044 i... Spring / '---- Summer 7659, ß... Autumn 6848 E:)._ :'... ' x.x.' "',... E,., ",,,. -.o 2.6 ' L.o '.,, E I ',.k X,,,, 'h, ". % 2..., I / ] \...' : / \/.:' / /! /7.." (c) ' \,.,, '7.; ' Figure 2. Mean profiles C at WS during (a) as a function of altitude, (b) as a function of distance from the tropopause, and (c) as in Figure 2b, except with all values normalized to the density of the standard atmosphere at 14 km (see text). hanced layers. Figure 4a shows the observed CN 2 polynomial fitted to the hourly profile are excluded values and the best fit polynomial. Figure 4b shows (rather than the 10 largest), it will be shown below in the deviations from the background. Because the Figure 7 that the percentage frequency distributions background values of CN 2 are decreasing with height of their lengths are about the same as for the scheme from -12 to 17 km, in this case the magnitude of the used here even though approximately twice as many enhancement at 16 km in Figure 4b is much larger layers are identified. Further, the calculations were than that at any other height even though the ob- made separately for the east beam and the north served magnitudes of CN 2 at 12 and 16 km in Figure 4a beam; since the results were nearly identical, only results for the east beam will be given here. are approximately equal. Profiles of enhancements were computed using the preceding method for every hour of the entire period of record. Persistent enhanced layers are defined as periods lasting 11 or more hours when the enhancement is over 7 db at the same layer or migrates only one range gate (150 m) from one hour to the next. A layer is considered broken if the enhancements fall below 7 db or if data for an hour are missing. Further, a layer is counted only if at least one enhancement exceeds 10 db. Results do not depend qualitatively on the details of the procedure _for finding enhanced layers. For example, when the 20 largest deviations from the Figure 5 shows examples of enhancements for 5 days during winter (Figure 5a) and summer (Figure 5b). Hours when radar data are present are indicated by small triangles at 12 km. Several layers with enhanced values at the same or adjacent levels and persisting 11 or more hours are present. In Figure 5a a persistent layer begins at hour 87 at 17.5 km and lasts until hour 109 at 16 km. In Figure 5b a persistent layer begins at hour 1 at 18 km and lasts until hour 37. Other persistent layers begin at hour 39 at 18 km and hour 94 at 17 km. The latter layer descends with time, crossing the tropopause (indicated by open squares) at hour 102.

5 NASTROM AND EATON: PERSISTENT LAYERS OF ENHANCED C Log C2N (2 decodes/hour) 6/26/1700-6/27/ ' u u5 Log C2N b c o ''M'ecJi Log C2N Figure 3. (a) 12 hour time series profiles C during June 26, 1991, 1700 UT, to June 27, 1991, 0500 UT. The abscissapplies for the first profile; succeeding profiles available at 3 min intervals are shifted 2 decades per hour. Note the persistent layer near 17.5 km. (b) Composite plot of all 183 profiles from Figure 3a plotted on the same abscissa, and (c) the mean and median of the 183 profiles. 3. Frequency of Persistent Layers that there are relatively few hours in the stratosphere Figure 6a shows the percent of hours in the strato- below 15 km during summer and autumn, consissphere when a persistent layer exits for each season. tent with Figure 1; thus the spikes in Figure 6a during Figure 6b shows the percent of all hours in which the autumn below 15 km are based on relatively few data altitude along the ordinate was above the tropopause points. For this reason the curve for summer is only for each season; the total number of hours available for each season is given in the inset. Figure 6b shows plotted above 14 km in Figure 6a. In Figure 6a the percent of hours with a layer I Log C2N i i i i i i i i i i i Deviotions of Log C2N Figure 4. A demonstration of the definition of enhanced C 2: (a) observed hourly median values for February 5, 1991, 1200 UT, and the second-order polynomial fitted to them and (b) Deviations from the polynomial curve except excluding the 10 largest values. See text.

6 142 NASTROM AND EATON: PERSISTENT LAYERS OF ENHANCED C 2 x x x X x x x x x fix x x Hour 120 o Hour Figure 5. Time series plots of deviations from the background C 2 eac hour: 5 days (a) beginning February 2, 1991, and (b) beginning June 26, Symbols: solid boxes, over 13 db above background; bold crosses, over 10 db; small crosses, over 7 db; open boxes, tropopause height at E1 Paso; small triangles along abscissa, hours that have data. Note the persistent layers of enhanced C 2 values. ranges between 1 and 2% in most cases, although the the 10 largest. The solid and dashed curves are nearly autumn values are usually less than 1%. The largest identical.) value (ignoring the spikes in autumn) is --2.7%, found during spring near 17.5 km. The probability of a layer is very small above 19 km during all seasons. 4. Associated Meteorology The total number of layers found is 259. The Seasonal mean profiles of several variables of atdistribution of layers by season is 48, 87, 78, and 46 for mospheric wind and temperature for those hours that winter (December, January, and February), spring, had a persistent layer present at 17 _ km were summer, and autumn. The cumulative frequency of compared with those for hours without a layer at lengths of persistent layers is given in Figure 7. By those heights. (Results using hours that had a persisdefinition, all of these layers are 11 or more hours in tent layer at 16 km were studied also and are not length; 39 (15%) are 20 hours or longer. The longest presented since they were very similar). The variables layer lasted 37 hours (the episode during June 1991 in used were zonal (u), meridional (v), and total hori- Figure 5). (The dashed curve in Figure 7 is the zontal wind speed, vertical velocity w, the hourly cumulative frequency of lengths of the 536 layers standard deviations of horizontal and vertical velocidetermined excluding the 20 largest deviations from ties Crw, the vertical shear of the horizontal wind the polynomial fitted to the hourly profile, rather than (S 2 -- (au/az) 2 + (Ov/Oz)2), computed over 300 rn

7 NASTROM AND EATON: PERSISTENT LAYERS OF ENHANCED C o ' ' -- Winter 3899 oo ----Spring 6247 ß - Summer Autumn ' O0 STRATOSPHERE Percent of Hours with Ioyer. Percent of hours in the strotosphere Figure 6. (a) Seasonal climatology of persistent layers of enhanced C 2 in the stratosphere and (b) percent of hours in the stratosphere each height. The total number of hours available is shown in the inset. See text. intervals, the Doppler spectral width (O't, corrected for beam.-, shear-, and wave broadening following Nastrom and Eaton [1997b], related to the eddy dissipation rate e by e = 0.3rrt2N), temperature (from. E1 Paso radiosondes), N 2, Ri -= N2/S 2, and the backscattered power on the vertical beam.. Even though much of the backscattered power on the vertical beam is from. processes other than Bragg scattering, the vertical power will be called vertical "C}" since it was calibrated using the same algorithm. as used with the oblique power, permitting direct comparison with the oblique beams' C}. Significant differences between the mean values of C 2 for hours with and without a persistent layer present at 17 km were found only in the region near 17 km as illustrated in the upper left panel of Figure 8 for the spring season; results for other seasons (not o o,, f/\ I Nortool bockground (No=259). I 36) I shown) were similar. Error bars extending +-2it/X/No, where rr is the standard deviation of the No hours in each group, are entered at 5.6, 11.3, and 17.0 km in Figure 8. Mean wind speeds in the troposphere are significantly greater during hours with layers, while mean wind speeds in the lower stratosphere are not significantly different between the two groups during spring. The patterns during winter and summer are similar to spring, but during autumn there is no significant difference in wind speeds at any altitude. Table 1 compares the mean values of wind speed at 5.6 km and other meteorological variables at 17 km of the two data groups for each season; note that a significant difference is present in wind speed during all seasons except autumn and in rr w during winter and spring. Nastrom and Eaton [1993] suggesthat strong winds over the rough topography near WS act as a significant source of gravity waves. Indeed, Figure 8 shows that rr w, taken as a proxy indicator of gravity wave intensity, is larger through a deep layer around 17 km during layer hours. The vertical wind shear S at 17 km is significantly larger during hours with layers during all seasons (Figure 8 and Table 1). However, there is no signifi- cant difference for N 2 (not shown) between the two data groups, perhaps because the radiosonde data do not contain sufficient resolution in space or time; a case study by Hermawan and Tsuda [1999] found that Length of Ioyer (hours) variations of N 2 from. local radio acoustic sounding system. (RASS) measurements were as important as Figure 7. Duration of persistent layers of enhanced C 2 in hours as a percent of total number of layers No. The dashed variations of radar spectral width for determining curve is for persistent layer determined using reduced changes in the eddy dissipation rate e. Thus, since background profiles as explained in the text. Ri = N2/S 2 and since N 2 in our data shows relatively

8 144 NASTROM AND EATON: PERSISTENT LAYERS OF ENHANCED C o Shear., SPRING] 4. ß "" ' '1 7.5 '1'6.5 4' '1 '2' ' '2'0' ' '2'8' ' '3'6' ' b.dl 6' 'O.024 Vertical 'C2N O- w ' 0:2' 0:6' 1'0' 1'4' 1 8 Figure 8. Profiles of the means for hours with and without a persistent layer present at 17 km (_+0.15 km) of log C 2, wind speed (ms- ), vertical shear of the horizontal wind (s- ), Doppler spectral width (ms- ), hourly standardeviation of vertical velocity (ms- ), and log of the vertical backscattered power calibrated as C 2. Error bars extending -+2rr/V'No are entered at 5.6, 11.3, and 17.0 km. little variation, it is not surprising that we find that the pattern for Ri resembles the inverse of that for S. Enhanced CN 2 has often been associated with en- hanced S [Smith et al., 1983; Sato, 1989; Worthington and Thomas, 1997]. Nastrom et al. [1997] found that large values of S in the stratosphere are a sufficient condition for large values of C (i.e., large values of shear usuall have large values of C 2) buthat large values of C also occur with small values of shear. The spectral width (corrected for beam-, shear-, Table 1. Mean Values of Meteorological Variables at 17 km During Hours Without and With a Persistent Layer at 17km Winter Spring Summer Autumn Without With Without With Without With Without With Layer Layer Layer Layer Layer Layer Layer Layer Number Wind speed (5.6 km) 15.6 _ _ o' w, m s _ _ _ 0.01 Shear, 10-2 s _ _ _ , 10-4 m 2 s _ _ (w), 10-2 m s _ _ _ Temperature, K _ _ _+ 0.6 Wind speed (17 km) 19.2 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 0.96

9 NASTROM AND EATON: PERSISTENT LAYERS OF ENHANCED CN and wave broadening following Nastrom and Eaton [1997b]) and e are larger at 17 km during hours when a layer is present in all seasons (Figure 8 and Table 1). Considering that the layers are relatively very long lasting (over 11 hours; median duration about 14 hours), the time-integrated effect of the increased eddy dissipation rate implies a very robust energy source for the layers. The enhancement in vertical C 2 during hours with a layer present is significant at 17 km. However, the magnitude of the increase is only -2 db, compared to -10 db for C. This difference in relative magnitude is smaller for the vertical beam because other reflec- tion and scattering processes act on the vertical beam. Table 1 includes the mean values for vertical velocity, temperature, and wind speed at 17 km for completeness, although these variables show no consistent differences when layers are present. The mean vertical velocity at 17 km is significantly larger during layer hours only during winter and spring, and the mean temperature is significantly different during layer hours only in winter. The differences in wind speed during summer are statistically significant, but because they are only 1 m s -, it is unlikely that they are physically important. 5. Discussion The physical mechanism responsible for the layers studied here is unclear but must be related to the product of turbulence intensity and stabili[y. As dis- cussed by Gage et al. [1980], the value of C sampled by the radar is given by C =Fa 2_,r a o 4/3.2 v, (1) where F is the fraction of the radar sampling volume that is actively turbulent, a is a constant, a' is the ratio of eddy diffusivities and is usually assumed to be unity, L o is the so-called outer scale of turbulence, and M is the gradient of the refractive index. Under dry conditions, such as prevail in the stratosphere, M - pn 2, where p is the density, and since the skil, 1971], where /= 1 is usually assumed. Using the relations given above and assuming that p is constant, (1) can be rewritten as C3 = (const) FRie 2/3N2. (2) It seems likely that there is an inverse relationship between Ri and F [e.g., VanZandt et al., 1978; Bertin et al., 1997]. Thus (2) indicates that the volume average C 2 in the stratosphere is proportional only to a constant, 82/3 and N Frozen Turbulence One suggestion is that the layers are embedded in the background flow and are being advected past the radar, like large pancakes of frozen turbulence. Layers of increased stability with large horizontal extent are often associated with the tropopause, as demonstrated by Danielsen [1959] and others, and could perhaps have counterparts in the lower stratosphere. In this case, the horizontal extent of the layers must range up to 900 km during winter on the basis of their duration (14 hours) and typical background wind speeds (15 m s- from Table 1). This suggestion is supported by the study of Eaton et al. [1999b], who compared measurements of C2 from the WS radar and an instrumented C-135 air- craft in a case study of a layer near the tropopause. They found that the layer had horizontal extent in excess of 100 km. Further, by assuming a "frozen turbulence" hypothesis they were able to maximize the correlation between the radar observations and the aircraft observations, which were spatially separated from the radar by up to 100 km. A key feature of a layer associated with "frozen turbulence" would be increased stability (N 2) as indicated by (2) and as suggested by the results of Danielsen [1959] and Dalaudier et al. [1994]. Figure 9 shows profiles of the hourly mean C 2, e, S, and horizontal winds from WS, in addition to profiles of temperature and S from radiosondes launched at the Oasis site (-3 km from the WS VHF radar) during three separate episodes of persistent layers. The radiosondes are launched at the Oasis site at irregular intervals in support of other activities at the missile range, and thus there are only a few ascents that coincide with a persistent layer. Rising at 5 ms-1 the balloon reaches 18 km -1 hour after launch. The radiosonde data have 305 m (1000 feet) vertical relative changes in p are small, the changes in M are resolution. most closely related to the changes in N 2. Under In Figure 9 a horizontal dotted line has been 2 locally isotropic conditions, Lo = 7 - ls1-3 [Tatar- entered in the four leftmost panels in each row at the altitude of the maximum C N. In each case there are corresponding maxima in e and in S from the radar (although the maxima in S are not at exactly the same altitude). However, increased stability is indicated by the temperature profile only in the center case while

10 146 NASTROM AND EATON: PERSISTENT LAYERS OF ENHANCED C ; '- -4 Log Ci -3-2 Log ½ '- :"' I-" Shear!...,'?:.,. o s,s! ' ".'Sz.'... : ' WS Shear [s- ] Temp[K] and Shear [10-3s- ] ' 10 ' 2'0 ' 3b ' 4( WS Winds (mrrj I > %: o s,s t ' '=' -4 Log ' Log ½ WS Shear Is-1] ' , 30,, 40 I o Temp(K] and Shear (10-3s- ], NS Winds (ms- _jl... /, ' '- -4 Log Log ½ ' ld ' lb 20 3b '40 ws Shear (s-i] Temp(K) and Shear (10-3r ] WS Winds Figure 9. Profiles hourly median C 2,, u, v, and wind shear based on WS radar data for August 10, 1992, 1300 UT (upper row), September 13, 1993, 0600 UT (center row), and October 8, 1993, 1300 UT (lowerow), plus temperature and wind shear based on radiosondes launched at WS at the times indicated. A dashed line is entered at the height of the tropopause interpolated time from the E1 Paso soundings. A dotted line indicates the level of the maximum C 2. The balloon-based shears have been multiplied by 103 and then shifted 200. the other two caseshow near-neutral stability. Fur- Several studies have found that the horizontal scales ther, note that the pattern of the shear based on the of inertial-gravity waves are on the order of a few radiosonde winds has little correlation with that from hundred kilometers [Sato, 1989, 1994; Thomas et al., the WS radar winds. Finally, the profiles of u and v 1992, 1999], which is large enough to account for the given in the rightmost panels show that persistent aircraft observations of Eaton et al. [1999b]. However, layers occur with winds from the east and west and the waves would have to have a continuous energy from the north and south and with highly variable source to survive the increased eddy dissipation rate profile shapes. Thus, in these three cases the coinci- (Table 1) in the layers. The total energy dissipation dent radiosondes give no clear indication of the during 14 hours during winter is -60 m 2 s -2 corresource of the persistent layers. sponding to the dissipation of a wind perturbation 5.2. Inertial-Gravity Waves amplitude of -11 ms-1. The amplitudes of most inertial-gravity waves that have been observed at WS Some studies have suggested that large-scale layers are much less than 11 m s -1 [e.g., VanZandt and could be generated by inertial-gravity waves [Cornish Nastrom, 1997], and thus they would require a conand Larsen, 1989; Worthington and Thomas, 1997]. tinuous regeneration. While several possible sources

11 NASTROM AND EATON: PERSISTENT LAYERS OF ENHANCED C 147 Table 2. Correlation of Hourly Median C} at 17 km With Meteorological Variables at the Altitude Indicated in Parentheses Number Wind Shear (17 km) Ri (17 km) Wind Speed (5.6 km) Winter Spring Summer Autumn of inertial-gravity waves have been discussed [e.g., Sato, 1994; Thomas et al., 1999], we are not aware of any source mechanism that would accomplish the regeneration required here Mountain Waves Another suggestion (following Hines [1989, 1995b]) is that the layers are due to instabilities in upward propagating mountain waves. Indeed, Nastrom and Eaton [1993] attributed the strong enhancement of CN 2 that they observed in the lower stratosphere during periods of high winds in the midtroposphere to gravity waves launched by flow over the terrain. Table 2 confirms their result, showing that CN 2 at 17 km is more strongly correlated with wind speed at 5.6 km than with shear or Ri at 17 km. It is also interesting to note in Figure 8 that the mean wind speeds in the midtroposphere are stronger during layer episodes than at other times. However, in order for a single wave event to produce a layer at a constant or slowly varying altitude, lasting for hours, the excitation mechanism and propagation environment for the wave would have to be nearly constant. A case study of a layer in June 1991 by Nastrom and Eaton [1997a] shows that the winds below the layer are highly variable with time, even changing direction, with no apparent effect on the layer. We have examined several other cases and find similar or even larger changes of the lower atmosphere winds. Also, the cases in Figure 9 show a wide range of speeds and directions for the tropospheric winds. Thus it seems unlikely that upward propagating mountain waves are an important source of the persistent layers seen at WS. 6. Summary and Conclusions Layers of enhanced C 2 in the lower stratosphere have been studied using observations from the VHF radar at WS over the period January 1991 to September A layer is defined to exist when C 2 is enhanced 7 db or more above the background values and the enhancement persists unbroken for 11 hours or more. Altogether, 259 layers were found, although this is a conservative count since missing data for even a single hour break a layer and there are a significant number of hours with missing data. The frequency of occurrence of layers is greatest in spring and summer and least in autumn and winter. The distribution with height of the probability of occurrence of layers averages from 1 to 2% below km and falls to near zero at 19 km. Wind speed at 5.6 km is significantly greater during hours when a layer is present at 17 km. Also, S,, and O'w at 17 km are all greater during layer hours. No consistently significant difference was found between hours with and without a layer for (w}, temperature, Ri, N 2, or hourly variance of the horizontal wind. The source of these layers is not known. Case studies of temperature and shear profiles from radiosonde ascents made at WS during persistent layer episodes do not show any unusual stability or shear signature. The observed conditions of wind and tem- perature are not consistent with any of the generation mechanisms considered here (frozen turbulence, inertial-gravity waves, and breaking mountain waves). Acknowledgments. We are grateful to H. Luce and an anonymous reviewer for helpful comments. G.D.N. was partially supported by the Air Force Office of Scientific Research. References Barat, J., Some characteristics of clear-air turbulence in the middle stratosphere, J. Atmos. Sci., 39, , Barat, J., and F. Bertin, Simultaneous measurements of temperature and velocity fluctuations within clear air turbulence layers: Analysis of the estimate of dissipation rate by remote sensing techniques, J. Atmos. Sci., 41, , Bertin, F., J. Barat, and R. Wilson, Energy dissipation rates, eddy diffusivity, and Prandtl number: An in situ experi-

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13 NASTROM AND EATON: PERSISTENT LAYERS OF ENHANCED C 149 Tsuda, T., P. T. May, T. Sato, S. Kato, and S. Fukao, Simultaneous observations of reflection echoes and re- fractive index gradient in the troposphere and lower stratosphere, Radio $ci., 23, , Tsuda, T., T. E. VanZandt, and J. Saito, Zenith-angle dependence of VHF specular reflection echoes in the lower atmosphere, J. Atmos. Sol. Terr. Phys., 59, , VanZandt, T. E., and G. D. Nastrom, Observations of inertial waves in the stratosphere using wind-profiling radars, paper presented at 8th Workshop on Technical and Scientific Aspects of MST Radars, Bangalore, India, Dec , VanZandt, T. E., J. L. Green, K. S. Gage, and W. L. Clark, Vertical profiles of refractivity turbulence structure constant: Comparisons of observations by the Sunset radar with a new theoretical model, Radio Sci., 13, , Worthington, R. M., and L. Thomas, Long-period unstable gravity-waves and associated VHF radar echoes, Ann. Geophys., 15, , F. D. Eaton, Air Force Research Laboratory, 3550 Aberdeen Street, S. E., Kirtland AFB, NM (frank.eaton@kirtland.af. mil) G. D. Nastrom, Department of Earth Sciences, St. Cloud State University, 720 Fourth Avenue South, St. Cloud, MN (nastrom@stcloudstate.edu) (Received January 11, 2000; revised September 26, 2000; accepted October 18, 2000.)