UV measurements in the m altitude region in Tibet

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2006jd007700, 2007 UV measurements in the m altitude region in Tibet Arne Dahlback, 1 Norsang Gelsor, 2 Jakob J. Stamnes, 3 and Yngvar Gjessing 4 Received 26 June 2006; revised 7 December 2006; accepted 19 December 2006; published 8 May [1] We present measurements of solar UV radiation performed with multichannel moderate-bandwidth NILU-UV filter instruments during winter and summer in 2003 in the altitude region from 3000 m to 5000 m at 29N in the Lhasa region in Tibet. During summer the UV index was found frequently to exceed 15 on clear days and occasionally to exceed 20 on partially cloudy days. High altitudes, low ozone column amounts, clean atmospheres, and relatively low latitudes are factors that contribute to the high UV levels on the Tibetan plateau. UV index values of 12 were measured in late winter for a solar zenith angle of 40 at a snow-covered 5000 m altitude site. This is a 35% increase compared to a corresponding snow-free surface. Our measurements show that the solar UV radiation increases with altitude. For clear-sky and snow-free conditions the altitude increase is 7 8% per km for erythemal UV dose rates and 3% per km at a wavelength of 340 nm. Results from clear-sky calculations using a multiple-scattering radiative transfer model were found to agree within 5% with clear-sky UV measurements. Radiative transfer calculations combined with measurements were used to estimate the influence of clouds on the UV radiation at the surface. On the average the variable cloud cover in Lhasa reduced the daily integrated erythemal UV dose by 25%. The NILU-UV instruments also provide total ozone column amounts. The mean difference between daily total ozone column amounts derived from NILU-UV measurements and from Earth Probe TOMS data was 1.4% ± 3.2% (1s). Citation: Dahlback, A., N. Gelsor, J. J. Stamnes, and Y. Gjessing (2007), UV measurements in the m altitude region in Tibet, J. Geophys. Res., 112,, doi: /2006jd Introduction [2] The sensitivity of surface UV amounts to a number of physical factors, e.g., stratospheric ozone, surface albedo, clouds and aerosols. This has recently been explored by Bais et al. [2007]. The Tibetan plateau receives high UV levels because of its high altitude (the average height is 4000 m above the sea level), relatively low latitude, and low total ozone column amounts [Ren et al., 1997]. [3] In general the biological effect of UV radiation depends on the wavelength. A UV dose rate D can be defined as D ¼ Z 1 0 AðlÞEðlÞdl: [4] Here A(l) is a weighting function that describes a certain biological effect and E(l) is the spectral irradiance at wavelength l. The UV dose rates presented in this paper are 1 Department of Physics, University of Oslo, Oslo, Norway. 2 Environmental Physics Institute, Tibet University, Lhasa, China. 3 Department of Physics and Technology, University of Bergen, Bergen, Norway. 4 Department of Geophysics, University of Bergen, Bergen, Norway. Copyright 2007 by the American Geophysical Union /07/2006JD ð1þ based on the widely used CIE action spectrum [McKinlay and Diffey, 1987] and represent erythemal UV dose rates. By using equation (1), we may express the dose rate in W/m 2. However, the UV dose rate may also be expressed as a UV index. The UV index [World Health Organization, 2002] is a dimensionless quantity and is defined as the erythemal UV dose rate in W/m 2 multiplied by 0.04 m 2 /W. We have chosen to present UV dose rates in terms of the UV index because this is a measure that many nonexperts are familiar with. [5] In this paper, we present results from measurements with multichannel moderate-bandwidth NILU-UV filter instruments [Høiskar et al., 2003] at four sites at altitudes between 3000 m and 5000 m in the Lhasa region in Tibet. The objectives of this work are to present UV index levels and total ozone column amounts based on measurements from a winter and a summer campaign that took place in 2003 and to estimate effects of cloud cover, snow cover, and altitude on the measured erythemal UV radiation. 2. Instruments and Locations 2.1. Instruments [6] The instruments used during the summer and winter campaigns on the Tibetan plateau in 2003 were 4 NILU-UV irradiance radiometers. The NILU-UV is a six-channel radiometer designed to measure hemispherical irradiances on a flat surface. UV doses with various action spectra and total ozone column amounts can be derived from the 1of10

2 measured irradiances by using a technique described by Dahlback [1996]. Five channels are in the UV spectral region with center wavelengths at 305 nm, 312 nm, 320 nm, 340 nm, and 380 nm, each having a bandwidth of approximately 10 nm FWHM (Full Width at Half Maximum). The sixth channel measures the visible radiation between 400 nm and 700 nm. The instrument has a built-in data logger and is temperature stabilized at 40C. For each of the channels the data logger automatically records 1-min averages every minute. At two of the mountain sites the instruments were run with solar cells and batteries. The instruments were calibrated against a Bentham D150 highwavelength resolution spectroradiometer (bandwidth 0.8 nm at FWHM) at the Norwegian Radiation Protection Authority (NRPA) in Oslo, Norway using a technique described in detail elsewhere [Dahlback, 1996]. The NILU-UV instrument provides results that are in close agreement with those obtained from state-of-the-art UV and ozone measuring instruments [Høiskar et al., 2003]. [7] Laboratory measurements show that the NILU-UV instruments have cosine responses that deviate from the ideal cosine response. For clear-sky erythemal UV dose rates this deviation results in an overestimation of about 3% at SZA = 0 and an underestimation of about 3% at a SZA of about 60 [Høiskar et al., 2003]. Their results were based on the measured angular responses and radiance distribution at sea level for a Rayleigh scattering atmosphere using a multiple scattering radiative transfer model. We have repeated their calculations for an altitude of 5000 m and found that the overestimation for an overhead sun increases to about 4% because of less diffuse radiation and more direct beam at these altitudes. The maximum underestimation is found at SZA = 75 and is 9%. However, the UV measurements presented in this paper are corrected for errors in the cosine response. [8] At the start and end of each of the summer and winter campaigns, we carried out comparisons in Lhasa of all instruments taking part in the campaigns. Two of the instruments were calibrated at NRPA in August 2002, and the other two were calibrated at NRPA in June 2003 (between the two campaigns). Only small and negligible changes in the sensitivity of the instruments were detected during the campaigns (1%) Sites and Campaigns [9] Four sites within a distance of 330 km in the eastern part of the Lhasa region were selected for the two campaigns: Linzhi at 3000 m altitude, Lhasa at 3700 m, Rodog at 4383 m, and Mt. Milha at 5060 m. The winter campaign took place in the period 19 February to 1 March 2003 and the summer campaign in the periods 25 June to 5 July and 26 July to 18 August (the interruption from 6 to 25 July was due to bad weather conditions). In addition, we have a continuous ozone record for Lhasa in the summer from 30 May to 7 October (However, the UV-dose record extends to 15 September because of problems with one of the channels.) The Linzhi site was not used in the summer campaign. Geographical coordinates and altitudes of the four sites are given in Table 1. [10] All sites are surrounded by mountains. The maximum elevations above the horizon from mountains or hills are about 5 for the Mt Milha site and about 10 for the other Table 1. Four Sites Used in the Campaigns Station Latitude Longitude Altitude Linzhi 29.66N 94.36E 3000 m Lhasa 29.63N 91.11E 3698 m Rodog 29.69N 92.23E 4383 m Mt. Milha 29.82N 92.34E 5063 m three sites. If we assume an isotropic radiance distribution and that the sky is blocked for all angles less than 10 above the horizon the irradiance would be lowered by 3% compared to a surface with a perfect horizon. However, this is only a crude estimate, since the sky radiance distribution usually is far from isotropic. We expect that the effect of the surrounding mountain tops is small for high sun but may be of greater importance for low sun. 3. Radiative Transfer Model [11] The UV radiation model used in this work is based on the discrete ordinate solution of the radiative transfer equation [Stamnes et al., 1988], which is modified to account for the curvature of the atmosphere [Dahlback and Stamnes, 1991] in order to obtain accurate results also at low solar elevations. The model includes all orders of multiple scattering and absorption, and the ground is treated as a Lambert reflector. The ozone, air pressure, and temperature profiles were taken from NOAA [1976]. The ozone cross sections at 46C were taken from Molina and Molina [1986], and the Rayleigh-scattering cross sections were obtained from an empirical formula by Nicolet [1984]. The extraterrestrial solar spectrum used in the calculations was the high-wavelength resolution Atlas3 solar spectrum (the Atlas3 extraterrestrial solar spectrum was taken from ftp://susim0.nrl.navy.mil/pub/atlas3/ in March 2001) interpolated to 1 nm bandwidth through convolution with a 1 nm wide rectangular slit function. All UV calculations were adjusted for the eccentricity of the Earth s orbit around the Sun. 4. Measurements 4.1. Ozone [12] The total ozone column amount W can be derived from measurements with multichannel moderate-bandwidth irradiance radiometers by comparing a measured ratio N given by Nðq 0 ; WÞ ¼ E i ðq 0 ; WÞ=E j ðq 0 ; WÞ ð2þ with the corresponding calculated ratio [Stamnes et al., 1991; Dahlback, 1996]. Here q 0 is the solar zenith angle (SZA), and E i and E j are the irradiances measured in channels i and j, which have different ozone absorption cross sections. A lookup table of N as a function of q 0 and W was generated using the radiative transfer model described in section 3. Sensitivity tests carried out by Dahlback [1996] showed that separately the presence of uniform clouds or a change in the surface albedo has an influence of less than 1% on the derived total ozone column amount. However, the error is larger for a combination of optically 2of10

3 Figure 1. Daily total ozone column amounts over Lhasa during the summer and winter campaigns in 2003 (left axis) measured with a NILU-UV instrument (solid line). NILU-UV/TOMS ratios (right axis) are shown as open squares. thick clouds and a high surface albedo [Dahlback, 1996]. The error in the angular response of the instrument has a minimal effect on the retrieved total ozone column amount because it is derived from a ratio between the outputs from two channels [Høiskar et al., 2003]. Our calculations show that N depends on the site altitude. Therefore we generated lookup tables for each of the four sites (i.e., for altitudes of 3000 m, 3700 m, 4400 m, and 5000 m). If a lookup table calculated for a site at the sea level were used to derive the total ozone column amount from measurements at an altitude of 5000 m, the total ozone column amount would be underestimated by approximately 2.5%. [13] Figure 1 shows total ozone column amounts derived from measurements with the NILU-UV instrument in Lhasa during the winter and summer campaigns in The total ozone column amounts varied between 240 and 280 DU during the summer. These low values are similar to those usually found in tropical regions. Low total ozone column amounts combined with high surface elevations and small SZAs in the summer contribute to the high levels of UV Figure 2. One-hour time averages of erythemal UV dose rates around local noon in Lhasa (3700 m, red line) and Mt. Milha (5000 m, blue line). 3of10

4 Figure 3. UV dose rates measured in Lhasa in June July One-minute averages every 10th minute from sunrise to sunset are shown. radiation observed in Tibet. The ozone values derived from NILU-UV measurements were found to agree well with those derived from TOMS data, both during winter and summer. Thus, for the measurements in 2003 the relative difference between ozone values derived from NILU-UV and TOMS measurements was found to be 1.4% ± 3.2% UV Dose Rates [14] Figure 2 shows daily 1-hour averages of the erythemal UV dose rates around local solar noon in Lhasa (3700 m) and Mt. Milha (5000 m). In June August the 1-hour noon-average UV index frequently exceeded 15 at both sites. This is considerably higher than typical summer maximum UV index levels around 9 11 measured at San Diego, U.S.A. at 32N and located at sea level [Biospherical Instruments, Inc., 2006]. The large day-to-day variations seen in Figure 2 are mainly due to changes in the cloud cover because the total ozone column amount over Tibet is quite stable. The June July average of the UV index in Lhasa is 12.2 ± 2.8 (1s). During the summer campaign the different UV index values observed at the sites were mainly due to different local variations in the cloud cover. Figure 3 Figure 4. UV dose rates measured at Mt. Milha in the period day One-minute averages every minute from sunrise to sunset are shown. 4of10

5 Figure 5. Calculated 1-hour time averages of clear-sky erythemal UV dose rates around local noon in Lhasa in the period A surface albedo of 0.05 and daily ozone column amounts from TOMS were used in the model. shows the measured UV dose rates in Lhasa in the period June July 2003 from sunrise to sunset with a 10-min time resolution as a function of the SZA. The maximum UV index value was 19 (does not appear in Figure 3 because of the time resolution used), and UV index values larger than 10 occurred frequently for SZAs below 40, suggesting that UV index values above 10 may occur from March to September during periods with normal total ozone column amounts and limited cloud cover. Measured UV dose rates with 1 min time resolution for a 10 day period (day ) from Mt. Milha is shown in Figure 4 where the maximum UV index exceeded 20. [15] Bodhaine et al. [1997] presented spectral irradiance measurements from Mauna Loa, Hawaii (19.5N and altitude 3400 m) for a period of about one year. The maximum UV index levels at summer solstice were around 18 as in Lhasa. They presented measured clear-sky UV index levels for a variety of total ozone column amounts for SZA = 45. This allows us to compare our UV index measurements with their measurements for the same atmospheric conditions and SZA. In Lhasa the measured UV index was 6.7 at SZA = 45 on 28 February. The sky was clear, the ground was snow-free and the total ozone column was 260 DU. The measured erythemal irradiance for the same conditions at Mauna Loa [Bodhaine et al., 1997] was 17 mw/cm 2 which corresponds to a UV index level of 6.8, essentially the same value as measured in Lhasa. [16] Using daily ozone values derived from TOMS data, we calculated daily 1-hour averages around local solar noon of the clear-sky UV index in Lhasa for the period The results shown in Figure 5 indicate that the UV index varied from between 4 and 6 at winter solstice to between 14 and 16 at summer solstice. These calculations show that a clear-sky UV index above 10 may occur frequently between 1 March and 1 October, in agreement with our suggestions based on the measurements presented in Figure 3. [17] During winter the surface albedo differed considerably between the four sites because of the presence of snow cover on the ground at some of the sites (Rodog and Mt. Milha). This gives larger differences between the UV dose rates observed at the four sites during winter than during summer. During the winter campaign in 2003 the clear-sky UV index measured at Mt. Milha (snow covered) exceeded 12, while the clear-sky UV index simultaneously measured in Lhasa (with no snow) did not exceed 9. [18] Figure 6 shows measured and calculated erythemal UV dose rates in Lhasa on day 172. The sky was clear after 0630 UTC. A surface albedo of 0.05 and the total ozone column amount derived from the NILU-UV measurements (265 DU) were used as input to the model. The calculated erythemal UV dose rates in Figure 6 (and in all other erythemal UV dose rate calculations in this paper) are multiplied by 0.95 to make the model results agree with several clear-sky UV measurements performed with one of the NILU-UV instruments in Oslo. The aerosol optical depths derived from direct sun measurements with a Brewer spectrophotometer for clear-sky in Oslo are typically 0.1 in the UV-B spectral range. Therefore we assume that the effect of aerosols on UV irradiance in Oslo is rather small. Thus the effect of scaling the model by a factor of 0.95 is mainly to tune the model to the calibration level of the instrument. Note that there are episodes before noon in Figure 6 with UV index values exceeding the clear-sky values. This is due to the presence of broken cloud cover. Then the unobscured direct beam from the Sun enters the instrument, and the diffuse radiation measured by the instrument is enhanced by Mie scattering from clouds near the solar disk. This is a well-known effect, and we observed episodes of short duration at Rodog and Mt. Milha in the 5of10

6 Figure 6. Measured (solid line) and calculated clear-sky (dashed line) UV dose rates in Lhasa (3700 m) on day 172. The calculations were based on an assumed surface albedo of 0.05 and a measured ozone column amount of 265 DU. period June with UV index values exceeding 20, corresponding to an increase of about 30% compared to the clear-sky value. The highest UV index value was 20.5 observed at Rodog at solar noon (SZA = 6.2). This is similar to the highest UV index level reported from Mauna Loa, Hawaii, which was 20.5 Bodhaine et al. [1997] Effects of Snow-Covered Surfaces [19] The UV-A channels of the NILU-UV instrument with center wavelengths at 340 nm and 380 nm are not sensitive to changes in the total ozone column amount because the ozone absorption cross section is very small in the UV-A spectral region. The combined effect of clouds, aerosols, and surface albedo may be expressed in terms of a simple ratio [e.g., Bodeker and McKenzie, 1996; Dahlback, 1996; Høiskar et al., 2003], i.e., CLT ¼ Emeas ðq 0 Þ E clear 100% ð3þ ðq 0 Þ where E meas (q 0 ) is the measured irradiance in the 340 nm channel and E clear (q 0 ) is the calculated clear-sky irradiance with no aerosols and zero surface albedo for a solar zenith angle q 0. For a fixed surface albedo, the variations in the CLT are mainly due to variations in the cloud cover. If the atmosphere contains only small column amounts of aerosols, the sky is clear, and the ground is not covered with snow, so that a low surface albedo of typically about 5% prevails, we expect the measured CLT to be close to 100%. If there is snow on the ground and the sky is clear, the CLT may exceed 100%. [20] For the high-altitude sites in Tibet we define the CLT in two ways: (1) CLT site, for which the denominator E clear (q 0 ) in equation (3) is calculated for a site at the same elevation as the measurement site, and (2) CLT sea, for which E clear (q 0 ) is calculated for a site at the sea level. During the winter campaign in 2003 there were 2 days (day 59 and day 60) with clear-sky conditions simultaneously at three of the sites, Linzhi (3000 m), Lhasa (3700 m), and Mt. Milha (5000 m). During the summer campaign Lhasa and Mt. Milha had clear-sky conditions simultaneously on day 218 and day 219. The CLT values for these 4 days are shown in Table 2. On the winter days 59 and 60 the surface at Mt. Milha was covered with fresh snow, and the CLT site value was 120 on day 59 and 115 on day 60. All other values in Table 2 correspond to a bare ground surface, and the CLT site values lie between 91 and 92 for these three Tibetan sites. We also measured clear-sky CLT values in Oslo, Norway (at the sea level) on several days with one of the instruments that was used in the two campaigns in Tibet, and found a typical CLT value of 95. Thus the bare ground CLT site values are similar for the Tibetan sites and Oslo. Further, all these CLT values are quite close to the modeled clear-sky CLT (100%) values. Therefore we may conclude that the atmospheric conditions at the Tibetan sites and in Oslo are similar and that aerosol effects are small. All measurements in Table 2 were taken at a SZA of 40. [21] Typical values of albedo for snow-covered surfaces are around 0.8 [e.g., McKenzie et al., 1998; Kylling et al., 2000b]. Snow albedos close to unity in clean environments in Antarctica have been reported by Grenfell et al. [1994] and Wuttke et al. [2006]. It has been shown that the measured downwelling UV irradiance is affected by reflection properties of surfaces several kilometers away [Ricchiazzi et al., 1995; Degünther et al., 1998; Kylling et al., 2000b]. Figure 7 shows measured and calculated UV dose rates for Mt. Milha (5000 m) on day 59. The surface was covered with fresh clean snow within a distance of more than 20 km away from the site. However, some small areas in the surroundings had little snow because of recent strong winds which might have affected the average albedo. To obtain agreement with the measured UV dose rates we used the ozone column amount derived from the NILU-UV measurement (254 DU) as input to the model calculations 6of10

7 Table 2. Clear-Sky CLT Values for the 340 nm Channel Measured at Three of the Tibetan Sites During Winter and Summer a Lhasa Altitude Reference Linzhi, Winter Winter Summer Winter b Summer Oslo, Summer Site altitude Sea level a The winter values are for day 59 and day 60 on which the sky was clear at all three Tibetan sites. The clear-sky summer values were measured on day 218 and day 219. Typical clear-sky measurements for Oslo, Norway have been included as a sea level reference. All measurements were performed at a SZA of 40. For CLT sea the clear-sky reference is for a site at the sea level. For CLT site the clear-sky reference is for a site at the same altitude as the measurement site. No measurements were performed in Linzhi during the summer campaign. Measurements at the Rodog site at an altitude of 4400 m were omitted from the table because the sky was not clear on the days considered. b The measured CLT was 120 on day 59 and 115 on day 60. This difference may result from a change in the snow quality and hence an altered surface albedo. Milha together with a surface albedo of The measured CLT site value was 120. The increase from the anticipated bare ground CLT site value of 92 to the measured value of 120 (a 30% increase) corresponds to an increase in the surface albedo from 0.05 to 0.75 in the model-calculated CLT site value in agreement with the albedo derived from the measured erythemal dose rate. The corresponding increase in the UV index was calculated to be 35%. Note that the UV index is more sensitive to changes in the surface albedo than the CLT (340 nm). This is due to the strong wavelength dependence of the Rayleigh scattering cross section causing a stronger scattering of radiation back to the surface in the UV-B than in the UV-A. Although all days from 58 to 61 were clear at Mt. Milha, we observed a steady decrease in the measured CLT site during this period from 123 to 113 (corresponding to CLT sea values ranging from 145 to 133). This decrease may have been caused by a change in the snow cover leading to a decrease in the surface albedo during these 4 days Altitude Effects on UV Radiation UV-A Radiation [22] In addition to CLT site values, Table 2 also shows the CLT sea values obtained by using clear-sky reference irradiances calculated for a site at the sea level. Then the modeled clear-sky reference is the same for all sites, so that we can determine how the CLT (or the irradiance in the 340 nm channel) depends on altitude. On the basis of the CLT sea values for bare ground surfaces in Table 2 including the Oslo values, we found the increase in irradiance with altitude (the altitude effect) to be about 3% per km. The calculated altitude effect with our radiative transfer model was 3.5% assuming 5% surface albedo, a pure Rayleigh scattering atmosphere and 340 nm irradiances at sea level and 3700 m altitude. On the basis of measurements at Lauder, New Zealand, and Mauna Loa Observatory, Hawaii, McKenzie et al. [2001] found the altitude effect to be about 4% for a clean atmosphere which is in close agreement with our result. They noted the importance of considering scattering from the atmosphere below Mauna Loa when using radiative transfer calculations to simulate the altitude effect. This was not considered in our model calculations because there are large distances to considerably lower altitudes from each site. Previous measurements in the Alps showed an altitude effect of 9% per km in the UV-A [Blumthaler et al., 1997]. However, their study may include effects of different surface albedos and aerosol Figure 7. Measured (solid line) and calculated clear-sky (dashed line) UV dose rates for Mt. Milha (5060 m) on day 59. The calculations were based on a surface albedo of 0.75 and a measured ozone column amount of 254 DU. 7of10

8 Figure 8. Local noon UV dose rates versus altitude. Solid triangles indicate Lhasa, Rodog and Mt. Milha on day 218. Solid squares indicate Lhasa and Linzhi on day 59. Open circles indicate Lhasa and Linzhi on day 60. column amounts at the selected sites, while our result is probably more closely related to a purely Rayleigh-scattering atmosphere with small effects of aerosols. Results from the Chilean Andes indicate altitude effects of 4 10% per km in rural areas [Cabrera et al., 1995]. [23] It should be noted that the altitude effect depends slightly on the solar zenith angle. This is discussed in detail by McKenzie et al. [2001]. They showed a peak increase in the altitude effect around 70 solar zenith angle. Further, the altitude effect depends on altitude. Our modelled altitude effect for a pure Ralyeigh scattering atmosphere based on 340 nm irradiances at sea level and at an altitude of 1000 m is 4.8%. The corresponding altitude effect based on calculated irradiances at 3700 m and at 5000 m is 2.8% Erythemal UV Dose Rates [24] Figure 8 shows how UV dose rates for snow-free surfaces vary with altitude on selected clear days during winter and summer in the Lhasa area. Winter UV dose rates are represented by measurements on the days 59 and 60 for which the sky was clear at Linzhi (3000 m) and Lhasa (3700 m). Day 218 represents a summer day with quite clear skies simultaneously at three of the sites: Lhasa (3700 m), Rodog (4400 m), and Mt. Milha (5000 m). All sites had snow-free surfaces. There is a glacier on a mountain top more than 30 km away from Mt. Milha but we do not expect that this has any significant effect on the measurements. All measurements in Figure 8 were taken at local solar noon. The noon solar zenith angle was 40 on days 59 and 60 and 13 on day 218. The measurements on the days 59 and 60 correspond to a 7.0% per km increase of the UV index with altitude. The measurements at the three sites in the altitude range from 3700 m to 5000 m on day 218 (see the solid triangles in Figure 8) correspond to an altitude increase in the UV index of about 8% per km. Blumthaler et al. [1997] found altitude increases in the UV dose rates in the Alps of more than 15% per km, which is considerably higher than our results. McKenzie et al. [2001] found the altitude effect to be 6.5% at SZA = 40 which is slightly lower than our results. Our model-calculated altitude effect for a purely Rayleigh-scattering and ozone-absorbing atmosphere is 5.1% per km for a SZA of 13 and 5.7% per km for a SZA of 40. The calculations were based on a surface albedo of 0.05 and a total ozone column amount of 260 DU at an altitude of 3000 m.for the higher-altitude sites the total ozone column amounts were reduced by 2.3 DU per km in agreement with the US76 standard ozone profile. The model calculations show an increase in the altitude effect with SZA in agreement with the investigations by McKenzie et al. [2001]. Note that our measurements, however, indicate a small decrease in the altitude effect with SZA. The days 59 and 60 were very clear while on day 218 some very thin clouds were present at all sites. Therefore the winter value (7%/km) is more reliable than the summer value (8%/km) and the apparent decrease in altitude effect with SZA may not be real UV Doses and Clouds [25] Our measurements show that variable cloud cover is an important UV controlling factor at all four sites considered (see, e.g., Figures 2 and 3). The NILU-UV instruments recorded 1-min averages of UV dose rates at a 1-min time resolution from sunrise to sunset and thus provided daily integrated UV doses. In order to quantify the influence of clouds on the daily integrated UV doses in Lhasa we compared the measured daily integrated UV doses with those calculated using our radiative transfer model for clearsky conditions. The calculated integrated UV doses were based on the daily measured ozone column amounts, a surface albedo of 0.05, and a time resolution of 10 min from sunrise to sunset. Figure 9 shows ratios of measured to calculated daily integrated UV doses for the complete 8of10

9 Figure 9. Comparison of measured and calculated clear-sky daily integrated erythemal UV doses in Lhasa. The dashed line shows the average measured to calculated ratio. record in Lhasa in The average reduction due to clouds (dashed line) is found to be 25% below the clear sky level (ratio = 1.0). We assume that the daily reductions compared to our clear sky calculations are mainly caused by clouds. [26] Kylling et al. [2000a] found the influence of clouds on monthly integrated UV doses for a high-latitude site (Tromsø, Norway, 70N) to vary between 20% and 40%. Tromsø is located close to the coast of northern Norway and the cloud climatology is expected to be very different from the cloud climatology in Tibet. Lhasa is strongly influenced by the large-scale monsoon in the summer months with more clouds compared to the rest of the year. Our measurements represent mainly summer months in The duration and the strength of the monsoon system vary from one year to another. A more thorough analysis of the effect of the cloud climatology on the surface UV radiation requires a much longer time series than presented in this paper. Therefore our result should be considered as a rough estimate. 5. Summary [27] We have measured UV radiation in the Lhasa area in Tibet at altitudes between 3000 m and 5000 m during winter and summer in 2003 using NILU-UV multichannel moderate-bandwidth radiometers. UV index values of 15 were found to appear frequently during the summer. The maximum UV index measured was Also, we have carried out clear-sky radiative transfer calculations based on daily ozone values derived from TOMS measurements in the period These calculations show that a UV index of 15 is typical on clear summer days and that UV index values above 10 appear frequently on clear days in the period from March to September. Clouds usually decrease the UV radiation, but on partially cloudy days, clouds may enhance the UV index by more than 30% compared to the clear-sky value. During winter high UV index values (above 12) were measured at an altitude of 5000 m with a snow-covered surface at a SZA of 40. These measurements correspond to a 35% increase compared to a snow-free surface. The increases in the UV index and the 340 nm irradiance with altitude for bare ground were found to be 7 8% per km and 3% per km, respectively, in good agreement with model calculations based on a purely Rayleigh-scattering and ozone-absorbing atmosphere. Thus the UV index values in Lhasa were found to be 22 30% higher than at a sea level site with similar surface albedo, latitude, and atmospheric optical conditions. On the basis of measurements and radiative transfer calculations we found the cloud cover in Lhasa to reduce the clear-sky daily integrated erythemal UV dose by 25% on the average. [28] Acknowledgments. Direct Overpass Earth Probe TOMS data were provided by NASA/GSFC. We thank Bjørn Johnsen at NRPA, Norway, for providing UV spectra from the UV reference spectroradiometer of the Norwegian UV monitoring network. The spectral responses for all instruments used in this paper were measured at the optical laboratory at NRPA. This work was sponsored by NORAD. We thank three anonymous reviewers for constructive criticisms. References Bais, A. F., et al. (2007), Surface ultraviolet radiation: Past, present and future, in Global Ozone Res. Monit. Proj. Rep. 50, chap. 7, World Meteorol. Organ., Geneva, Switzerland. Biospherical Instruments, Inc. (2006), NSF Polar Programs UV monitoring Network, San Diego, Calif., Sept. (Available at com/nsf/updates/boreal/euvindex.aspx) Blumthaler, M., W. Ambach, and R. Ellinger (1997), Increase in solar UV radiation with altitude, J. Photochem. Photobiol., Ser. B, 39, Bodeker, G. E., and R. L. McKenzie (1996), An algorithm for inferring surface UV irradiance including cloud effects, J. Appl. Meteorol., 35, Bodhaine, B. A., E. G. Dutton, D. J. Hofmann, R. L. McKenzie, and P. V. Johnston (1997), UV measurements at Mauna Loa: July 1995 to July 1996, J. Geophys. Res., 102, 19,265 19,273. 9of10

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