Significant Variations of Surface Albedo during a Snowy Period at

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(Received 27 September 2008; revised 20 April 2009) ... For example, over grassland the surface albedo was relatively lower after snowfall, and as a result, more solar ... shaded areas, snow generally melts away and disap- pears in .... 3a, b, and c. ... Weather conditions ..... radiative heating: Influence on Tibetan Plateau.
ADVANCES IN ATMOSPHERIC SCIENCES, VOL. 27, NO. 1, 2010, 80–86

Significant Variations of Surface Albedo during a Snowy Period at Xianghe Observatory, China YU Yu1,2 (余 予), CHEN Hongbin∗1 (陈洪滨), XIA Xiangao1 (夏祥鳌), XUAN Yuejian1 (宣越健), and YU Ke1 (喻 珂) 1

Laboratory for middle Atmosphere and Global Environment Observation, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029 2

Graduate University of Chinese Academy of Sciences, Beijing 100049 (Received 27 September 2008; revised 20 April 2009) ABSTRACT

Surface albedo over typical types of surfaces in the North China Plain was observed using a Multi-field Albedo Observation System before and after several snowfalls from 13 to 27 February 2005. Dramatic variations of the surface albedos of bare land, a frozen pond, and withered grassland during that period were analyzed. Under cloudy sky, the mean surface albedo of bare land was about 0.23, but it immediately rose to 0.85 when the surface was covered by fresh snow. The albedo decreased gradually to normal levels afterwards. The melting processes were different depending on the characteristics of the underlying surfaces. For example, over grassland the surface albedo was relatively lower after snowfall, and as a result, more solar energy could be absorbed and consequently the snow melting process was accelerated. Significant variations of surface albedo cannot be easily captured by satellite observations; therefore, detailed measurements of surface albedo and related parameters are essential for determining the impact of snow on the energy budget of the Earth. Key words: surface albedo, snow, snow melting Citation: Yu, Y., H. B. Chen, X. A. Xia, Y. J. Xuan, and K. Yu, 2010: Significant variations of surface albedo during a snowy period at Xianghe observatory, China. Adv. Atmos. Sci., 27(1), 80–86, doi: 10.1007/s00376-009-8151-2.

1.

Introduction

Broadband surface albedo is an important parameter for determining the energy budget of the Earth (Baret et al., 2005). To understand surface albedo variation, particularly its complex spatial and temporal variability, ground-based measurements of surface albedo over different types of surfaces are still in urgent demand. In addition, the ground measurements are valuable with regard to validation of satellite remote sensing data. Detailed measurements of surface albedo, in combination with measurements of soil and atmospheric factors, are needed to determine to what extent these factors influence the surface albedo (Dickinson, 1995; Grant et al., 2000; Duchon and Hamm, 2006; Bao et al., 2008). Snow can change surface albedo by a factor of 2–4, ∗ Corresponding

author: CHEN Hongbin, [email protected]

depending on the surface type (Betts and Ball, 1997; Jin et al., 2002). Consequently, hydrologic and atmospheric processes may be significantly influenced by snow, especially at high altitudes or latitudes (Perovich et al., 2002; Pirazzini, 2004; Flanner and Zender, 2005; Hudson et al., 2006). It has also been demonstrated that snow-covered underlying surfaces can greatly affect the surface net radiation and the ultraviolet (UV) irradiance (Renaud et al., 2000; Arola et al., 2002, 2003; Lenoble et al., 2004; Sicart et al., 2004; Wendler et al., 2004; Xia et al., 2008). Much attention has been paid to the effect of snow and its parameterization (Baker and Ruschy, 1989; Baker et al., 1990; Betts and Ball, 1997; Greuell et al., 2002; Barlage et al., 2005; Stroeve et al., 2005; Molotch and Bales, 2006; Li et al., 2008), but the impurities contained in snow, and the effects of blowing snow, wind

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ventilation, and frost formation need to be studied further for a thorough understanding of snow albedo evolution (Aoki et al., 2003; Oleson et al., 2003; Flanner and Zender, 2006). Over the North China Plain, snow accounts for the majority of precipitation in winter. Except over some shaded areas, snow generally melts away and disappears in one or two weeks, leading to great variations in the surface albedo. Although satellite remote sensing platforms such as MODIS (Schaaf et al., 2002) can offer a broad view of surface albedo, the details of albedo change during the snow melting process cannot be captured by 16-day albedo products such as this. Little attention has been paid to the observations of snow and the albedo of snow-covered land over the North China Plain. The objective of this paper is to present our study of dramatic variations in the surface albedo of three typical types of surface before and after several snowfalls from 13 to 27 February 2005. This is achieved by using measurements of surface albedo over different types of surfaces during these snow events. The differences of snow albedo evolution are shown and the qualitative effects of surface type on snow melting are discussed in this paper. 2.

Measurements

A Multi-field Albedo Observation System (MAOS, see Fig. 1), designed by the Laboratory for middle Atmosphere and Global Environment Observation (LA-

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GEO) of the Institute of Atmospheric Physics (IAP), is utilized to quasi-simultaneously measure surface albedo over different types of surfaces. The system is installed at the Xianghe Atmospheric Observation Station of the IAP, which is located in the central part of the North China Plain [35◦ –45◦ N, 110◦ –132◦ E; also see Fig. 1 in Liu et al. (2005)] and is about 70 km east of Beijing. As shown in Fig. 1, the observation field, which is about 36 m long and 6 m wide, is divided into 6 parts. This includes, from right (west) to left (east), plastic film covered land, bare clay land, grassland I (a plot of unsmoothed sparse withered lawn), a pond (with water depth of about 2 meters), grassland II (a plot of unsmoothed withered lawn), and a crop field respectively. A motor driven platform moves in sequence at 1.2 m above the ground and stops at the center of each field for a short period so that a field is measured for 3 minutes every half hour. A pair of Apogee precision pyranometer sensors (PPSs) are mounted on the platform—one on the upper side facing toward the sky, the other on the bottom side staring at the ground. They measure solar irradiance between 300 and 1100 nm in wavelength. Measurements at solar zenith angles (θ) larger than 85◦ were discarded because they are contaminated by surrounding trees and buildings. Note that the cosine response of the instruments also comes into play at large solar zenith angles. In order to check the performance of a PPS, its data are compared to the combined measurements from two Eppley in-

Fig. 1. Photographs of the MAOS and PPSs.

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Fig. 2. Comparison of the upward looking PPSmeasured daytime-averaged global irradiance to the sum of NIP and B&W measurements. The estimated uncertainty of PPS is about 2.8%. (Note: N—sample size; R2 —square of the correlation coefficient; MAE—mean absolute error; RMSE—root mean square error)

struments mounted on an EKO STR-22 solar tracker— a normal incident pyrheliometer (NIP) for direct solar irradiance, and a black-and-white pyranometer (B&W, Model 8–48) for diffuse sky irradiance, both covering 285–2800 nm wavelengths. The field measurement uncertainties of NIP and B&W were estimated to be 3% and 6% respectively, complying with the Baseline Surface Radiation Network standards (Xia et al., 2007). Figure 2 shows the PPS measurements scaled to match the sum of NIP and B&W. The uncertainty of a PPS is estimated to be about 2.8%. We assume

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that the two PPSs have the same uncertainties and that the uncertainty of the derived albedo is less than 5.6%. Air temperature, Ta , which is measured at 2 m above the ground on a tower located about 30 meters away, is also used in the analysis. It snowed at Xianghe from dawn to sunset on 15 February 2005 with an accumulated snow depth of about 5 cm. During the nights of 17 and 21 February, light snow fell again. Total sky imager (TSI) measurements showed that it was very clear on 19 February and also on 20 February, except for some cirrus passing through the sky from 1330 to 1430 LST. It was also clear on 21 February, except for some occasional small patches of cirrus quickly passing by. There was some haze on the morning of 22 February but it was blown away rapidly and the sky became clear from 0900 LST onward. Light snow fell during the early morning of 23 February and stopped at about 1000 LST. It restarted at 1400 LST that day and continued into the evening. A summary of the weather conditions from 13 to 27 February 2005 is given in Table 1. It should be noted that due to the low air temperature, the pond was frozen during these days. 3.

Results and discussion

Based on the MAOS measurements from 13 to 27 February 2005, variations of the surface albedos observed over three typical types of surfaces, i.e. the bare land, pond, and grassland II, are shown in Figs. 3a, b, and c. Daily mean albedo is calculated as the average of a whole day’s measured upward irradiance divided by its downward counterpart. The results are listed in Table 1. Note that under clear sky the inci-

Table 1. Daily mean and median albedo of bare land, frozen pond, and withered grassland. The daily maximum and minimum air temperature and weather conditions are also given in the last two columns. Date 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Feb Feb Feb Feb Feb Feb Feb Feb Feb Feb Feb Feb Feb Feb Feb

Daily mean albedo/Daily median albedo Bare land Frozen pond Withered grassland 0.23/0.23 0.24/0.24 0.24/0.24 0.22/0.22 0.22/0.22 0.23/0.23 0.90/0.91 0.89/0.90 0.83/0.84 0.85/0.86 0.84/0.84 0.81/0.81 0.83/0.82 0.81/0.81 0.78/0.78 0.86/0.86 0.83/0.82 0.79/0.79 0.81/0.81 0.79/0.79 0.73/0.73 0.80/0.79 0.77/0.77 0.68/0.68 0.72/0.73 0.73/0.74 0.52/0.52 0.42/0.47 0.59/0.58 0.27/0.26 0.65/0.67 0.76/0.76 0.40/0.43 0.34/0.32 0.65/0.69 0.27/0.27 0.20/0.20 0.42/0.43 0.22/0.23 0.20/0.20 0.28/0.26 0.22/0.22 0.20/0.20 0.22/0.22 0.22/0.22

Ta (◦ C) Maximum/Minimum 3.9/−9.9 6.1/−2.3 0.4/−2.1 3.4/−5.3 0.2/−5.5 −2.7/−9.1 −4.7/−12.1 −1.7/−14.1 1.8/−13.6 3.0/−5.9 −1.6/−4.8 1.1/−5.1 0.5/−7.8 5.8/−10.1 9.7/−3.9

Weather conditions cloudy cloudy snow fog/cloudy cloudy/light snow partly cloudy sunny sunny fog/light snow fog/sunny snow fog/cloudy sunny partly cloudy cloudy/sunny

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Fig. 3. Surface albedos over three types of surfaces from 13 to 27 February 2005 are given in (a), (b), and (c). Comparison of daily mean albedos is shown in (d). Air temperature is given in (e). Horizontal dashed lines in (a), (b), and (c) indicate mean surface albedo before snowfall, and indicates 0◦ C in (e). Vertical dashed lines represent the beginning of the four snow events.

dent irradiance and reflected irradiance when θ > 75◦ account for only about 5% of total energy during the daytime. By excluding this part of data, the uncertainty of mean surface albedo is less than 0.5%. As a comparison, the results using median instead of average are also shown, which indicate that the results are not impacted by outliers. On 13 and 14 February (two cloudy days) the albedos were relatively stable and the value for bare land was about 0.23. When it snowed on 15 February, the albedos increased from about 0.23 to above 0.83 due to the high reflectivity of snow. However, falling snowflakes might have influenced the measurements of upward and downward looking PPSs and could have resulted in a maximum bias of 0.15 in the daily mean albedo during that day. Because snow could not fully cover the withered grass, the albedo was lower over grassland. The daily mean albedo of the frozen pond

was very close to that of the bare land. On the following two days (16 and 17 February), the daily mean albedos of the three terrains decreased gradually. As listed in Table 1, the daily mean albedo of the snowcovered bare land on 16 February was 0.85, which represents the fresh snow albedo under a cloudy sky. It snowed slightly on the night of 17 February, which led to very small increases (0.02–0.04) in the surface albedo on 18 February. On 22 February, the surface albedos were close to 0.8 and 0.5, respectively, for bare land and grassland in the morning. Reflectivity decreased dramatically to the background level in the afternoon when snow melted away completely, with the maximum temperature rising up to about 3◦ C. The surface albedo for the frozen pond remained at a relatively high level on the afternoon of 22 February because it was still covered by some snow. However, the daily mean surface albedos all showed decreases for

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that day—by about 0.14, 0.25, and 0.30 for the pond, grassland, and bare land, respectively. From 19 to 22 February, the albedo of grassland varied from about 0.73 to 0.27, which was the largest decrease among the three terrains. The increase of surface albedo on 23 February was due to snowfall. The surface albedo decreased to the background level on 25 February when snow melted away completely over the bare land and grassland. However, the surface albedo of the frozen pond did not decrease to the level observed before snowfall occurred until 27 February. Observations from Terra (EOS-AM) obtained from the MOD43B3 Albedo Product (MODIS/Terra Albedo 16-Day L3 Global 1 km SIN Grid; http:// www-modis.bu.edu/brdf/userguide/albedo.html) showed that the black-sky albedo was about 0.14 around the Xianghe site from 18 February to 6 March. In contrast, our analyses show that surface albedo changed greatly during those days and that there were remarkable differences among the variations of albedos over the bare land, frozen pond, and withered grassland surfaces following snowfall. Note that the MODIS albedo data are produced every 16 days, and if only a few days during 16-day period are snow covered, a snow-free retrieval is made. So, it is not surprising that the snow effects on albedo were not captured by MODIS. As seen from Table 1, the surface albedo for the grassland returned nearly to the background level on 22 February. The surface albedo decreased by about 0.08 per day after 15 February. This rate of decline was higher than that observed for the bare land and the frozen pond, which had rates of decline of 0.068 and 0.043 per day, respectively. Under the same weather conditions and solar daylight illumination, it seems that the underlying surface characteristic is the major player in determining these differences. Snow albedo evolution is a very complex process. Sunshine duration and cloudiness affect the surface solar radiation, while relative humidity and wind ventilation influence snow grain growth. The research of Flanner and Zender (2006) also indicates that when there is a high vertical temperature gradient within snow that exceeds 20 K m−1 , this is the dominant factor in albedo decay. Soot and dust contained in snow and their accumulation levels have strong impacts on solar radiation extinction. Furthermore, the surface albedo is closely related to the characteristics of the underlying surface that determine the quantity of energy absorbed by the ground. One would expect quite different speeds of snow melting for different surfaces, mainly due to different surface characteristics. Baker et al. (1990) pointed out that snow depth greater than 10.2 cm can exclude the effect of the underlying sur-

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face on surface albedo. In the cases we studied, the amount of snow was not large enough to fully cover the ground, so the underlying surface played an important role from the beginning. Covered by vegetation, the grassland has a lower albedo than the bare land and the frozen pond after snowfall, and obtained more solar energy; as a result, snow on the grass began to melt at the earliest time of the sites, and consequently the largest amount of shortwave energy was absorbed, which in return further accelerated the melting speed and thereby formed a positive feedback. The melting rate of the snow was lowest on the frozen pond. The albedo of snow covered surfaces varied with solar zenith angle on clear days. For example, between 18 and 20 February albedo generally decreased from sunrise, and reached a minimum at solar noon and then increased towards evening. When snow started to melt, for example as on 22 and 24 February, the surface albedo linearly decreased over time. It should be noted that the daily mean albedos of the bare land and the grassland from 25 to 27 February, were about 0.02 lower than the measurements on 13 and 14 February, which is believed to be caused by the relatively high water content in soil after the snow melt. Visually the color of the surface in the two fields was darkened when the soil moisture increased. A systematic decrease in albedo in response to rainfall events has also been observed for sites with a large amount of bare soil in the Southern Great Plains of America (Duchon and Hamm, 2006). Land-use in the North China Plain during 1990– 2000 has been changed by human activities, and 7.9 × 108 m2 of grassland and 7.5 × 108 m2 of water bodies have been transformed into cropland, while 0.4 × 108 m2 of cropland was occupied due to urbanization (Liu et al., 2005). The distinct difference in the surface albedo variations for the bare land and grassland observed by the MAOS indicate that human activities will exert significant impact on the surface albedo. Further, over the past 40 years, the observed total amount and frequency of precipitation during winter in the region of the North China Plain both show declining trends with decreasing snow magnitude (Liu et al., 2005). Additionally, the seasonal mean air temperature in winter has increased by about 0.5◦ C per decade (Qian and Qin, 2006), which may have promoted snow melt speeds. Both climatic factors contribute to the significant decrease in snow-cover duration over the North China Plain. If these have led to significant variation of the surface albedo in winter, the question arising here is to what extent the regional climate is influenced by the variation of the surface albedo. Further studies on this issue are urgently required.

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Conclusions

This study investigates the significant variations of surface albedo during a snowy period at Xianghe observatory of the IAP. The main results are summarized as follows: (1) Under cloudy sky the surface albedo of bare land without snow cover in winter is about 0.23, while the fresh snow albedo is about 0.85. Surface albedo changes greatly in one or two weeks after snowfall, which cannot be captured by current satellite observations, such as those from MODIS. (2) Underlying surfaces that are not fully covered by snow will continue to play an important role in the variation of albedo. Compared to the fully covered bare land and frozen pond, the partially covered grassland absorbed more solar energy, which accelerated melting and formed a stronger positive feedback. (3) High soil water content can lead to a decrease in the albedos of bare land and grassland shortly after snow melts away. Acknowledgements. The data used in this paper were all obtained from the Xianghe Atmospheric Observation Station of the Institute of Atmospheric Physics. We thank WU Qinghong, NAN Weidong, and ZHOU Dajun for their maintenance of the MAOS. We also thank two anonymous reviewers for their constructive comments on the manuscript. This work was supported by the National Natural Science Foundation of China (40675017). REFERENCES Aoki, T., A. Hachikubo, and M. Hori, 2003: Effects of snow physical parameters on shortwave broadband albedos. J. Geophys. Res., 108(D19), 4616, doi: 10.1029/2003JD003506. Arola, A., and Coauthors, 2002: Assessment of four methods to estimate surface UV radiation using satellite data, by comparison with ground measurements from four stations in Europe. J. Geophys. Res., 107(D16), 4310, doi: 10.1029/2001JD000462. Arola, A., and Coauthors, 2003: A new approach to estimating the albedo for snow-covered surfaces in the satellite UV method. J. Geophys. Res., 108(D17), 4531, doi: 10.1029/2003JD003492. Baker, D. G., and D. L. Ruschy, 1989: Winter Albedo Characteristics at St. Paul, Minnesota. J. Appl. Meteor., 28, 227–232. Baker, D. G., D. L. Ruschy, and D. B. Wall, 1990: The Albedo Decay of Prairie Snows. J. Appl. Meteor., 29, 179–187. Bao, Y., S. L¨ u, Y. Zhang, X. Meng, and S. Yang, 2008: Improvement of surface albedo simulations over arid regions. Adv. Atmos. Sci., 25(3), 481–488, doi: 10.1007/s00376-008-0481-y.

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