Improvement of surface albedo simulations over arid regions

To improve the simulation of the surface radiation budget and related thermal processes in arid regions, three sophisticated surface albedo schemes designed for such regions were incorporated into the Biosphere- Atmosphere Transfer Scheme （BATS）. Two of these schemes are functions of the solar zenith angle （SZA）, where the first one has one adjustable parameter defined as SZA1 scheme, and the second one has two empirical parameters defined as SZA2 scheme. The third albedo scheme is a function of solar angle and soil water that were developed based on arid-region observations from the Dunhuang field experiment （DHEX） （defined as DH scheme）. We evaluated the performance of the original and newly-incorporated albedo schemes within BATS using the in-situ data from the Oasis System Energy and Water Cycle Field Experiment that was carried out in JinTa, Gansu arid area （JTEX）. The results indicate that a control run by the original version of the BATS generates a constant albedo, while the SZA1 and SZA2 schemes basically can reproduce the observed diurnal cycle of surface albedo, although these two schemes still underestimate the albedo when SZA is high in the early morning and late afternoon, and overestimate it when SZA is low during noontime. The SZA2 scheme has a better overall performance than the SZA1 scheme. In addition, BATS with the DH scheme slightly improves the albedo simulation in magnitude as compared to that from the control run, but a diurnal cycle of albedo is not produced by this scheme. The SZA1 and SZA2 schemes significantly increase the surface absorbed solar radiation by nearly 70 W m^-2, which further raises the ground temperature by 6 K and the sensible heat flux by 35 W m^-2. The increased solar radiation, heat flux, and temperature are more consistent with the observations that those from the control run. However, a significant improvement in these three variables is not found in BATS with the DH scheme due to the neglect of the diurnal cycle of albedo. Further analysis indi     

CHARACTERISTICS OF VEGETATION COVER,ROUGHNESS AND ALBEDO DISTRIBUTION OVER CHINA

To build land surface dataset for climate model,with application of remote sensing techniqueas well as the Geographic Information System(GIS),the data of surface type,roughness andalbedo over China in 1997 were retrieved,resolutions being 10 km×10 km.Based on these data,an analysis is conducted on the geographic distributions and seasonal variations of surfacevegetation cover and roughness as well as albedo over China.Results show that surface vegetationcover is mainly located to the south of Yangtze River,in Southwest and Northeast China andsparse vegetation cover is in the Northwest.The variation of land surface cover affects thevariations of land surface roughness and albedo.High albedo occurred in the north of XinjiangAutonomous Region,the north of Northeast China and the Qinghai-Xizang Plateau in winter,incorrespondence with the location of snow cover.For most part of China,surface roughness decreases and albedo increases in winter,while theroughness increases and the albedo decreases in summer,which could mainly result from landsurface cover(snow cover and vegetation cover)and soil moisture changes.This shows that thegeographic distribution and seasonal variation of the albedo are almost opposite to those of theroughness,in agreement with theoretical results.Temporally,the amplitude of surface roughnesschange is quite small in comparison with the roughness itself.     

Four-stream Radiative Transfer Parameterization Scheme in a Land Surface Process Model

Accurate estimates of albedos are required in climate modeling. Accurate and simple schemes for radiative transfer within canopy are required for these estimates, but severe limitations exist. This paper developed a four-stream solar radiative transfer model and coupled it with a land surface process model. The radiative model uses a four-stream approximation method as in the atmosphere to obtain analytic solutions of the basic equation of canopy radiative transfer. As an analytical model, the four-stream radiative transfer model can be easily applied efficiently to improve the parameterization of land surface radiation in climate models. Our four-stream solar radiative transfer model is based on a two-stream short wave radiative transfer model. It can simulate short wave solar radiative transfer within canopy according to the relevant theory in the atmosphere. Each parameter of the basic radiative transfer equation of canopy has special geometry and optical characters of leaves or canopy. The upward or downward radiative fluxes are related to the diffuse phase function, the G-function, leaf reflectivity and transmission, leaf area index, and the solar angle of the incident beam. The four-stream simulation is compared with that of the two-stream model. The four-stream model is proved successful through its consistent modeling of canopy albedo at any solar incident angle. In order to compare and find differences between the results predicted by the four- and two-stream models, a number of numerical experiments are performed through examining the effects of different leaf area indices, leaf angle distributions, optical properties of leaves, and ground surface conditions on the canopy albedo. Parallel experiments show that the canopy albedos predicted by the two models differ significantly when the leaf angle distribution is spherical and vertical. The results also show that the difference is particularly great for different incident solar beams. One additional experiment is carried out to evaluate the simulations of the BATS land surface model coupled with the two- and four-stream radiative transfer models. Station observations in 1998 are used for comparison. The results indicate that the simulation of BATS coupled with the four-stream model is the best because the surface absorbed solar radiation from the four-stream model is the closest to the observation.     

CHARACTERISTICS OF VARIATION IN SURFACE ALBEDO AND SNOW FORCING OVER THE TIBETAN PLATEAU

The combination of field experiments and satellite observations is the fundamental way tounderstand the characteristics of spatial-temporal variation in surface albedo over the Tibetan(Qinghai-Xizang) Plateau. Under the condition without snow cover, the relatively regular annualvariation cycle of the surface albedo can be expressed by an empirical formula. The effect of snowcover on the surface albedo in winter can be expressed by introducing two variables of snow forcingand sensitivity parameter. The existing satellite retrieved results of surface albedo may provide thedigital grid data for describing the geographical distribution. However, some satellite retrievedsurface albedos available over the Tibetan Plateau are obviously too low in winter. Taking thesatellite derived results in summer as the background field representative of geographicaldistribution and combining the empirical formula of annual cycle based on the surface observations,a dynamic model of surface albedo is developed for the need of modeling the climatic influence ofthe underlying surface forcing of the Tibetan Plateau.     

Effect of The Atmosphere on UVB Radiation Reaching The Earth’s Surface:Dependence on Solar Zenith Angle

The atmosphere protects humans,plants,animals,and microorganisms from damaging doses of ultraviolet-B(UVB) solar radiation(280-320 nm) because it modifies the UVB reaching the Earth’s surface.This modification is a function of the solar radiation’s path length through the atmosphere and the amount of each attenuator along the path length.The path length is determined by solar zenith angle(SZA).The present work explains the dependence of hemispherical transmittance of UVB on SZA.The database used consists of five years of hourly UVB and global solar radiation measurements.From 2001 to 2005,the South Valley University(SVU) meteorological research station(26.20°N,32.75°E) carried out these measurements on a horizontal surface.In addition,the corresponding extraterrestrial UVB(UVBext) and broadband solar radiation(Gext) were estimated.Consequently,the hemispherical transmittance of UVB(KtUVB) and the hemispherical transmittance of global solar radiation(Kt) were estimated.Furthermore,the UVB redaction due to the atmosphere was evaluated.An analysis of the dependence between KtUVB and SZA at different ranges of Kt was performed.A functional dependence between KtUVB and SZA(KtUVB=-a(SZA)+b) for very narrow Kt-ranges(width of ranges was 0.01) was developed.The results are discussed,and the sensitivity of △KtUVB to △SZA for very narrow Kt-ranges was studied.It was found that the sensitivity of △KtUVB to △SZA slightly increases with increased Kt,which means KtUVB is sensitive to SZA as Kt increases.The maximum correlation(R) between KtUVB and SZA was equal to-0.83 for Kt= 0.76.     

Albedo of coastal landfast sea ice in Prydz Bay,Antarctica: Observations and parameterization

The snow/sea-ice albedo was measured over coastal landfast sea ice in Prydz Bay, East Antarctica(off Zhongshan Station)during the austral spring and summer of 2010 and 2011. The variation of the observed albedo was a combination of a gradual seasonal transition from spring to summer and abrupt changes resulting from synoptic events, including snowfall, blowing snow, and overcast skies. The measured albedo ranged from 0.94 over thick fresh snow to 0.36 over melting sea ice. It was found that snow thickness was the most important factor influencing the albedo variation, while synoptic events and overcast skies could increase the albedo by about 0.18 and 0.06, respectively. The in-situ measured albedo and related physical parameters(e.g., snow thickness, ice thickness, surface temperature, and air temperature) were then used to evaluate four different snow/ice albedo parameterizations used in a variety of climate models. The parameterized albedos showed substantial discrepancies compared to the observed albedo, particularly during the summer melt period, even though more complex parameterizations yielded more realistic variations than simple ones. A modified parameterization was developed,which further considered synoptic events, cloud cover, and the local landfast sea-ice surface characteristics. The resulting parameterized albedo showed very good agreement with the observed albedo.     

Satellite-based estimation of daily average net radiation under clear-sky conditions

ABSTRACT Daily average net radiation （DANR） is an important variable for estimating evapotranspiration from satellite data at regional scales, and is used for atmospheric and hydrologic modeling, as well as ecosystem management. A scheme is proposed to estimate the DANR over large heterogeneous areas under clear-sky conditions using only remotely sensed data. The method was designed to overcome the dependence of DANR estimates on ground data, and to map spatially consistent and reasonably distributed DANR, by using various land and atmospheric data products retrieved from MODIS （Moderate Resolution Imaging Spectroradiometer） data. An improved sinusoidal model was used to retrieve the diurnal variations of downward shortwave radiation using a single instantaneous value from satellites. The downward shortwave component of DANR was directly obtained from this instantaneous value, and the upward shortwave component was estimated using satellite-derived albedo products. Four observations of air temperature from MOD07_L2 and MYD07_L2 data products were used to derive the downward longwave component of DANR, while the upward longwave component was estimated using the land surface temperature （LST） and the surface emissivity from MOD1 l_L2. Compared to in situ observations at the cropland and grassland sites located in Tongyu, northern China, the root mean square error （RMSE） of DANR estimated for both sites under clear-sky conditions was 37 W m-2 and 40 W m-2, respectively. The errors in estimation of DANR were comparable to those from previous satellite-based methods. Our estimates can be used for studying the surface radiation balance and evapotranspiration.     

An Overview of the Studies on Black Carbon and Mineral Dust Deposition in Snow and Ice Cores in East Asia

Black carbon （BC） is the most eff ective insoluble light-absorbing particulate （ILAP）, which can strongly absorb solar radiation at visible wavelengths. Once BC is deposited in snow via dry or wet process, even a small amount of BC could signifi cantly decrease snow albedo, enhance absorption of solar radiation, accelerate snow melting, and cause climate feedback. BC is considered the second most important component next to CO2 in terms of global warming. Similarly, mineral dust （MD） is another type of ILAP. So far, little attention has been paid to quantitative measurements of BC and MD deposition on snow surface in the midlatitudes of East Asia, especially over northern China. In this paper, we focus on reviewing several experiments performed for collecting and measuring scavenging BC and MD in the high Asian glaciers over the mountain range （such as the Himalayas） and in seasonal snow over northern China. Results from the surveyed literature indicate that the absorption of ILAP in seasonal snow is dominated by MD in the Qilian Mountains and by local soil dust in the Inner Mongolian region close to dust sources. The detection of BC in snow and ice cores using modern techniques has a large bias and uncertainty when the snow sample is mixed with MD. Evidence also indicates that the reduction of snow albedo by BC and MD perturbations can signifi cantly increase the net surface solar radiation, cause surface air temperature to rise, reduce snow accumulation, and accelerate snow melting.     

Development of Land Surface Model BCC＿AVIM2.0 and Its Preliminary Performance in LS3MIP/CMIP6

The improvements and validation of several parameterization schemes in the second version of the Beijing Climate Center Atmosphere–Vegetation Interaction Model(BCC_AVIM2.0) are introduced in this study. The main updates include a replacement of the water-only lake module by the common land model lake module(Co LM-lake) with a more realistic snow–ice–water–soil framework, a parameterization scheme for rice paddies added in the vegetation module, renewed parameterizations of snow cover fraction and snow surface albedo to accommodate the varied snow aging effect during different stages of a snow season, a revised parameterization to calculate the threshold temperature to initiate freeze(thaw) of soil water(ice) rather than being fixed at 0°C in BCC_AVIM1.0, a prognostic phenology scheme for vegetation growth instead of empirically prescribed dates for leaf onset/fall, and a renewed scheme to depict solar radiation transfer through the vegetation canopy. The above updates have been implemented in BCC_AVIM2.0 to serve as the land component of the BCC Climate System Model(BCC_CSM). Preliminary results of BCC_AVIM in the ongoing Land Surface, Snow, and Soil Moisture Model Intercomparison Project(LS3 MIP) of the Coupled Model Intercomparison Project Phase 6(CMIP6) show that the overall performance of BCC_AVIM2.0 is better than that of BCC_AVIM1.0 in the simulation of surface energy budgets at the seasonal timescale. Comparing the simulations of annual global land average before and after the updates in BCC_AVIM2.0 reveals that the bias of net surface radiation is reduced from-12.0 to-11.7 W m-2 and the root mean square error(RMSE) is reduced from 20.6 to 19.0 W m-2; the bias and RMSE of latent heat flux are reduced from 2.3 to-0.1 W m-2 and from 15.4 to14.3 W m-2, respectively; the bias of sensible heat flux is increased from 2.5 to 5.1 W m-2 but the RMSE is reduced from 18.4 to 17.0 W m-2.     

Influence of Clouds on UV Irradiance at Ground Level and Backscattered Exittance

The influence of various cloud parameters and the interactions with the ground albedo and the solar zenith angle have been studied by means of model simulations. The radiative transfer model suitable for a cloudy atmosphere as well as for a clear atmosphere has been developed on the basis of the Discrete Ordinate Method. This study leads to a general understanding for cloudy atmospheres: in the presence of a uniform cloud, the cloud scattering is dominant to molecular and aerosol scattering, and it is also wavelength-independent; the ratio of transmitted irradiance in a cloudy atmosphere to that in the background clear atmosphere is independent of cloud height and solar zenith angle. That’s to say, the radiation downwelling out of a cloud is quite isotropic; it decreases approximately exponentially with the cloud optical depth at a rate related to the ground albedo; the reflected irradiance at the top of the atmosphere is dependent on cloud optical depth as well as on solar zenith angle, but not on ground albedo for clouds of not very thin optical depth.     

Satellite-Based Monitoring of Decadal Soil Salinization and Climate Effects in a Semi-arid Region of China

Soil salinization is a common phenomenon that affects both the environment and the socio-economy in arid and semi-arid regions; it is also an important aspect of land cover change. In this study, we integrated multi-sensor remote sensing data with a field survey to analyze processes of soil salinization in a semi-arid area in China from 1979 to 2009. Generally, the area of salt-affected soils increased by 0.28% per year with remarkable acceleration from 1999 to 2009 (0.42% increase per year). In contrast, the area of surface water bodies showed a decreasing trend (-0.08% per year) in the same period. Decreases in precipitation and increases in aridity due to annual (especially summer) warming provided a favorable condition for soil salinization. The relatively flat terrain favored waterlogging at the surface, and continuous drought facilitated upward movement of soil water and accumulation of surface saline and calcium. Meanwhile, land-use practices also played a crucial role in accelerating soil salinization. The conversion to cropland from natural vegetation greatly increased the demand for groundwater irrigation and aggravated the process of soil salinization. Furthermore, there are potential feedbacks of soil salinization to regional climate. The salinization of soils can limit the efficiency of plant water use as well as photosynthesis; therefore, it reduces the amount of carbon sequestrated by terrestrial ecosystem. Soil salinization also reduces the absorbed solar radiation by increasing land surface albedo. Such conversions of land cover significantly change the energy and water balance between land and atmosphere.     

Urban Heat Island and Boundary Layer Structures under Hot Weather Synoptic Conditions: A Case Study of Suzhou City, China

A strong urban heat island (UHI) appeared in a hot weather episode in Suzhou City during the period from 25 July to 1 August 2007. This paper analyzes the urban heat island characteristics of Suzhou City under this hot weather episode. Both meteorological station observations and MODIS satellite observations show a strong urban heat island in this area. The maximum UHI intensity in this hot weather episode is 2.2℃, which is much greater than the summer average of 1.0℃ in this year and the 37-year (from 1970 to 2006) average of 0.35℃. The Weather Research and Forecasting (WRF) model simulation results demonstrate that the rapid urbanization processes in this area will enhance the UHI in intensity, horizontal distribution, and vertical extension. The UHI spatial distribution expands as the urban size increases. The vertical extension of UHI in the afternoon increases about 50 m higher under the year 2006 urban land cover than that under the 1986 urban land cover. The conversion from rural land use to urban land type also strengthens the local lake-land breeze circulations in this area and modifies the vertical wind speed field.     

SATELLITE OBSERVATION OF SURFACE ALBEDO OVER THE QINGHAI-XIZANG PLATEAU REGION

A method has been developed to determine the surface albedo over the Qinghai-Xizang Plateau region from NOAA polar orbiter AVHRR (Advanced Very High Resolution Radiometer) data. The empirical relationship between clear-sky planetary and surface albedos is established the basis of surface global radiation measurements and the specified ratio between atmospheric reflection and absoption of solar radiation. The method is applied to the Qinghai-Xizang region with several measurements during the period of Sep. to Nov., 1985. A comparison is presented between the estimated surface albedos and that of surface observation. The results show that the presented method is suitable to detecting the spatial and temporal variation of surface albedo and is relevant for climatological studies. The possible error sources and improvements are discussed as well.     

A Modeling Study of the Effects of Anomalous Snow Cover over the Tibetan Plateau upon the South Asian Summer Monsoon

The effect of anomalous snow cover over the Tibetan Plateau upon the South Asian summer monsoon is investigated by numerical simulations using the NCAR regional climate model (RegCM2) into which gravity wave drag has been introduced. The simulations adopt relatively realistic snow mass forcings based on Scanning Multi-channel Microwave Radiometer (SNINIR) pentad snow depth data. The physical mechanism and spatial structure of the sensitivity of the South Asian early summer monsoon to snow cover anomaly over the Tibetan Plateau are revealed. The main results are summarized as follows. The heavier than normal snow cover over the Plateau can obviously reduce the shortwave radiation absorbed by surface through the albedo effect, which is compensated by weaker upward sensible heat flux associated with colder surface temperature, whereas the effects of snow melting and evaporation are relatively smaller.The anomalies of surface heat fluxes can last until June and become unobvious in July. The decrease of the Plateau surface temperature caused by heavier snow cover reaches its maximum value from late April to early May. The atmospheric cooling in the mid-upper troposphere over the Plateau and its surrounding areas is most obvious in May and can keep a fairly strong intensity in June. In contrast, there is warming to the south of the Plateau in the mid-lower troposphere from April to June with a maximum value in May.The heavier snow cover over the Plateau can reduce the intensity of the South Asian summer monsoon and rainfall to some extent, but this influence is only obvious in early summer and almost disappears in later stages.     

Numerical Simulation of the Evolution of Snow Cover and Its Sensitivity Experiments

By using Comprehensive Land Surface Model (CLSM), three snow cases, i.e., France Col de Porte 1993/1994, 1994/1995 and BOREAS SSA-OJP 1994/1995, were simulated. The simulated results were compared with the observations to examine the capability of the model to describe the evolutions of snow cover under two different land cover conditions. Several sensitivity experiments were performed to investigate the effects of the parameterization schemes of some snow cover internal processes and vegetation on the model results. Results suggest that the CLSM simulates the basic processes of snow cover accurately and describes the features of snow cover evolutions reasonably, indicating that the model has the potential to model the processes related to the snow cover evolution. It is also found that the different parameterization schemes of the snowfall density and snow water holding capacity have significant effects on the simulation of snow cover. The estimation of snowfall density mainly impacts the simulated snow depth, and the underestimation (overestimation) of the snowfall density increases (decreases) the snow depth simulated significantly but with little effect on the simulated snow water equivalent (SWE). The parameterization of the snow water holding capacity plays a crucial role in the evolution of snow cover, especially in the ablation of snow cover. Larger snow water holding capacity usually leads to larger snow density and heat capacity by storing more liquid water in the snow layer, and makes the temperature of snow cover and the snow ablation vary more slowly. To a smaller snow water holding capacity, contrary is the case. The results also show that the physical processes related to the snow cover variation are different, which are dependent on the vegetation existed. Vegetation plays an important role in the evolution of soil-snow system by changing the energy balance at the snow-soil surface. The existence of vegetation is favorable to the maintenance of snow cover and delays the increase of underlying soil temperature.     

有关雪盖模型内部及界面过程的参数化方案的敏感试验

In order to develop a seasonal snow model of land surface process as accurately as possible for climatic study, it is necessary to fully understand the effects of important snow internal processes and interaction with air and to get an insight into influence of several relevant parameterization schemes with parameters＇ uncertainty to some degree. Using the snow model (SAST) developed by first author and other one and some useful field observation data, this paper has conducted a series of sensitivity studies on the parameterization schemes. They are relative to compaction process, snow thermal conduction, methodology of layering snow pack and to key parameters such as snow albedo, water holding capacity. Then, based on the results from the sensitivity studies, some useful conclusions for snow cover model improvement are ob tained from the analysis of the results.     

Interannual and decadal variations of snow cover over Qinghai-Xizang Plateau and their relationships to summer monsoon rainfall in China

Interannual and decadal variations of winter snow cover over the Qinghai-Xizang Plateau (QXP) are analyzed by using monthly mean snow depth data set of 60 stations over QXP for the period of 1958 through 1992. It is found that the winter snow cover over QXP bears a pronounced quasi-biennial oscillation, and it underwent an obvious decadal transition from a poor snow cover period to a rich snow cover period in the late 1970’s during the last 40 years.It is shown that the summer rainfall in the eastern China is closely associated with the winter snow cover over QXP not only in the interannual variation but also in the decadal variation. A clear relationship exists in the quasi-biennial oscillation between the summer rainfall in the northern part of North China and the southern China and the winter snow cover over QXP. Furthermore, the summer rainfall in the four climate divisions of Qinling-Daba Mountains, the Yangtze-Huaihe River Plain, the upper and lower reaches of the Yangtze River showed a remarkable transition from drought period to rainy period in the end of 1970’s, in good correspondence with the decadal transition of the winter snow cover over QXP.     

Atmospheric Corrections Using MODTRAN for TOA and Surface BRDF Characteristics from High Resolution Spectroradiometric/Angular Measurements from a Helicopter Platform

High-resolution spectral radiance measurements were taken by a spectral radiometer on board a heli copter over the US Oklahoma Southern Great Plain near the Atmospheric Radiation Measurements (ARM) site during August 1998. The radiometer has a spectral range from 350 nm to 2500 nm at 1 nm resolution. The measurements covered several grass and cropland scene types at multiple solar zenith angles. Detailed atmospheric corrections using the Moderate Resolution Transmittance (MODTRAN) radiation model and in-situ sounding and aerosol measurements have been applied to the helicopter measurements in order to re trieve the surface and top of atmosphere (TOA) Bidirectional Reflectance Distribution Function (BRDF) characteristics. The atmospheric corrections are most significant in the visible wavelengths and in the strong water vapor absorption wavelengths in the near infrared region. Adjusting the BRDF to TOA requires a larger correction in the visible channels since Rayleigh scattering contributes significantly to the TOA reflectance. The opposite corrections to the visible and near infrarred wavelengths can alter the radiance dif ference and ratio that many remote sensing techniques are based on, such as the normalized difference vege tation index (NDVI). The data show that surface BRDFs and spectral albedos are highly sensitive to the veg etation type and solar zenith angle while BRDF at TOA depends more on atmospheric conditions and the vi ewing geometry. Comparison with the Clouds and the Earth＇s Radiant Energy System (CERES) derived clear sky Angular Distribution Model (ADM) for crop and grass scene type shows a standard deviation of 0.08 in broadband anisotropic function at 25° solar zenith angle and 0.15 at 50° solar zenith angle, respectively.     

RESPONSE OF THE ATMOSPHERIC GENERAL CIRCULATION TO WINTER SNOW COVER ANOMALY

In this paper,the response of the atmospheric general circulation to winter anomalous snowcover was investigated through observations studies and model simulation.Results from the observations show that:(1)the anomalous winter snow cover in theextratropics of Eurasian Continent bears an intimate relation to the contemporary atmosphericgeneral circulation.The positive anomaly of winter snow cover is usually accompanied by positiveatmospheric EUP teleconnection pattern and stronger East Asian winter monsoon:or vice versa.(2)The linkage between them suggests that the abnormal winter snow cover has an importantimpact on winter atmospheric general circulation.The anomalous snow cover pattern can lead tothe anomaly of winter atmospheric EUP teleconnection pattern and thus influence East AsianWinter monsoon.With NCAR CCM2 including BATS land surface scheme,three groups of experiments wereperformed to examine the atmospheric response to the anomalous snow cover pattern and explorethe relevant mechanism.Simulated results agree well with the observations,which testify thesignificant response of the atmosphere to snow cover anomaly.It is found that the radiative coolinginduced by anomalous snow cover plays an important role in above processes,and the feedback oflong-wave radiation can not be neglected.     

Observation Study of Aerosol Radiative Properities over China

With a simplified radiation balance model, study is performed of aerosol direct radiation forcing in relation to its optical properties and surface reflectance, indicating that with the thickened aerosol layer the earth-atmosphere system may increase or weaken the solar radiation albedo, depending upon different combinations of aerosol single scattering albedo (SSA,ω0), asymmetry factor (g), and surface albedo (αg) rather than relying directly on the aerosol optical depth (δ), which has its value just in proportion to the changed range of albedo alone. As indicated by the model results, systematic observations of aerosol radiative properties are required to make quantitative study of aerosol direct radiative forcing. Observational research of the properties has been undertaken based on ground and space measurements over China, including ground-based sunphotometer-aerosol optical depth (AOD), nephelometer-aerosol scattering coefficients, aethalometer-aerosol absorption coefficients, and MODIS products-retrieved AOD. The satellite retrieved AOD is validated against in situ sun photometer measured AOD, indicating that for eastern China remote sensing given AODs are acceptable owing mainly to lower surface reflectance there whereas for poor vegetation in the north of China the surface reflectance may be underestimated in AOD retrieval. However, appropriate modification of the scheme of aerosol remote sensing is likely to improve the retrieval accuracy. The aerosol single scattering albedo in dry condition is around 0.80 from surface-measured scattering and absorption coefficients. It requires further studies based on more observations to improve our understanding of the issue.