Zhian Sun

Precipitable water vapor on the Tibetan Plateau estimated by GPS, water vapor radiometer, radiosonde, and numerical weather prediction analysis and its impact on the radiation budget

[1]xa0Precipitable water vapor amounts (PW) determined by Global Positioning System (GPS), radiosonde and operational numerical weather prediction (NWP) system analysis at three stations (Naqu, Gaize, and Deqin) on the Tibetan Plateau are compared. PW measured by water vapor radiometer at Naqu and a low-elevation site, Xian, is used for calibration. The results show that the PW determined by NWP analysis in these regions is comparable with that of the radiosonde measurements but that they both are systematically smaller than those determined by the GPS measurements. The averaged difference of PW between GPS and radiosonde estimates is about 1.75 mm, and that between GPS and NWP analysis can be as large as 7.75 mm. These differences are relatively larger than those reported in the literature because the absolute PW in this region is much smaller. The effect of such large differences on the surface radiation budget is evaluated using a radiation model. The results show that both longwave and shortwave radiative fluxes at the surface determined using the model analysis profiles with the water vapor corrected by the GPS PW are closer to the observations compared with those without water vapor correction. The flux difference at the surface with and without water vapor correction is about 20 W m−2 in the shortwave and 30 W m−2 in the longwave. These differences are much larger than that caused by doubling the concentration of carbon dioxide in the atmosphere in this region.

Parameterization of instantaneous global horizontal irradiance: Cloudy‐sky component

[1]xa0Radiation calculations in global numerical weather prediction (NWP) and climate models are usually conducted in 3-hourly time interval in order to reduce the computational cost. This treatment can lead to an incorrect solar radiation at the Earths surface which could be one of the error sources in modeled convection and precipitation. In order to improve the simulation of the diurnal cycle of solar radiation a fast scheme has been developed based on detailed radiative transfer calculations for a wide range of atmospheric conditions and can be used to determine the surface solar radiation at each model integration time step with affordable costs. This scheme is divided into components for clear-sky and cloudy-sky conditions. The clear-sky component has been described in a companion paper. The cloudy-sky component is introduced in this paper. The input variables required by this scheme are all available in NWP and climate models or can be obtained from satellite observations. Therefore, the scheme can be used in a global model to determine the surface GHI. It can also be used as an offline scheme to calculate the surface GHI using data from satellite measurements. SUNFLUX scheme has been tested using observations obtained from three Atmospheric Radiation Measurements (ARM) stations established by the U. S. Department of Energy. The results show that a half hourly mean relative error of GHI under all-sky conditions is less than 7%. An important application of the scheme is in global climate models. The radiation sampling error due to infrequent radiation calculations is investigated using the SUNFLUX and ARM observations. It is found that errors in the surface net solar irradiance are very large, exceeding 800 W m−2at many non-radiation time steps due to ignoring the effects of clouds. Use of the SUNFLUX scheme can reduce these errors to less than 50 W m−2.

A Study on Sulfate Optical Properties and Direct Radiative Forcing Using LASG-IAP General Circulation Model

The direct radiative forcing (DRF) of sulfate aerosols depends highly on the atmospheric sulfate loading and the meteorology, both of which undergo strong regional and seasonal variations. Because the optical properties of sulfate aerosols are also sensitive to atmospheric relative humidity, in this study we first examine the scheme for optical properties that considers hydroscopic growth. Next, we investigate the seasonal and regional distributions of sulfate DRF using the sulfate loading simulated from NCAR CAM-Chem together with the meteorology modeled from a spectral atmospheric general circulation model (AGCM) developed by LASG-IAP. The global annual-mean sulfate loading of 3.44 mg m−2 is calculated to yield the DRF of −1.03 and −0.57 W m−2 for clear-sky and all-sky conditions, respectively. However, much larger values occur on regional bases. For example, the maximum all-sky sulfate DRF over Europe, East Asia, and North America can be up to −4.0 W m−2. The strongest all-sky sulfate DRF occurs in the Northern Hemispheric July, with a hemispheric average of −1.26 W m−2. The study results also indicate that the regional DRF are strongly affected by cloud and relative humidity, which vary considerably among the regions during different seasons. This certainly raises the issue that the biases in model-simulated regional meteorology can introduce biases into the sulfate DRF. Hence, the model processes associated with atmospheric humidity and cloud physics should be modified in great depth to improve the simulations of the LASG-IAP AGCM and to reduce the uncertainty of sulfate direct effects on global and regional climate in these simulations.

Estimation of hourly solar radiation at the surface under cloudless conditions on the Tibetan Plateau using a simple radiation model

In this study, the clear sky hourly global and net solar irradiances at the surface determined using SUNFLUX, a simple parameterization scheme, for three stations (Gaize, Naqu, and Lhasa) on the Tibetan Plateau were evaluated against observation data. Our modeled results agree well with observations. The correlation coefficients between modeled and observed values were > 0.99 for all three stations. The relative error of modeled results, in average was < 7%, and the root-mean-square variance was < 27 W m−2.The solar irradiances in the radiation model were slightly overestimated compared with observation data; there were at least two likely causes. First, the radiative effects of aerosols were not included in the radiation model. Second, solar irradiances determined by thermopile pyranometers include a thermal offset error that causes solar radiation to be slightly underestimated.The solar radiation absorbed by the ozone and water vapor was estimated. The results show that monthly mean solar radiation absorbed by the ozone is < 2% of the global solar radiation (< 14 W m−2). Solar radiation absorbed by water vapor is stronger in summer than in winter. The maximum amount of monthly mean solar radiation absorbed by water vapor can be up to 13% of the global solar radiation (95 W m−2). This indicates that water vapor measurements with high precision are very important for precise determination of solar radiation.