Outgoing longwave radiation and cloud radiative forcing of the Tibetan Plateau

In order to study the energy balance and the cloud radiative forcing (CRF) of the Tibetan Plateau in detail, 2 years of GMS5 satellite data are employ ed to analyze the monthly mean outgoing longwave radiation (OLR) and CRE It should be noted that the temporal resolution of GMS5 data is 1 hour, so th e data can be used to study the diurnal variations of OLR. First, a method is presented to retrieve the OLR from split-window channels (10.5-11.5 and 11.5-12.5 mu m) and the water vapor channel (6.5-7.0 mu m) of GMS5. The met hod applies the discrete ordinates radiative transfer (DISORT) model togeth er with the radiosonde profiles of the Tibetan Plateau to simulate radiance s and fluxes of the three channels. A regression relationship is then devel oped to calculate the OLR from the observations of the three channels. Sinc e the Tibetan Plateau is located nearly out of the effective observational range of the GMS5 satellite, the regression results of GMS5's split-window channels and water vapor channel are corrected by using simultaneously retr ieved results from TIROS Operational Vertical Sounder (TOVS). The correlati on coefficient of GMS5 and TOVS results is 0.8510, which is targe enough fo r 1% significant level. The OLR distributions are calculated for the Tibeta n Plateau using 2 years of GMS5 data and the regression and correction meth ods. The average of the OLR images for the same month and same time gives t he monthly mean OLR distribution for each hour. The 24-hour OLR distributio ns of the same month are then averaged to yield the monthly mean OLR distri bution for that month. Then our monthly mean OLR distributions are compared with the Clouds and the Earth's Radiant Energy System (CERES) results, and they are generally in good agreement with differences of <10% for January and 5% for July. Analyzing the monthly mean OLR distributions for different seasons, we find that during the winter season the OLR distribution exhibi ts low values over the Tibetan Plateau but high values for areas off the Ti betan Plateau. During the summer season the OLR of the southern part is sma ller than that of the northern part. Studying the monthly mean diurnal vari ations of OLR, we Find that the diurnal variations of OLR are affected by d iurnal cycles of cloud quantity and surface temperature. The relief of the Tibetan Plateau is very high, and the radiative healing is intense after su nrise. The OLR is greatly influenced by the surface and reaches a maximum v alue soon after sunrise, but the time the minimum OLR emerges varies. After the OLR distributions of the Tibetan Plateau are obtained, the role of clo uds in the climate system is also studied. In order to calculate the CRF th e international Satellite Cloud Climatology Project (ISCCP) cloud detection algorithm is used to detect the clear pixels for each image. The clear-sky components of OLR and albedo for different months and hours are then deriv ed and averaged over a month to obtain the monthly mean clear-sky OLR and a lbedo for each hour. Finally, data are averaged over 24 hours to give the m onthly mean shortwave CRF (SWCRF), longwave CRF (LWCRF), and CRE The result s show that the CRF over the Tibetan Plateau is negative most of the time. This means the CRF is dominated by cooling effects, and the distribution pa ttern is mainly determined by the SWCRF component. While the CRFs to the so uth and the north of the Tibetan Plateau are different, there are obvious a nnual variations with heating effects in the summer-autumn season and cooli ng effects in the winter-spring season.