Observational Analysis Of Summer Atmospheric Downward Longwave Radiation In Tibetan Plateau

The summer Downward Longwave Radiation(DLR)observed in Naqu,Lhasa,Nyingchi and Ali,is presented.The averaged DLR at 4 sites is respectively 299±35,319±31,368±14and 305±26 W/m~2.DLR is lower in the local morning and subsequently significantly increases in the afternoon.DLR mean diurnal variation at Naqu and Ali is about 30 W/m~2,while is respectively 9 and 19W/m~2 in Lhasa and Nyingchi.To view the situation as a whole,DLR is proportional to the lower layer temperature and water vapor pressure,and is inverse proportion to pressure.DLR increases with the cloud amount.The most common cloud basement height in Tibetan Plateau is 2-3km(as about 17.3%,34.5%and 17.2%in Naqu,Nyingchi and Ali),then followed by 1-2km(as about 16%,10.9%and 7.9%in Naqu,Nyingchi and Ali),while 3-4km has high frequency in Nyingchi and Ali(as about 11.4%and 8%separately).DLR increases when the cloud is lower than 1km,as the result of the lower surface temperature and higher water vapor pressure.Based on solar shortwave radiation observations,a method to determine the daytime sky cloud-coverage index(CLoud Fraction:CLF)is presented by the solar radiation comparisons between the empirically calculated for cloud-free and the observed ones.CLF of-5%-5%is used as daytime cloud-free to test the suitability of current 10 empirical formula of DLR in the Plateau based on proven practices..It shows that the empirical formula of Prata(1995)is most suitable for the Nyingchi where the vapor pressure is high,the empirical formula of Idso(1981)is most suitable for the Lhasa,while the empirical formula of Konzelmann(1994)is most suitable for Naqu and Ali.The errors of the calculated daytime cloud-free DLR from the observed in Naqu,Lhasa,Nyingchi and Ali are respectively 2.4%,1.6%,5.7%and 2.5%.The errors decreased when fitting the parameterizations to each station separately,which are 2%,1.5%,3.5%and 2.4%.The cloud-induced DLR enhancement effect(measured DLR minus the calculated cloud-free values given the surface temperature and humidity)clearly shows that the mean DLR enhancement effect in Naqu,Lhasa,Nyingchi and Ali is respectively30.8±28.2,22.1±23.4,38.8±15.4 and 15.6±21.3 W/m~2 with median values of 24.4,17.3,42.7 and 6.8 W/m~2.With the increase of artificially visual total cloud amount,the increasing trend of DLR enhancement is obvious,especially when cloud amount from less than 2 to 7-10,the corresponding DLR enhancement effects rapidly increases from above 20 to more than 50 W/m~2at the 4 sites.Given the same visual cloud mount,the DLR enhancement effects induced by the low clouds in Lhasa and Ali are obviously higher than those induced by the total cloud.The effect from cloud coverage and height on the DLR enhancement is further confirmed by the aerosol lidar cloud-bottom height(CBH)at zenith direction and the CLF.The decreasing CBH(no available CBH data in Lhasa)corresponding to increasing trends of CLF and DLR enhancement is detected.The DLR enhancement effects are only about 5 W/m~2(Nyingchi is close to 20 W/m~2)at cloud-free condition,but they may rise to 60 W/m~2(Nyingchi is nearly 50W/m~2)when the CLF is above 90%(average CBH<3.5km).Given the fixed CBH,the DLR enhancement obviously increases with the growth of CLFs.CLF,significantly more than zenith CBH,affects the DLR enhancement in the Tibetan Plateau.