The lifetime of Mei- Yu fronts over southern China ranges from 3 to 22 days (Chen, 1983), with a nearly continuous cloud band accompanying the front. It is the goal of this study to estimate the cloud and radiative effects on the Mei-Yu front during the TAMEX IOP-2 periods.
The present radiative transfer model is based on a broadband method and involves the transfer of IR and solar radiation for both clear and cloudy regions. To perform radiative transfer calculations and to compare this with observed cloudiness, a diagnostic cloud scheme is adopted in th paper. This cloud scheme is similiar to Geleyn's(1981) approach. After we obtained the model-predicted cloud covers, we then divided into low, middle and high clouds, which overlapped each other to a maximum or minimum degree.
Predicted cloudiness during the TAMEX IOP-2 periods, with the use of ECMWF/WMO analysis data sets, has shown a reasonable degree of success. The nearly continuous cloud band over the Mei-Yu front is well simulated. This cloud band is dominated by middle-level cloud with several high-level deep cumulus clouds embedded in it. However, the cloudness in general is underestimated by the model, especially for high-level cloud.
Observing the model predicted cloud cover and OLR fields, we find that the OLR patterns are basically related to the predicted cloud cover. In the Mei-Yu frontal zone the computed OLR value is less than. 220 wm-2 and it also has several minimum centers within it. Comparing the predicted OLR patterns with the satellite derived OLR data, we find that simulated patterns are in agreement with the observations. Nevertheless, the OLR fluxes over the Mei-Yu frontal zone and the tropical convection areas, are slightly larger than the observations.
The distributions of the computed radiative heating and cooling rate have very different vertical profiles. For the net radiative heating rates, it is seen that maximum heating for the three computed periods is less than 3.S°C/day and all located below the high-level cloud base. Nevertheless, the biggest net cooling along the Mei-Yu front is much greater than the heating(> 6°C/day) rate and all concentrated in the upper troposphere. It is also noted that cooling over the Taiwan area is greater than 6°C/day for both the 1200 UTC 16 and 0000 UTC 17 periods.