Test of the Fixed Anvil Temperature Hypothesis

Abstract

he fixed anvil temperature (FAT) hypothesis is examined based on the Aqua Moderate Resolution Imaging Spectroradiometer (MODIS)-based cloud-top temperature (CTT) in conjunction with the tropical atmospheric profiles and sea surface temperature (SST) from the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalysis. Consistent with the physical governing mechanism of the FAT hypothesis, the peak clear-sky diabatic subsidence and convergence profiles are located at roughly the same level (200 hPa) as the peak in the cloud profile, which is fundamentally determined by the rapid decrease of water vapor concentration above this level. The geographical maxima of cloud fraction agree well with those of water vapor, clear-sky cooling rates, and diabatic convergence at 200 hPa. The use of direct CTT measurements suggests the CTT in specific Pacific basins exhibit different characteristics as the frequency distribution of the tropical SST varies from boreal winter to summer. When averaging over the tropics as a whole, the CTT distributions are approximately unchanged primarily because of cancellation by the variations associated with individual regions. An analysis of the response of the tropical mean CTT anomaly time series to the SST indicates that a possible negative relationship is present, whereas the relationship tends to be positive over the tropical western Pacific and Indian Oceans. In addition, it is suggested to interpret the FAT hypothesis, and the more recent proportionately higher anvil temperature (PHAT) hypothesis, by using the temperature at the maximum cloud detrainment level instead of the CTT.