The Convective Cold Top and Quasi Equilibrium

To investigate dominant vertical structures of observed temperature perturbations, and to test the temperature implications of the convective quasi-equilibrium hypothesis, the relationship of the tropical temperature profile to the average free tropospheric temperature is examined in Atmospheric Infrared Sounder (AIRS) satellite data, radiosonde observations, and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. The spatial scales analyzed extend from the entire tropics down to a single reanalysis grid point or radiosonde station, with monthly to daily time scales. There is very high vertical coherence of free tropospheric temperature perturbations. There is also fairly good agreement throughout the free troposphere between observations and a theoretical quasi-equilibrium perturbation profile calculated from a distribution of moist adiabats. The boundary layer is fairly independent from the free troposphere, especially for smaller scales. A third vertical feature of the temperature perturbation profile is here termed the "convective cold top:" a robust negative correlation between temperature perturbations of the vertically averaged free troposphere and those of the upper troposphere and lower stratosphere. The convective cold top is found for observations and reanalysis at many temporal and spatial scales. Given this prevalence, the literature is reviewed for previous examples of what is likely a single phenomenon. One simple explanation is proposed: hydrostatic pressure gradients from tropospheric warming extend above the heating, forcing ascent and adiabatic cooling. The negative temperature anomalies thus created are necessary for anomalous pressure gradients to diminish with height.