Thursday, 18 January 2007: 9:30 AM
The Convective Cold Top and Quasi-Equilibrium
214B (Henry B. Gonzalez Convention Center)
To investigate dominant vertical structures of observed temperature perturbations, and to test the temperature implications of the convective quasi-equilibrium hypothesis, we examine how the tropical temperature profile covaries with the average free tropospheric temperature 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. Some effects of masking these temperature analyses with moisture variables such as precipitation are explored, particularly in relation to the boundary layer behavior.
A third vertical feature of the temperature perturbation profile, besides the fairly coherent free troposphere and somewhat independent boundary layer, is what we call 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, though it generally has larger amplitude at smaller scales. 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. Some implications of a general and robust convective cold top relationship are considered for models that assume a moist adiabat-like structure for tropospheric temperature profiles and perturbations.