The Convective cold top and quasi-equilibrium

To test the extent to which temperature perturbations can be simplified in the vertical, as well as specifically testing the convective quasi-equilibrium (QE) 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 data. There is remarkably good agreement through most of the free troposphere between various observations and the theoretical quasi-equilibrium perturbation profile calculated from a distribution of typical moist adiabats. The degree to which temperature perturbations are well-correlated throughout the free troposphere improves as the space and time scales analyzed become larger. These spatial scales extend from the entire deep tropics down to a single reanalysis gridpoint or radiosonde station, with monthly to daily time scales. The boundary layer also tends to become more correlated with the free troposphere at larger scales, although the boundary layer temperature is not necessarily on the same moist adiabat as the free troposphere. 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. We propose one simple explanation: that hydrostatic pressure gradients from tropospheric warming extend above the heating, forcing ascent and adiabatic cooling. The negative temperature anomalies thus created are necessary in any circumstance in which anomalous pressure gradients 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.