Abstract:

To improve understanding of convective processes, especially those important for convective parameterization, the vertical temperature and moisture structure of the tropical atmosphere is investigated with respect to its interaction with deep convection, mainly using observational data. The analyses test assumptions and implications of convective quasi-equilibrium, the main theoretical basis for convective parameterizations which posits that small-scale convection quickly acts to reduce vertical instability caused by slower large-scale forcing. Temperature perturbations are found to be largely in agreement with a moist-adiabatic curve, especially in the free troposphere on large enough space and time scales, in broad agreement with quasi-equilibrium thinking. Reanalysis of meteorological data as well as climate model output appears to be overly constrained to the moist-adiabatic curve, especially in the boundary layer where observations show less agreement--this is likely due to convective parameterizations that too closely couple the boundary layer and free troposphere via deep convection. A robust negative correlation between free-tropospheric and upper-tropospheric-lower-stratospheric temperature perturbations, termed the "convective cold top", is hypothesized to derive from simple hydrostatic pressure perturbations causing divergence and quantifiable adiabatic cooling above convective heating.

Water vapor perturbations from radiosondes at Nauru in the western tropical Pacific are found to be maximized in the lower free troposphere. That layer, around 800 hPa, also contains the most variance represented by the first vertical principal component, which correlates almost perfectly with column water vapor (CWV). The sharp pick-up of precipitation at high CWV shown in previous satellite-based studies and confirmed in these radiosonde data is therefore particularly associated with a moistening of this lower-free-tropospheric layer. Entraining plume analysis shows that this transition to deep convection at high CWV is associated with higher buoyancy due to moisture entrained air in the lower troposphere. Plumes using entrainment profiles based on a simple increasing mass flux profile, and including the effects of freezing, are especially skillful. An overall view of CWV as a predictor variable with relatively long autocorrelation scales that greatly increases probability of shorter-lived precipitation events near and several hours after high enough CWV values is supported by temporal analyses of microwave radiometer CWV and its relationship to precipitation.