Towards an improved understanding of deep convection patterns over the tropical oceans
We quantitatively assess thermodynamic characteristics shared by deep convection over the tropical oceans, and develop a simple model to explain the observed precipitation distribution, as well as geographic variability in the vertical development of deep convection.Relationships between humidity, surface wind speed and precipitation are analyzed using four years of daily satellite passive microwave retrievals. We find that throughout the ITCZ, at high-column relative humidities (conditions under which deep convection is likely to occur), faster winds are associated with substantially more precipitation, explaining a small, but highly statistically significant fraction of daily rainfall variability. The observed increases in precipitation are much greater than evaporation changes associated with the increased wind speed which suggests a convergence feedback.Several atmospheric reanalyses are used to compute daily column-integrated MSE budgets, and seperate the MSE export by the circulation into horizontal and vertical MSE advection terms to examine the extent to which observationally-derived data is consistent with the idea of a positive gross moist stability. Primarily due to substantial geographic variability in vertical motion profiles (and latent heating), the implied gross moist stability is negative in a 2000-km region in the central-eastern Pacific ITCZ. In the west Pacific warm pool rainy regions, mean horizontal convergence extends above 400mb, while in parts of the Pacific ITCZ where meridional SST gradients are strong, the vertical motion profile is "bottom-heavy", with convergence below 800mb and divergence above.We examine what combination of local column thermodynamic and non-local dynamic reasoning can lead to a simple quantitative understanding of the similarities and differences between the convection in the narrow eastern-central Pacific ITCZ with that in the west Pacific warm pool. A simple dynamical boundary layer model is used to understand the "bottom-heavy" component of the convection, which we argue is primarily due to SST gradients. This model is combined with a SST-based prediction of the amount of free tropospheric vertical motion from the surface convergence to yield a simple two-mode model that predicts precipitation and vertical motion profiles from SST, 850mb temperature, geopotential height and horizontal winds.
- Atmospheric sciences