Intensity and Amplification of Tropical Deep Convection

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This research seeks to understand the basic mechanisms which control the intensity and aggregation of deep convection in the tropics. Deep convection refers to the overturning motions found in cumulus clouds and associated with showers and intense precipitation. A central issue addressed by this research is the contrast in convective intensity between land and ocean regions, which can be inferred from the difference in lightning flash rate per unit rainfall seen in satellite observations from the tropics. The higher flash rate is generally accepted as an indication that the updraft and downdraft speeds above the melting level are are greater for convective precipitation over land, thus the flash rate serves as a proxy for the intensity of convection. It is sometimes assumed that convection is more intense over land due to higher values of Convective Available Potentially Energy (CAPE), but evidence for this claim is somewhat ambiguous. Other hypotheses have been proposed, including one that the difference is due to the more intense surface heating that occurs over land than ocean for the same temperature and insolation due to the dryness of the surface, another that the heterogeneity of land surfaces is at fault, and a third that the greater abundance of cloud and ice condensation nuclei over land are responsible. The working hypothesis here is that greater instability over land is in fact the cause of the intensity difference, but that a systematic examination of the full distribution of CAPE values is required, rather than a simple comparison of land versus ocean mean values.Further research would address the aggregation of convection in the tropics, meaning the tendency of precipitating convective clouds in the tropics to occur in large organized aggregates. The work begins from the observation that episodes of organized convection typically start out with a "bottom heavy" structure, in which most of the latent heat of condensation (released as water vapor condenses into cloud droplets) occurs at low to middle levels of the troposphere. As the convection intensifies the clouds deepen and take on a "top heavy" structure, eventually forming a broad stratiform region near the tropopause. The PI hypothesizes that clouds with a bottom heavy structure are effective in drawing moisture and energy into the region, thereby intensifying convection and leading to the development of deep convective clouds. The PI argues that this self-amplification can be understood in terms of a "drying efficiency" which determines the gross moist stability of the atmosphere is above or below a critical level. The research also considers variations in the effectiveness of this amplification between short-lived and longer-lasting convective aggregates.The work is conducted through a combination of observational data analysis and numerical model simulations. Simulations use a cloud resolving model, often in a channel-shaped periodic domain with an island on one end but otherwise covered by an ocean surface. Observational data comes from reanalysis products and weather balloon soundings, and includes an examination of CAPE over land and oceans.The research has broader impacts due to the value of understanding, simulating, and forecasting the intensity and organization of tropical convective precipitation. This is clearly an issue of local concern in the tropics, and large regions of tropical convection can also influence weather in middle and high latitudes. A better understanding of the aggregation and organization of convection could thus lead to improvements in weather prediction over much of the globe. The work also has educational broader impacts, as the project will support and train two graduate students, thereby providing for the next generation of scientists in this field. The PI also works with the Expanding Your Horizons (EYH) program in Madison, WI. EYH holds a one-day conference for that provides middle school-aged girls the opportunity to explore careers that use math and science.