An empirical study of the relationship between soil moisture and convection in the southern Great Plains
Frye, John Dale
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Land surface properties, particularly soil moisture, have been shown to alter the atmosphere’s planetary boundary layer (PBL) at many spatial and temporal scales. Changes in the PBL can lead to conditions that promote or inhibit the development of convective thunderstorms. This dissertation examines the relationship between soil moisture and two aspects of the convective environment in the southern Great Plains from 1998 to 2004: (1) diurnal growth of the planetary boundary layer, and (2) measures of atmospheric stability, including (convective available potential energy (CAPE), lifted index (LI), and convective inhibition (CIN)). This work also examines the relationship between soil moisture, gradients in soil moisture and the location of convective development. The synoptic-scale environment can also promote or inhibit convective development. Therefore, this work also examines how these relationships change under various synoptic-type days. This research is enhanced by the use of soil moisture estimates from the Tropical Rainfall Measuring Mission Microwave Imager and atmospheric soundings from the North American Regional Reanalysis. Moderate soil moisture (15-30%) and moderate soil moisture gradient (1.5-2.0% km-1) are shown to be critical values when examining the role of this surface property on convection. The diurnal growth of the boundary layer is greatest within this range of soil moisture values, allowing more turbulent thermals to reach the level of free convection. Within this range of soil moisture values, CAPE is lower and LI is higher than over wetter and drier soils and the CIN values are highest. The increase in CIN indicates a stronger cap to convection exists over areas with moderate soil moisture, allowing a greater accumulation of convective energy during the day. Once the stronger caps are eroded, the convection tends to develop more quickly and become more organized. The moderate soil moisture and soil moisture gradient values are also shown to represent a “tipping point” for the probability of convection initiation. These elements of convective development are also related to the synoptic-scale environment, particularly the presence or absence of the low-level jet (LLJ).