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Unformatted text preview: 570 V OLUME 4 J O U R N A L O F H Y D R O M E T E O R O L O G Y q 2003 American Meteorological Society Atmospheric Controls on Soil MoistureBoundary Layer Interactions. Part II: Feedbacks within the Continental United States K IRSTEN L. FINDELL Geophysical Fluid Dynamics Laboratory, Princeton University, Princeton, New Jersey E LFATIH A. B. ELTAHIR Parsons Laboratory, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts (Manuscript received 6 March 2002, in final form 26 November 2002) ABSTRACT The CTP-HI low framework for describing atmospheric controls on soil moistureboundary layer interactions is described in a companion paper, Part I. In this paper, the framework is applied to the continental United States to investigate how differing atmospheric regimes influence local feedbacks between the land surface and the atmosphere. The framework was developed with a one-dimensional boundary layer model and is based on two measures of atmospheric thermodynamic properties: the convective triggering potential (CTP), a measure of the temperature lapse rate between approximately 1 and 3 km above the ground surface, and a low-level humidity index, HI low . These two measures are used to distinguish between three types of early-morning atmospheric conditions: those favoring moist convection over dry soils, those favoring moist convection over wet soils, and those that will allow or prevent deep convective activity, independent of the surface flux partitioning. Analyses of multiyear CTP-HI low scatterplots from radiosonde stations across the contiguous 48 United States reveal that during the summer months (June, July, and August) positive feedbacks between soil moisture and moist convection are likely in much of the eastern half of the country. Over the western half of the country, atmospheric conditions and the likelihood of moist convection are largely determined by oceanic influences, and land surface conditions in the summer are unlikely to impact convective triggering. The only area showing a potential negative feedback is in the dryline and monsoon region of the arid Southwest. This potential arises because of the topography of this and surrounding regions. A relatively narrow band of stations lies in between the eastern and western portions of the country, in some years behaving like the stations to the west and in other years behaving like the stations to the east. 1. Introduction a. Motivation Feedbacks from the earths surface to the atmosphere are an instrumental part of global climatic processes. Extensive research on the El Nin oSouthern Oscillation phenomenon connects anomalous sea surface temper- atures (SSTs) in the eastern Pacific Ocean with dramatic shifts in weather patterns over much of the globe. Like SSTs, vegetation cover and soil moisture content control the partitioning of energy fluxes at the earths surface, and, like SSTs, land surface conditions in some regions...
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