FindellEltahir-03-framwork - 552 V OLUME 4 J O U R N A L O...

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Unformatted text preview: 552 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 Moisture–Boundary Layer Interactions. Part I: Framework Development 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 This paper investigates the influence of soil moisture on the development and triggering of convection in different early-morning atmospheric conditions. A one-dimensional model of the atmospheric boundary layer (BL) is initialized with atmospheric sounding data from Illinois and with the soil moisture set to either extremely wet (saturated) or extremely dry (20% of saturation) conditions. Two measures are developed to assess the low- level temperature and humidity structure of the early-morning atmosphere. These two measures are used to distinguish between four types of soundings, based on the likely outcome of the model: 1) those soundings favoring deep convection over dry soils, 2) those favoring deep convection over wet soils, 3) those unlikely to convect over any land surface, and 4) those likely to convect over any land surface. Examples of the first two cases are presented in detail. The early-morning atmosphere is characterized in this work by the newly developed convective triggering potential (CTP) and a low-level humidity index, HI low . The CTP measures the departure from a moist adiabatic temperature lapse rate in the region between 100 and 300 mb (about 1–3 km) above the ground surface (AGS). This region is the critical interface between the near-surface region, which is almost always incorporated into the growing BL, and free atmospheric air, which is almost never incorporated into the BL. Together, these two measures form the CTP-HI low framework for analyzing atmospheric controls on soil moisture–boundary layer interactions. Results show that in Illinois deep convection is trigged in the model 22% of the time over wet soils and only 13% of the time over dry soils. Additional testing varying the radiative conditions in Illinois and also using the 1D model with soundings from four additional stations confirm that the CTP-HI low framework is valid for regions far removed from Illinois. 1. Introduction Feedbacks between the land surface and the atmo- sphere have been the focus of much recent inquiry into questions ranging from the maintenance of extreme drought or flood conditions, to the influence of defor- estation on rainfall, to responses to increases in atmo- spheric concentrations of greenhouse gases. Many stud- ies of the midwestern U.S. drought of 1988 and flood of 1993, for example, suggest that the soil moisture condition in these cases helped to sustain the extreme circumstances throughout the summer (Trenberth and...
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FindellEltahir-03-framwork - 552 V OLUME 4 J O U R N A L O...

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