Near-Tropopause Vertical Motion within the Trailing Stratiform Region of a Midlatitude Squall Line

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  • 1 Atmospheric Science Department, Colorado State University, Fort Collins, Colorado
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Abstract

Rawinsonde observations have been used to determine the flow structure in the vicinity of the tropopause atop the trailing stratiform precipitation region of an intense midlatitude squall line. Computations of vertical motion using the kinematic and thermodynamic methods show (i) upward motion in the mid- to upper troposphere within the stratiform cloud and (ii) downward motion in a thin layer 2–3 km deep centered near the tropopause at cloud top. The latter feature, which has received little attention and explanation, has recently been reported to exist above stratiform rain areas in the tropics, as measured directly by a wind profiler on the tropical western Pacific island Pohnpei.

The thin layer of sinking at the top of the trailing stratiform region occurs along the sloping upper cloud boundary and is associated with downward-sloping isentropes in the lower stratosphere to the rear of the convective line. The deformation of the isentropes is associated with an upward bulging of the tropopause, presumably caused by strong ascent in the convective line and/or mesoscale ascent aloft in the squall line system. In computations involving the thermodynamic method, the diagnosed sinking is greatest when cloud-top radiative cooling is included but it occurs even without it. Comparison of results from the various methods, however, suggests that radiative cooling at cloud top exists and is important, and it may have an amplitude on the order of 0.5°C h−1 as determined by Webster and Stephens. The strong cooling at the top of mesoscale convective systems such as these suggests a possible mechanism for dehydration of the lower tropical stratosphere that does not require a primary transport of water vapor from a particularly cold tropopause region such as the Indonesian maritime continent.

Abstract

Rawinsonde observations have been used to determine the flow structure in the vicinity of the tropopause atop the trailing stratiform precipitation region of an intense midlatitude squall line. Computations of vertical motion using the kinematic and thermodynamic methods show (i) upward motion in the mid- to upper troposphere within the stratiform cloud and (ii) downward motion in a thin layer 2–3 km deep centered near the tropopause at cloud top. The latter feature, which has received little attention and explanation, has recently been reported to exist above stratiform rain areas in the tropics, as measured directly by a wind profiler on the tropical western Pacific island Pohnpei.

The thin layer of sinking at the top of the trailing stratiform region occurs along the sloping upper cloud boundary and is associated with downward-sloping isentropes in the lower stratosphere to the rear of the convective line. The deformation of the isentropes is associated with an upward bulging of the tropopause, presumably caused by strong ascent in the convective line and/or mesoscale ascent aloft in the squall line system. In computations involving the thermodynamic method, the diagnosed sinking is greatest when cloud-top radiative cooling is included but it occurs even without it. Comparison of results from the various methods, however, suggests that radiative cooling at cloud top exists and is important, and it may have an amplitude on the order of 0.5°C h−1 as determined by Webster and Stephens. The strong cooling at the top of mesoscale convective systems such as these suggests a possible mechanism for dehydration of the lower tropical stratosphere that does not require a primary transport of water vapor from a particularly cold tropopause region such as the Indonesian maritime continent.

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