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Dingying Wei
,
Alan M. Blyth
, and
David J. Raymond

Abstract

The buoyancy of convective clouds in TOGA COARE was calculated from the NCAR Electra in situ measurements of temperature, humidity, liquid water content, and two-dimensional images of raindrops. Most of the measurements were made at 700 mb (10°C) although some were made at 850 (18°C) and 600 mb (2°C). The temperature was measured with the Ophir radiometer, which does not have the wetting problem that has degraded many previous measurements of in-cloud temperature in warm clouds.

On average, the in-cloud virtual temperature excess was found to be less than the adiabatic value by about 2 K, while the negative influence of total water content on buoyancy was less than 0.5 K. Furthermore, the total water content was highly variable and much smaller than the adiabatic value. The authors conclude, therefore, that entrainment and mixing was usually a much larger factor in reducing the buoyancy than water loading.

The average buoyancy in downdrafts was positive and similar to the value in updrafts.

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Richard C. Igau
,
Margaret A. LeMone
, and
Dingying Wei

Abstract

An examination of the properties of updraft and downdraft cores using Electra data from TOGA COARE shows that they have diameters and vertical velocities similar to cores observed over other parts of the tropical and subtropical oceans. As in previous studies, a core is defined as having vertical velocity of the same sign and greater than an absolute value of 1 m s−1 for at least 500 m. A requirement that the core contain either cloud or precipitation throughout is added, but this should not affect the results significantly.

Since the Electra was equipped with the Ophir III radiometric temperature sensor, it was also possible to make estimates of core buoyancies. As in TAMEX and EMEX, where core temperatures were estimated using the modified side-looking Barnes radiometer on the NOAA P3s, a significant fraction of both updraft and downdraft cores had apparent virtual temperatures greater than their environments. In fact, the average virtual temperature deviation from the environment for downdraft cores was +0.4 K.

Sixteen of the strongest downdraft cores were examined, all of which had positive virtual-temperature deviations, to find the source of this surprising result. It is concluded that the downdraft cores are artificially warm because 100% relative humidity was assumed in calculating virtual temperature. However, reducing core mixing ratios to more physically realistic values does not eliminate warm virtual potential temperature downdraft cores, nor does water loading make all cores negatively buoyant. Thus positively buoyant convective downdrafts do exist, though probably in smaller numbers than previously suggested.

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