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Updraft and Downdraft Cores in TOGA COARE: Why So Many Buoyant Downdraft Cores?

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  • 1 Texas A&M University, College Station, Texas, and National Center for Atmospheric Research,* Boulder, Colorado
  • | 2 National Center for Atmospheric Research,Boulder, Colorado
  • | 3 Department of Physics and Geophysical Research Center, New Mexico Institute of Mining and Technology, Socorro, New Mexico
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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.

* Current affiliation: El Paso Energy Marketing Company, Houston, Texas.

The National Center for Atmospheric Research is sponsored by the National Science Foundation.

Current affiliation: University Advancement, University of California, Irvine, Irvine, California.

Corresponding author address: Margaret A. LeMone, NCAR, P.O. Box 3000, Boulder, CO 80307.

Email: lemone@ucar.edu

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.

* Current affiliation: El Paso Energy Marketing Company, Houston, Texas.

The National Center for Atmospheric Research is sponsored by the National Science Foundation.

Current affiliation: University Advancement, University of California, Irvine, Irvine, California.

Corresponding author address: Margaret A. LeMone, NCAR, P.O. Box 3000, Boulder, CO 80307.

Email: lemone@ucar.edu

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