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Richard C. Igau
John W. Nielsen-Gammon


The evolution of the southerly low-level jet (LLJ) during a return flow event is studied using output from the Penn State/NCAR Mesoscale Model (Version 4). Three geographically different southerly LLJs develop in the simulation: one over the southern Plains of the United States, a second southwest of Brownsville, Texas, and a third over the western Gulf of Mexico. The LLJ over the Plains is found to form first as an inertial oscillation and later as a response to lee troughing and an elevated mixed layer that develops over the region. Over Mexico, the temperature structure over the Altiplanicie Mexicana (Mexican High Plain) is responsible for a locally intense low-level pressure gradient east of the High Plain which remains nearly stationary over two diurnal cycles. The LLJ over the western Gulf of Mexico results largely from topographic blocking of the low-level southerly flow by the eastern end of the Neovolcanic Cordillera northwest of Veracruz, Mexico.

The evolution of the lower troposphere over the southern Plains resembles the Carlson and Ludlam conceptual model for a severe storm environment, but the structure of the return flow is complex. When midlevel westerlies are weak, mesoscale and boundary layer processes govern the development of LLJs. As the west and southwesterly winds increase with an approaching upper-level disturbance, synoptic influences overwhelm the mesoscale processes leading to a single, larger-scale LLJ.

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


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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