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Mary Beth Whitfield and Steven W. Lyons

Abstract

National Meteorological Center 200-mb analyses are used to develop an abridged six-year climatology of the tropical upper-tropospheric trough (TUTT) over the Gulf of Mexico. The climatology reveals large intraseasonal and interannual variability in TUTT axis position. The summer of 1988 is identified as having had an active TUTT near Texas, and is examined in detail. Satellite imagery and 200-mb winds are used to identify and track TUTT lows.

A TUTT low that remained quasi-stationary over Texas is selected for detailed examination. Horizontal and vertical cross sections of wind, temperature, vorticity, and relative humidity illustrate that 1) the maximum circulation around the low occurs near 200-mb, 2) the cold anomaly is largest near 300 mb, and 3) the troposphere moistens as the TUTT low strengthens over Texas. The initial vorticity source for the TUTT low is attributed to positive vorticity advection from midlatitudes, conservation of absolute vorticity, and vorticity convergence along the TUTT axis.

Calculations reveal a net direct circulation in and around the TUTT low, indicating it is an energy-generating system. Vertical motion and temperature fields display warm air rising on the eastern side of the low and cool air sinking near the center and western side of the low in the middle and upper troposphere. Precipitation is maximum in the SE quadrant relative to the TUTT low center.

Examination of the temporal distribution of rainfall over Texas reveals that the TUTT low was one of six synoptic systems that resulted in significant rainfall over the state during July and August 1988. The amount of cold convective cloud as seen from satellite imagery associated with the TUTT low displays significant diurnal variation with a maximum (minimum) observed in afternoon and early evening (late night and early morning) hours, consistent with the diurnal heating cycle over land. It is shown that TUTT lows can be significant sources of summer precipitation in Texas on synoptic space and time scales.

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