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Michael W. Douglas, Robert A. Maddox, Kenneth Howard, and Sergio Reyes

Abstract

The pronounced maximum in rainfall during the warm season over southwestern North America has been noted by various investigators. In the United States this is most pronounced over New Mexico and southern Arizona; however, it is but an extension of a much larger-scale phenomenon that appears to be centered over northwestern Mexico. This phenomenon, herein termed the “Mexican monsoon,” is described from analyses of monthly mean rainfall, geostationary satellite imagery, and rawinsonde data. In particular, the authors note the geographical extent and magnitude of the summer rains, the rapidity of their onset, and the timing of the month of maximum rainfall. Finally, the difficulty in explaining the observed precipitation distribution and its timing from monthly mean upper-air wind and moisture patterns is discussed.

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David J. Stensrud, Robert L. Gall, Steven L. Mullen, and Kenneth W. Howard

Abstract

The Mexican monsoon is a significant feature in the climate of the southwestern United States and Mexico during the summer months. Rainfall in northwestern Mexico during the months of July through September accounts for 60% to 80% of the total annual rainfall, while rainfall in Arizona for these same months accounts for over 40% of the total annual rainfall. Deep convection during the monsoon season produces frequent damaging surface winds, flash flooding, and hail and is a difficult forecast problem. Past numerical simulations frequently have been unable to reproduce the widespread, heavy rains over Mexico and the southwestern United States associated with the monsoon.

The Pennsylvania State University/National Center for Atmospheric Research mesoscale model is used to simulate 32 successive 24-h periods during the monsoon season. Mean fields produced by the model simulations are compared against observations to validate the ability of the model to reproduce many of the observed features, including the large-scale midtropospheric wind field, southerly low-level winds over the Gulf of California, and the heavy rains over western Mexico. Preliminary analysis of the mean model fields also suggest that the Gulf of California is the dominant moisture source for deep convection over Mexico and the southwestern United States, with upslope flow along the Sierra Madre Occidental advecting low-level gulf moisture into western Mexico during the daytime and southerly flow at the northern end of the gulf advecting gulf moisture into Arizona on most days. These results illustrate the usefulness of four-dimensional data assimilation techniques to create proxy datasets containing realistic mesoscale features that can be used for detailed diagnostic studies.

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Andrew J. Negri, Robert F. Adler, Robert A. Maddox, Kenneth W. Howard, and Peter R. Keehn

Abstract

A three-year climatology of satellite-estimated rainfall for the warm season for the southwest United States and Mexico has been derived from data from the Special Sensor Microwave Imager (SSM/1). The microwave data have been stratified by month (June, July, August), yew (1988, 1989, 1990), and time of day (morning and evening orbits). A rain algorithm was employed that relates 86-GHz brightness temperatures to rain rate using a coupled cloud-radiative transfer model.

Results identify an early evening maximum in rainfall along the western slope of the Sierra Madre Occidental during all three months. A prominent morning rainfall maximum was found off the western Mexican coast near Mazatlan in July and August. Substantial differences between morning and evening estimates were noted. To the extent that three years constitute a climatology, results of interannual variability are presented. Results are compared and contrasted to high-resolution (8 km, hourly) infrared cloud climatologies, which consist of the frequency of occurrence of cloud colder than −38°C and −58°C. This comparison has broad implications for the estimation of rainfall by simple (cloud threshold) techniques.

By sampling the infrared data to approximate the time and space resolution of the microwave, we produce ratios (or adjustment factors) by which we can adjust the infrared rain estimation schemes. This produces a combined micro wave/infrared rain algorithm for monthly rainfall. Using a limited set of raingage data as ground truth, an improvement (lower bias and root-mean-square error) was demonstrated by this combined technique when compared to either method alone. The diurnal variability of convection during July 1990 was examined using hourly rain estimates from the GOES precipitation index and the convective stratiform technique, revealing a maximum in estimated rainfall from 1800 to 2100 local time. It is in this time period when the SSM/1 evening orbit occurs. A high-resolution topographic database was available to aid in interpreting the influence of topography on the rainfall patterns.

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