Michael W. Douglas

THE SOUTH AMERICAN LOW-LEVEL JET EXPERIMENT

Abstract Moisture is transported in South America westward from the tropical Atlantic Ocean to the Amazon basin, and then southward toward the extratropics. A regional intensification of this circulation to the east of the Andes Mountains is called the South American low-level jet (SALLJ), with the strongest winds found over eastern Bolivia. SALLJ is present all year and channels moisture to the La Plata basin, which is analogous to the better-known Amazon basin in terms of its biological and habitat diversity, and far exceeds the latter in its economic importance to southern and central South America in terms of hydroelectricity and food production. The relatively small SALLJ spatial scale (compared with the density of the available sounding network) has a limited understanding of and modeling capability for any variations in the SALLJ intensity and structure as well as its possible relationship to downstream rainfall. The SALLJ Experiment (SALLJEX), aimed at describing many aspects of SALLJ, was carried...

The Summertime Low-Level Jet over the Gulf of California

Abstract Special pilot balloon and aircraft observations made during the 1990 Southwest Area Monsoon Project (SWAMP-90) are used to describe the structure of a low-level jet (LLJ) observed in the southerly flow over the Gulf of California and surroundings. Mean wind fields based on the 35 days of pilot balloon observations reveal an LLJ strongest over the northern Gulf of California, with southerly flow approximately parallel to the axis of the gulf. The diurnal variation of the mean wind fields is also shown; there is upslope flow, away from the gulf, in the afternoon and downslope flow, toward the gulf, in the morning. Over land, the LLJ is strongest at Yuma, Arizona, where it shows a strong diurnal variation. Here the morning maximum wind speeds in the jet reach 20 m s−1 and are strongest approximately 300–600 m above the surface. The low-level flow undergoes synoptic timescale fluctuations in intensity, but the jet is present at Yuma on about 75% of the mornings during SWAMP-90. SWAMP aircraft observa...

The NAME 2004 Field Campaign and Modeling Strategy

The North American Monsoon Experiment (NAME) is an internationally coordinated process study aimed at determining the sources and limits of predictability of warm-season precipitation over North America. The scientific objectives of NAME are to promote a better understanding and more realistic simulation of warm-season convective processes in complex terrain, intraseasonal variability of the monsoon, and the response of the warm-season atmospheric circulation and precipitation patterns to slowly varying, potentially predictable surface boundary conditions. During the summer of 2004, the NAME community implemented an international (United States, Mexico, Central America), multiagency (NOAA, NASA, NSF, USDA) field experiment called NAME 2004. This article presents early results from the NAME 2004 campaign and describes how the NAME modeling community will leverage the NAME 2004 data to accelerate improvements in warm-season precipitation forecasts for North America.

THE AMMA RADIOSONDE PROGRAM AND ITS IMPLICATIONS FOR THE FUTURE OF ATMOSPHERIC MONITORING OVER AFRICA

In the face of long-term decline, the AMMA research program has reactivated the radiosonde network over West Africa. The lessons learned in AMMA have significance for the upper-air network throughout the continent.

Diurnal Variation and Horizontal Extent of the Low-Level Jet over the Northern Gulf of California

Abstract This note describes aspects of the diurnal variation and geographical location of the summertime low-level jet over the Gulf of California, based on wind soundings made at two sites near the northern end of the gulf during a two-week summer field program in 1995. The low-level wind observations from Puerto Penasco, a site along the northern shore of the gulf, showed a strong diurnal cycle decreasing in amplitude to near 0 by the 1800-m altitude. A weaker, but deeper diurnal cycle was observed at Mexicali, an inland site some 200 km west-northwest of Puerto Penasco. The strongest winds were observed at the coastal site; the observations suggest that the strongest southerly winds in the northern gulf lie closer to the eastern shore of the gulf.

Characteristics of Wet and Dry Spells over the Pacific Side of Central America during the Rainy Season

This paper describes the mean atmospheric conditions associated with synoptic-scale rainfall fluctuations over Central America during the rainy season. The study is based on composites of wet and dry spells; these composites are generated from six years (1990‐94 and 1997) of daily rainfall observations from select Central American stations, one year (1997) of upper-air wind data from an enhanced sounding network over the region, National Center for Environmental Prediction (NCEP) reanalysis data, and outgoing longwave radiation (OLR) data. Wet spells, defined as days when 75% or more of the stations along the Pacific side of Nicaragua, Costa Rica, and Panama reported rainfall, are associated with weaker trade winds over the Caribbean and stronger cross-equatorial flow northward over the eastern Pacific. During wet spells the intensity of eastern Pacific cross-equatorial flow exceeds by several meters per second the seasonal mean in the lower and middle troposphere, and is strongest and deepest one day before the wettest day. Dry spells, defined as the days when 35% or less of these stations reported rainfall, are associated with stronger trade winds over Central America and weaker and shallower crossequatorial flow. The basic flow patterns seen in the observation-based composites agree well with similar composites produced using reanalysis data, except that the observations show stronger cross-equatorial flow in the lower-mid troposphere over the eastern Pacific. OLR data shows that convective cloudiness anomalies associated with the wet and dry spells extend westward from Central America into the eastern tropical Pacific.

Diurnal Variation of the Lower-Tropospheric Flow over the Arizona Low Desert from SWAMP-1993 Observations

Abstract This paper describes the tropospheric circulation over the lower deserts of Arizona, California, and northwestern Mexico using observations from a special rawinsonde network operated during July and August as part of the 1993 Southwest Area Monsoon Project. The observations show that the flow over the low desert is diffluent, divergent, and upslope during the afternoon and confluent, weakly convergent, and downslope or slope parallel in the early morning hours. This diurnal cycle may help to explain the observed tendency for summer thunderstorms to occur most frequently over the low desert during the nighttime. Despite the relatively short observation interval (18 days), the authors show that this period is similar to multiyear mean conditions during July and August. A heat budget indicates that radiative heating in the lower troposphere over the desert is approximately balanced by the influx of cooler air from over the Gulf of California but uncertainties in the estimates of vertical motion and ...

Polar low structure over the northern Gulf of Alaska based on research aircraft observations

Abstract During the 1987 Alaska Storms Program, a polar low that developed over the northern Gulf of Alaska was investigated during two flights by a WP-3D research aircraft on successive days. Analyse based on data obtained from omega dropwindsondes during the first flight showed the small (∼300 km) horizontal scale of the vortex The vortex center was characterized by suppressed cloudiness on both days and was decidedly warmer at low levels than its surroundings. The vorticity associated with the polar low was largest new the surface and decreased rapidly with height on the first day, with only a very weak circulation evident by 700 mb. Coldest 500-mb temperatures and lowest static stabilities were found directly above the surface low. On the second day the vortex was more intense at midtropospheric levels than on the previous day, and convective clouds extended to higher levels.

Structure and Dynamics of Two Monsoon Depressions. Part I: Observed Structure

Abstract Part I of a two-part paper describes the wind, thickness, and cloud-field evolution of two monsoon depressions observed during the 1979 Summer Monsoon Experiment (SMONEX). One depression was associated with the June Arabian Sea monsoon onset; the other formed over the Bay of Bengal in early July. The wind-field analyses showed the southwesterly tilt with height of both depressions. Using the subjectively produced wind-field analyses, height and thickness fields were derived using the balance equation. The thickness fields clearly showed the evolution of key features of both depressions, such as the development of a cold lower troposphere southwest of the depressions surface center and the upper-tropospheric warm-air pool above the region of maximum rainfall, west of the surface center. East-west cross sections showed that the thermal anomalies associated with both depressions tilted westward with height. Mesoscale analyses using research aircraft data showed that key structures evident from the ...

Research Aircraft Observations of a Polar Low at the East Greenland Ice Edge

Abstract The structure of a subsynoptic-scale cyclone (polar low) that formed along the cast Greenland ice edge during the 1989 Coordinated Eastern Arctic Research Experiment (CEAREX) is described using NOAA WP-3D research aircraft and satellite observations. Satellite imagery showed a well-defined 400-km-wide comma cloud pattern during the time of the aircraft observations. Frontal zones with marked wind shifts and thermal gradients near the surface were associated with the polar low. Although the polar lows vorticity decreased rapidly with height between 950 and 800 mb, a secondary vorticity maximum was found in the upper troposphere associated with a short-wave trough. Doppler radar and aircraft observations showed the structure of the main precipitation band to be similar to that of other polar lows observed by research aircraft. In general, the structure of the polar low resembled, except for horizontal scale, the structure of midlatitude cyclones at a similar stage of cloud field evolution.