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Michael W. Douglas

Abstract

This paper discusses selected aspects of the vorticity and thermal fields of the two depressions described in Part I. The growth rate of the onset vortex is shown to be about three times that of the July depression. The most significant difference between the evolution of the two depressions lay in the growth rate at 500 mb, with the onset vortex showing a large vorticity increase and the July monsoon depression only a small increase. Vorticity budgets of the rain areas west of each depression's center showed that the observed westward motion of the depressions resulted from dominance of the divergence term in the lower troposphere. Heat-budget calculations for the rain areas showed an approximate balance between warm advection and adiabatic cooling at 850 mb, though diabatic heating was large above this level. For both depressions the region of maximum rainfall was coincident with the location of the maximum warm advection in the lower middle troposphere. In the suppressed northeast quadrant of both depressions subsidence warming was balanced by cold advection. Implications of this study's findings for eye formation in tropical storms and for improved satellite-imagery interpretation are discussed.

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Michael W. Douglas

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 depression's 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 synoptic-scale analyses were also reflected in the mesoscale structure of the depressions. Notable was a feature resembling a weak tropical cyclone eye during the mature stage of both depressions.

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Michael W. Douglas

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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 observations show the attitude of the maximum winds to be nearly constant over the Gulf of California (approximately 300 m above the surface), and the jet is present on days with widely varying synoptic flow regimes.

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Michael W. Douglas

This article discusses the feasibility of using dropwindsonde-equipped aircraft to obtain meteorological observations over oceanic data-void areas for operational forecasting applications. Such in-situ meteorological observations would provide measurements of wind, temperature, and moisture at a higher vertical resolution and greater accuracy than currently available from satellite-derived observations. Such airborne observations, if globally obtained, would greatly enhance global datasets for both numerical weather prediction and for research studies of large-scale phenomena, such as the El Niño/Southern Oscillation. The aircraft could also serve as platforms for collecting frequent oceanic thermal and current information. The advantages and limitations of a dropwindsonde-based observing system are compared with current and proposed ground- and satellite-based systems.

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Michael W. Douglas and Shuhua Li

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 horizontal temperature advection make the budget uncertain in the middle troposphere.

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MalaquÍas Peña and Michael W. Douglas

Abstract

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 cross-equatorial 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.

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Michael W. Douglas and David J. Stensrud

The current modernization and restructuring of the National Weather Service (NWS) to date has not involved the rehabilitation of the upper-air observing network over North America. The authors discuss the need for a thorough evaluation of the current upper-air observing system that examines both the old (current) rawinsonde network and newer remote sensing networks. The key concern is that the meteorological community has given insufficient attention to the scientific questions underlying the choices of observing systems as related to the needs of operational forecasting. The authors argue that the principal basis for a scientific and objective evaluation of observing strategies is the ability of a particular observing system (or component of a system) to improve numerical weather prediction in the modernized NWS. Given this starting point, a number of possible new (and old) observing systems and observing strategies are discussed that offer the possibility for much improved initialization of mesoscale numerical forecast models. Perhaps the key shift in observation strategy suggested is the establishment of many additional radiosonde and pilot balloon sounding stations that rely on contract observers rather than the NWS workforce. However, a comprehensive and satisfactory upgrading of the upper-air observing system will be possible only after more substantial participation of the scientific community in the planning and implementation processes.

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Michael W. Douglas and Javier Murillo

A research effort primarily involving pilot balloon observations was carried out during the summer of 1997 to study rainfall variability over Central America. This activity, supported by NOAA's Pan American Climate Studies (PACS) program, grew in scope in response to the strong El Nino event of 1997/98 and subsequently evolved into a network ranging from Mexico to Paraguay. The overall goal of the PACS-Sounding Network (PACS-SONET) was to obtain relatively inexpensive wind profiles for describing climate variability over parts of the intertropical Americas that were not well covered by routine radiosonde observations. Major portions of the project supported climate research programs focusing on both the South and North American monsoon systems, while other parts of the network provided multiyear observations across important gaps in the Central American cordillera and also helped to describe cross-equatorial flow variations in the eastern Pacific. Approximately 50,000 observations were made by the PACS-SONET over its 10-yr operation.

This paper describes the motivation for and evolution of the network, the logistical complications that were involved in establishing and operating a long-term multinational network, and some of the important results from analysis of the data. We conclude by discussing some of our perspectives on why the network was unable to make a transition from research funding to one supported by meteorological services of the region.

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Michael W. Douglas and Juan Carlos Leal

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This paper describes aspects of summertime northward surges of low-level moisture over the Gulf of California, based on 9 yr (1980–88) of radiosonde observations and also from NCEP reanalyses. These events are usually referred to as “gulf surges” by forecasters in the southwestern United States. A composite structure of 38 well-marked surge passages at Empalme, Mexico, during this period is presented. Radiosonde observations were composited to obtain the synoptic-scale structure of the surge from 2 days before to 2 days after the surge passage at Empalme. The composites reveal that the surges, strongest in the lower troposphere, are associated with low-latitude cyclonic perturbations that pass south of the Gulf of California. The composite cyclonic perturbation associated with the surges can be traced back to the western Gulf of Mexico 2 days prior to surge passage at Empalme. Composites based on the NCEP reanalyses for the same dates also show a similar evolution, though with somewhat weaker amplitude. Rainfall data from Mexican stations along the eastern side of the Gulf of California show that the surges modulate the climatological daily rainfall totals by ∼15%–30%. The evolution of surges is shown to be related to the propagation of tropical storms over the eastern Pacific Ocean.

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John F. Mejia, Michael W. Douglas, and Peter J. Lamb

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This paper describes aspects of a strong moisture surge over the Gulf of California that was observed during the 2004 North American Monsoon Experiment. Although a variety of special observation platforms aid the analyses, the authors focus on observations collected during two NOAA research aircraft flights made on 12 and 13 July. These flights sampled the initial and mature phases of a strong surge associated with Tropical Storm Blas. The first flight is identified by both a convective outflow and another feature, both deeper and with larger spatial scale, ahead of the outflow in association with the surge’s leading edge. The surge speed, ~18 m s−1, was identified from anomaly analysis of surface station pressure data. Observations show interesting multiscale features associated with the surge during its initial stages but do not allow for unambiguous identification of the surge’s forcing mechanism or dynamical properties. Data from the second flight were used to describe the along- and cross-gulf structure of the enhanced low-level flow associated with the surge event. The strongest winds were over the northernmost gulf, with weaker winds over the surrounding coastal areas. The kinematic moisture flux increased toward the northern gulf; wind speed is the main control on the flux as the moist layer shows only small horizontal gradients. Over the northern gulf, the combination of a very shallow moist layer and a shallow low-level jet yield maximum moisture fluxes near 950 hPa that are almost an order of magnitude larger than those at 850 hPa.

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