Search Results

You are looking at 1 - 10 of 17 items for

  • Author or Editor: R. A. Maddox x
  • Refine by Access: All Content x
Clear All Modify Search
J. M. Fritsch
and
R. A. Maddox

Abstract

High-resolution visible imagery for the eastern GOES satellite is used to document a convectively driven mesoscale weather system which propagates against the mean atmospheric flow and produces an apparent spiraling anvil outflow.

Full access
J. M. Fritsch
and
R. A. Maddox

Abstract

Examination of NMC upper tropospheric analyses (300, 200 and 150 mb charts) indicates that significant perturbations are present in the wind fields in the vicinity of intense meso-α scale (250–2500 km) thunder-storm complexes (identified utilizing enhanced IR satellite imagery). This effect is investigated for each of 10 mesoscale convective complexes. Since the LFM convective adjustment procedure cannot infuse large amounts of mass, momentum and moisture into the upper troposphere and lower stratosphere, the 12 h LFM predicted winds are used as an indication of the unperturbed environmental flow. An estimate of the convective perturbation is obtained by subtracting the LFM predicted 200 mb winds from the observed winds. A large anticyclonic flow perturbation is present in each of the 10 events. Wind speed perturbations at individual sounding locations are commonly 10–20 m s−1 with maximum values as great as 38 m s−1. Detailed case analyses for two events are presented to illustrate these effects.

The predicted and observed fields are objectively analyzed over a common grid to develop a composite field for the 10 cases. The composite difference field is not only similar to that of the individual cases but it is also found that significant perturbations occur only in the vicinity of the convective complexes. Macroscale and mesoscale characteristics of these composite flow fields are also examined utilizing an objective technique for scale separation. Other characteristics of the perturbed fields are presented and implications of the convectively forced perturbations are discussed.

Full access
J. M. Fritsch
and
R. A. Maddox

Abstract

A fine-mesh, 20-level, primitive equation model is used to study the generation of convectively driven weather systems in the vicinity of the tropopause. In a test simulation, a high-level (∼200 mb) mesoscale high pressure system forms in conjunction with the development of a convective complex. In response to this high-level mesohigh, winds aloft rapidly decelerate as they approach the convective complex. On the other hand, downstream of the convective system the mesoscale pressure gradient accelerates the wind to generate a jet maximum which is stronger than any wind speed prior to the development of the convection.

The formation of the high-level mesohigh appears to be linked to the convectively forced production of a layer of cold air above the tropopause. The cold layer of air is generated by cloud-scale cooling from overshooting tops and from adiabatic cooling by strong (∼0.5 m s−1) mesoscale lifting in response to the convective cloud warming below the tropopause.

The model-generated high-level convective system is compared to observed systems and briefly discussed in light of the interaction of these systems with their larger scale environment.

Full access
R. A. Maddox
,
C. F. Chappell
, and
L. R. Hoxit

Meteorological conditions associated with more than 150 intense convective precipitation events have been examined. These heavy rainfalls caused flash floods and affected most geographic regions of the conterminous United States. Heavy rains associated with weather systems of tropical origin were not considered. Analyses of surface and standard level upper-air data were undertaken to identify and define important synoptic and mesoscale mechanisms that act to intensify and focus precipitation events over specific regions. These analyses indicated that three basic meteorological patterns were associated with flash flooding in the central and eastern United States. Heavy convective precipitation episodes that occurred in the West were considered as a separate category event. Climatological characteristics, composite analyses, and upper-air data are presented for these four classifications of events.

The large variability of associated meteorological patterns and parameters (especially winds aloft) makes identification of necessary conditions for flash flood-producing rainfall quite difficult; however, a number of features were common to many of the events. An advancing middle-level, short-wave trough often helped to trigger and focus thunderstorm activity. The storm areas were often located very near the mid-tropospheric, large-scale ridge position and occurred within normally benign surface pressure patterns. Many of the intense rainfalls occurred during nighttime hours. These elusive characteristics further complicate a difficult forecast problem.

Full access
R. L. Thompson
,
J. M. Lewis
, and
R. A. Maddox

Abstract

The return of tropical air from the Gulf of Mexico is examined in the autumnal cool season. Results from the thermodynamic equilibrium model of Betts and Ridgway are used to calculate the equilibrium equivalent potential temperature (θ e ) over the gulf and the northwestern Caribbean Sea. With a climatological study as a backdrop, a case of severe weather outbreak in mid-November 1988 is analyzed with emphasis on the analysis of low-level θ e that flowed into the storm region from the Gulf of Mexico.

The primary results of the study are the following:

  1. The climatological distribution of equilibrium θ e over the gulf and the Caribbean in November serves as a useful tool for the analysis of the 1988 case study.

  2. Between 5 and 15 November 1988, equilibrium in the marine layer was established over the gulf due to the absence of any deep cold-air penetrations during this period.

  3. The high-valued θ e that streamed into the severe storm region on 15 November 1988 tracked from the Yucatán straits and the northwestern Caribbean over a three-day period.

  4. This air was able to maintain its high-θ e property because of an anomalously warm gulf.

  5. Significant increases in available energy for deep convection could have been anticipated by means of the upper bounds on coastal θ e predicted by the Betts and Ridgway formulation, which was supported by observations along the Texas coast.

Full access
K. E. Nielsen
,
R. A. Maddox
, and
S. V. Vasiloff

Abstract

The character of cloud-to-ground lightning is examined during the life cycle of a distinct mesoscale segment of the 10–11 June 1985 mesoscale convective system (MCS). Three phases of lightning activity are identified and related to both the radar-observed structure of the convection and to the severe weather produced by the MCS. Positive strikes to ground are dominant when the MCS is first developing. Negative strikes then dominate during a period of intense leading-line convective activity with high storm tops. Finally, a period of relatively frequent positive strikes within the trailing stratiform region occurs during the demise of the MCS. This last phase begins after the vertical extent of the leading convective line decreases rapidly and markedly, with moderate intensity echoes (i.e., 30–40 dBZ) occurring mostly below the freezing level. The first period of frequent positive flashes results from the lightning associated with a single severe thunderstorm in southwest Kansas; however, a second severe storm occurs nearby and produces mainly negative strikes. An extended period of strong surface winds does not appear to have any direct relationship with the observed character of the lightning activity.

Full access
R. A. Maddox
,
D. M. Rodgers
, and
K. W. Howard

Abstract

Satellite images are used to document the life cycles of Mesoscale Convective Complexes (MCCs) which occurred over the United States during the warm season months of 1981. These systems were found to exhibit characteristics similar to aspects of MCCs discussed recently in the literature; however, the behavior of several of the convective systems poses questions that can only be answered through detailed studies. The systems did produce a variety of significant weather events ranging from severe thunderstorms to locally heavy rains and flooding. Information is also provided for a number of other significant mesoscale convective systems that, although they did not meet the stringent MCC definition criteria, caught the investigators' attention. This documentation should provide a useful starting point for scientists who might wish to pursue studies of mesoscale convective weather systems.

Full access
R. A. Maddox
,
D. J. Perkey
, and
J. M. Fritsch

Abstract

An intense mesoscale convective complex developed over the central Mississippi Valley during the night and early morning hours of 24 and 25 April 1975. Analyses of upper tropospheric features during this period indicate strong changes in temperature, wind and pressure-surface heights occurred over the convective system in a period of only 6 h. It is hypothesized that the convective system is responsible for these changes. The question of whether the diagnosed changes reflect a natural evolution of large-scale meteorological fields or are a result of widespread deep convection is considered utilizing two separate numerical forecasts produced by the Drexel-NCAR mesoscale primitive equation model. A “dry” forecast, in which no convective clouds are permitted, is considered representative of the evolution of the large-scale environment. This forecast is contrasted with a “moist” forecast which, through the use of a one-dimensional, sequential plume cumulus model, includes the effects of deep convection. Differences between the forecasts are substantial and the perturbations produced by the convection are quite similar to diagnosed features. The numerical results support the contention that mososcale, convectively driven circulations associated with large thunderstorm complexes can significantly alter upper tropospheric environmental conditions.

Full access
J. Li
,
X. Gao
,
R. A. Maddox
, and
S. Sorooshian

Abstract

In this article, four continually processed sea surface temperature (SST) datasets, including the Reynolds SST (RYD), the global final analysis of skin temperature at oceans (FNL), and two Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua SSTs retrieved from thermal infrared imagery (TIR) and midinfrared imagery (MIR), were compared. The results show variations from each other. In comparison with the RYD SST, the FNL data have −0.5° ∼ 0.5°C perturbations, while the TIR and MIR SSTs possess larger deviations of −2° ∼ 1°C, mainly due to algorithm and/or sensor differences in these SST datasets.

A regional model, the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (Penn State–NCAR) Mesoscale Model (MM5), was used to investigate whether model atmospheric predictions, especially those concerning precipitation during the North American monsoon season, are sensitive to these SST variations. A comparison of rainfall, atmospheric height, temperature, and wind fields produced by model results, reanalysis data, and observations indicates that, at monthly scale, the model shows changes in the simulations for three consecutive years; in particular, rainfall amounts, timing, and even patterns vary at some specific regions. Forced by the MODIS Aqua midinfrared SST (MIR), which includes large regions with SST values lower than the conventional Reynolds SST, the MM5 rain field predictions show reduced errors over land and oceans compared to when the model is forced by other SST data. Specifically, rainfall estimates are improved over the offshore of southern Mexico, the Gulf of Mexico, the coastal regions of southern and eastern Mexico, and the southwestern U.S. monsoon active region, but only slightly improved over the monsoon core and the high-elevated Great Plains. Using MIR SST data, one is also capable of improving geopotential height and temperature fields in comparison with the reanalysis data.

Full access
J. Li
,
X. Gao
,
R. A. Maddox
, and
S. Sorooshian

Abstract

Rainfall evolution and diurnal variation are important components in the North American monsoon system (NAMS). In this study these components are numerically studied using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) with high resolution (12-km grids) in contrast to most previous model studies that used relatively coarse spatial resolutions (>25 km grids). The model was initialized at the start of each month and allowed to run for 31 days.

The study shows that, in general, the model results broadly matched the patterns of satellite-retrieved rainfall data for monthly rainfall accumulation. The rainfall timing evolution in the monsoon core region predicted by the model generally matched the gauge observations. However, the differences among the three precipitation estimates (model, satellite, and gauge) are obvious, especially in July. The rainfall diurnal cycle pattern was reproduced in the monsoon core region of western Mexico, but there were differences in the diurnal intensity and timing between modeled and observed results. Furthermore, the model cannot capture the diurnal variation over Arizona.

Modeling results showed heavy monsoon rains shift northward along the western Mexico coast in association with the northward evolution of the subtropical highs. This is consistent with previous data analyses. The rainfall diurnal cycle was associated mainly with sea–land/mountain–valley circulations over western Mexico and adjacent oceans.

The simulations show that the model has deficiencies in predicting precipitation over the Gulf of Mexico. The model cannot reproduce the low-level inversion above the marine boundary layers and thus does not generate enough convective inhibition (CIN) to suppress the convection. The model also cannot produce realistic variations of day-to-day atmospheric conditions with only a single initialization at the start of the month.

Full access