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Donna F. Tucker

Abstract

Orographic forcing of diurnal precipitation variations in south-central New Mexico is examined. Harmonic analysis reveals a strong diurnal cycle in precipitation frequency at all stations studied. In addition, relatively high amplitudes in the second, third, and fourth harmonics were present at several stations in the region. Cumulant methods confirm the importance of the higher harmonies and can also divide the stations into precipitation regimes.

At each of the stations one of the maxima in the precipitation frequencies appears to be due to surface convergence caused by a mountain-valley circulation system. Surface wind data support this explanation. All stations have a maximum near midnight local time, which seems to have its source in larger-scale forcing. A possible cause is diurnal variations in the plateau circulation system of the western United States. Upper-air wind data indicate that such variations could result in the formation of a low-level jet that would destabilize the atmosphere near midnight local time.

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Donna F. Tucker

In recent years the western United States has gone from being a region with relatively sparse surface observations to one that has a number of mesoscale networks maintained by a variety of interests. Expanding population and increased agricultural activity have spawned a greater appreciation for meteorological measurements in this region. Yet the meteorological community is not generally aware of these data sources, and some potential research uses for the data are not realized. A survey of these mesonets is presented to illustrate the number of stations and the variations in their characteristics. Even though the networks have been set up by different agencies, there is very little overlap in their locations. Some areas in the region, however, are still lacking an abundance of surface data. Information sources for the networks are provided so that readers can obtain additional information. It can be difficult to work with data from several different sources. Nonetheless, it is recommended that the meteorological implications of these data be further explored. Additional efforts are also needed to establish standards for surface observations so that the data collected by various sources will be more uniform.

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Donna F. Tucker

Abstract

Average July heights and winds over a 10-yr period are computed at 850 mb over Mexico using both surface and radiosonde data. During the day central Mexico at this level is dominated by low pressure, with some smaller areas of high pressure. The pressure systems were less pronounced in 1990 and the smaller high pressure areas were notably weaker during 1993, showing that there is important interannual variability in these patterns. At night, the low pressure weakens and there are generally expanded areas of high pressure.

This area of low pressure is caused by the high plateau, which acts as an elevated heat source for the atmosphere during the day. Thus, the circulation is an extension of a similar one previously observed in the United States and is of about the same magnitude. Although computation of 850-mb heights from surface data helps improve horizontal resolution at this level, additional data would be needed to assess the vertical structure of the plateau circulation system as well as to provide more details on the diurnal variations.

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Donna Tucker, Donna K. Ginther, and Julie A. Winkler

Abstract

No Abstract available.

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Donna F. Tucker and Kristine S. Zentmire

Abstract

Evidence is presented to support the hypothesis that mesoscale convective complexes (MCCs) near the Rocky Mountains are more likely to form when the middle-tropospheric relative humidity is greater than average and the lower-tropospheric relative humidity is less than average. Radiosonde data for MCC events are chosen at the nearest place to first storm development and at the nearest time before first storms occurred. A sounding representing an average seasonally adjusted climatological location of orogenic MCC first storms was used to represent non-MCC days. The 500-hPa relative humidities were significantly higher for MCC events than for non-MCC days. The 700-hPa relative humidity was significantly lower for MCC events than for non-MCC days. MCC days also have somewhat less stability than non-MCC days but this factor appears to be related to higher temperatures at 500 hPa on days when the 500-hPa relative humidity is low. The values of various quantities used to assess the utility of this information for weather forecasting indicate that this method needs to be combined with other MCC forecasting methods to be useful.

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N. Andrew Crook and Donna F. Tucker

Abstract

The flow past heated topography is examined with both linear and nonlinear models. It is first shown that the forcing of an obstacle with horizontally homogenous surface heating can be approximated by the forcing of an obstacle with surface heating isolated over the obstacle. The small-amplitude flow past an obstacle with isolated heating is then examined with a linear model. Under the linear approximation, the flow response to heated topography is simply the addition of the separate responses to thermal and orographic forcing. These separate responses are first considered individually and then the combined response is examined. Nondimensional parameters are developed that measure the relative importance of thermal and orographic forcing. Nonaxisymmetric forcing is then considered by examining the flow along and across a heated elliptically shaped obstacle. It is shown that the low-level lifting is maximized when the flow is along the major axis of the obstacle.

The linear solutions are then tested in a nonlinear anelastic model. The response to a heat source and orography are first examined separately. Good agreement is found between nonlinear and linear models for the individual responses to thermal and orographic forcing. The case of uniformly heated flow past an obstacle is then examined. In these simulations, the thermal response is isolated by subtracting the orographic-only response from the full thermal–orographic response. The numerical simulations are able to capture the main features of the thermal response. Finally, numerical simulations of the flow along and across an elliptically shaped heated obstacle are examined, where it is verified that the lifting is maximized when the flow is along the major axis of the obstacle.

These results are extended in Part II of this study to examine the moist convective response to flow over both idealized terrain and the complex terrain of the Rocky Mountains of the United States.

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Donna F. Tucker and N. Andrew Crook

Abstract

A mesoscale convective system (MCS) that formed just to the east of Denver is investigated with a nonhydrostatic numerical model to determine which processes were important in its initiation. The MCS developed from outflow from previous convective activity in the Rocky Mountains to the west. Model results indicate that this outflow was necessary for the development of the MCS even though a convergence line was already present in the area where the MCS developed. A simulation with a 3-km grid spacing more fully resolves the convective activity in the mountains but the development of the MCS can be simulated with a 6.67-km grid. Cloud effects on solar radiation and ice sedimentation both influence the strength of the outflow from the mountain convection but only the ice sedimentation makes a significant impact on the development of the MCS after its initiation.

The frequent convective activity in the Rocky Mountains during the warm season provides outflow that would make MCS generation favorable in this region. Thus, there is a close connection between mountain convective activity and MCS generation. The implications of such a connection are discussed and possible directions of future research are indicated.

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Donna F. Tucker and N. Andrew Crook

Abstract

Previous studies have shown that thunderstorms in the Rocky Mountain region have preferred areas in which to form. There has been some indication that these areas depend on the midtropospheric wind direction. A nonhydrostatic model with a terrain-following horizontal grid is employed to investigate the initiation of precipitating convection over heated topography. Horizontally homogeneous meteorological conditions with no directional shear in the vertical wind profile are used.

The numerical simulations indicate that precipitating convection was more likely to be generated downwind of ridges than upwind of them. Initiation of these storms was more likely downwind of ridges with their long axis parallel to the wind direction than downwind of ridges with their long axis perpendicular to the wind direction. In Part I of this study it was shown that heating-induced convergence is larger downwind of a ridge with its longer axis parallel to the wind direction. For the orographic configuration of the Rocky Mountains, total precipitation is maximized for southerly and northwesterly winds. Slower wind speeds are more likely and faster wind speeds are less likely to produce convective storms. Soundings with larger instability are more likely to produce convection. The soundings with a greater temperature lapse rate produce more initiation locations, and soundings with greater moisture produce greater amounts of precipitation.

Even though a number of assumptions were made for this study, the authors believe the results explain a significant amount of the observed variability in the initiation locations of precipitating convection in the Rocky Mountains during the summer. Because of the theoretical basis for this work, detailed in Part I of this study, the authors believe it should explain convective initiation in other mountainous areas that are subject to strong solar heating.

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Julie A. Winkler, Donna Tucker, and Anne K. Smith

Zevin and Seitter's analyses of the 1993 American Meteorological Society membership survey indicated that university/college employees had the largest difference in salary by gender when controlling for experience and age. Further analyses of the membership survey presented here indicate that a large salary discrepancy exists for female full professors in atmospheric science. In addition, the small number of women at the associate professor rank suggests a “leaky pipeline” for female atmospheric science faculty. A comparison of tenure-stream faculty to Ph.D.-level atmospheric scientists outside of academia suggests that female Ph.D.'s have fared better in nonuniversity positions in terms of senior-level salaries and advancement from entry- to midlevel positions. Possible explanations for the salary differential at the full professor level and for the small number of female associate professors in atmospheric science are explored, although no conclusive explanation can be given at this time. Possible actions to remediate the salary differential and poor advancement of faculty are proposed. These remediative actions are directed to heads and chairs of atmospheric science departments who are often in a position to initiate change within their departments and universities.

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