Abstract

The time interval between initiation of surface convergence and the subsequent response of visible cloud growth to this convergence was examined for nine cases of convection that occurred over the FACE 1973 and 1975 mesonetworks in south Florida. Clouds ranged in size from small echoes with a few towers to merged lines or large clusters of towers, but they met a series of observational criteria that specified them as belonging to a similar set of clouds, and were not representative of the entire range of clouds in the area. Visible clouds first formed 10 to 55 min after the associated surface convergence began, and grew rapidly upward 20 to 100 min after convergence started.

This highly variable response could be understood better by taking into account the duration of the cloud, which is defined as the time from first surface convergence to complete dissipation. The same nine cases were examined as were chosen initially for the visible cloud study. When duration was considered, first visible cloud response occurred at an average of 15% through the cloud duration, and rapid upward cloud growth at 36%. Other parameters derived from divergence, radar- and gage-measured rainfall also tended to cluster within specific portions of the total duration of the cloud. The data for each event for the nine clouds are presented and described in terms of the cloud duration.

Abstract

Total area divergence is related to area rainfall using data collected during the Florida Area Cumulus Experiment (FACE) 1975 field experiment over a network that covered 1440 km². A convergence event is defined as a monotonic decrease in total area divergence of more than 25×10⁻⁶ s⁻¹ for more than ten minutes. This change in total area divergence is related to the total amount of area rainfall considered to be associated with the convergence event. For 121 convergence events during July and August 1975, a correlation coefficient of −0.59 is found between change in convergence and rainfall amount. When the ensemble is subdivided, it is found that for slow moving convective systems, or when low-level winds are weak, there is twice the amount of rainfall per convergence event. When middle-level (850–500 mb) relative humidity is in the range 50–65%, the correlation coefficient between convergence and rainfall is −0.81. Data are also partitioned according to stability and buoyancy. Convective outflow and its reflection in total area divergence are examined, and relationships are developed for determining the amount of precipitation for each divergence event. For the 75 rain events during FACE 1975, a correlation coefficient of 0.75 is found between the change in divergence and the rainfall amount.

The 1996 Summer Olympics will be held in the Atlanta, Georgia, vicinity and several other sites in the southeast United States between 19 July and 4 August 1996. This period coincides with the peak thunderstorm season, so the threat of lightning casualties cannot be taken lightly since Georgia and surrounding states with Olympic venues rank quite high in the United States in annual lightning casualties and the casualty rate per population. Flash density and thunderstorm day estimates for July and August show that the greatest cloud-to-ground (CG) lightning activity (> 5 flashes per square kilometer) occurs near the seacoasts and over the Florida peninsula. Cloud-to-ground flash activity decreases as distances increase from the coastlines, reaching a minimum (< 1 flash per square kilometer) along the North Carolina–Tennessee border. However, there are also important maxima (> 3 flashes per square kilometer) in and around Atlanta. The frequency of CG lightning reaches a maximum in the late afternoon and early evening hours in the Atlanta area. Lightning reaches a maximum in the midafternoon at Wassau Sound near Savannah Beach. As expected, the lowest chances of lightning are from midnight to noon, while probabilities are slightly greater during these hours along the coasts. Flash densities for July and August may vary as much as 2–3 flashes per square kilometer from year to year in most locations. When lightning begins, the chances are greater than 1 in 3 that lightning will still be occurring 1 h later at most venue sites; the chances are 1 in 5 that lightning will be present after 2 h.

Abstract

Network-detected cloud-to-ground lightning coincident with mainly frozen precipitation (freezing rain, sleet, snow) was studied over the central United States during two outbreaks of arctic air in January 1994. During the first event, the ratio of positive to total flashes was 59%, flashes were few and disorganized in area, and no surface observer reported thunder. For the other event the ratio was 52% during the first few hours in subfreezing surface air, then decreased when flashes formed in the nearby region above freezing. Also, flashes in this case were linearly aligned and coincided with conditional symmetric instability; thunder was heard infrequently by surface observers. On radar, reflectivity cores grew from weak to moderate intensity within a few hours of the lightning during both cases. Echo area increased greatly before flashes in one case, while the area increase coincided with flashes in the other. Some base-scan reflectivities were strong in both thunderstorm regions due to the radar beam intersecting the melting level. Regions with lightning often could be identified better by high echo tops than reflectivity. Analyses on the scale of one or two states diagnosed the strength of low-level warming that contributed to formation of thunderstorms and significant frozen precipitation. Quasigeostrophic analyses showed that 850-mb temperature advection and 850–500-mb differential vorticity advection were similar in magnitude in the lightning area during both events. Once convection formed, lightning and echo-top information identified downstream regions with a potential for subsequent frozen precipitation.

Abstract

The relationship of vertical motion to the occurrence of precipitation from the convective and stratiform regions of a mesoscale convective system (MCS) is presented. On 20–21 May 1979, an MCS developed over portions of Oklahoma, Texas, and Arkansas. The uniqueness of this system was its lack of squall-line characteristics and development of a large stratiform precipitation region. The evolution of the system is detailed by rawinsonde observations, radar cross sections, 15-min composite analyses of six NWS WSR-57 radars, and by raingages. The genesis stage of the MCS was described by strong convection along an east-west cold front that was reinforced by outflow generated by two mesoscale convective complexes (MCCS) that formed tile night before in Kansas and Missouri. The mature stage of the MCS was characterized by the development of a large stratiform precipitation region while convection was limited to the southern and eastern flanks of the system. Finally, in the dissipative stage, a moderate north-south squall line that developed over west Texas in the afternoon moved rapidly to the cast apparently associated with a short-wave aloft and appeared to sweep the entire system out of Oklahoma.

A modified Cheng and Houze technique is applied to the radar composites to determine stratiform and convective regions utilizing temporal as well as areas considerations. For the system as a whole, the stratiform region generated 30–50% of the total precipitation. The vertical-motion profiles hold the key to the precipitation characteristics over the storm-scale network. The genesis period was characterized by a strongly convective profile. As the system matured, low-level upward motion cased, while middle-level upward motion was sustained. A large area of stratiform rain developed as the deep convection weakened. Water-budget considerations suggest that the stratiform region was maintained by a combination of mesoscale middle-level updraft and by horizontal transfer of convective debris.

Abstract

Cloud-to-ground (CG) lightning shows great variability across Arizona from one year to the next as well as from one day to the next. Availability of moisture, location of the subtropical ridge axis, transitory troughs in both the westerlies and easterlies, and low-level moisture surges from the Gulf of California can affect thunderstorm occurrence, which, in turn, will affect lightning production. Diurnal CG lightning patterns in Arizona are also determined by daily heating cycles and topography. Six years of Bureau of Land Management CG flash data are used in this investigation.

In Arizona, lightning usually starts first, on a daily basis, in the plateau region and extends in an arc from the White Mountains of eastern Arizona westward across the Mogollon Rim and then northward onto the Kaibab Plateau of northern Arizona. Flash activity moves in a more or less continuous fashion off the plateau, south and westward down the topography gradient, and enters the lower desert by early evening. At the same time, flash activity develops in the highlands of southeast Arizona and moves west-northwestward, reaching the lower desert by late afternoon. Cloud-to-ground activity across Arizona is at a minimum at 1000 MST and rapidly reaches a peak at 1600 MST in central and southeast Arizona. Maximum CG activity in the Phoenix vicinity is at 2200 MST and appears to be the result of the intersection of both plateau-generated and southeast highlands-generated convection. Precipitation and lightning are well correlated, except that precipitation seems to linger longer than lightning, probably due to the occasional development of mesoscale convective systems, which product light stratiform precipitation during their dissipation stage.

Abstract

Convective bursts and breaks in the southwest U.S. monsoon are investigated in a lightning context because cloud-to-ground (CG) lightning is an excellent indicator of deep convection. Bursts and breaks are identified using six years of Bureau of Land Management CG lightning information. Composited upper-air analyses for 12 bursts and 10 breaks are developed to examine the synoptic-scale differences between these two regimes. Anomaly patterns are investigated, and average burst and break regimes am presented.

This investigation shows the importance of moisture, the location of the subtropical ridge axis, and the high-plateau thermal low. For the burst, die ridge axis is displaced northward across Arizona and New Mexico and moisture is usually abundant in the southwestern United States. During the break, the ridge retreats southward into northern Mexico, giving way to dry westerly winds across Arizona. The high-plateau thermal low is firmly in place during July and August, and it pulls low-level moist air upslope into the Great Basin from the Gulf of California through the only opening available, which is the lower desert of Arizona.

Abstract

Anthropogenic aerosol interacts strongly with incoming solar radiation, perturbing Earth’s energy budget and precipitation on both local and global scales. Understanding these changes in precipitation has proven particularly difficult for the case of absorbing aerosol, which absorbs a significant amount of incoming solar radiation and hence acts as a source of localized diabatic heating to the atmosphere. In this work, we use an ensemble of atmosphere-only climate model simulations forced by identical absorbing aerosol perturbations in different geographical locations across the globe to develop a basic physical understanding of how this localized heating impacts the atmosphere and how these changes impact on precipitation both globally and locally. In agreement with previous studies we find that absorbing aerosol causes a decrease in global-mean precipitation, but we also show that even for identical aerosol optical depth perturbations, the global-mean precipitation change varies by over an order of magnitude depending on the location of the aerosol burden. Our experiments also demonstrate that the local precipitation response to absorbing aerosol is opposite in sign between the tropics and the extratropics, as found by previous work. We then show that this contrasting response can be understood in terms of different mechanisms by which the large-scale circulation responds to heating in the extratropics and in the tropics. We provide a simple theory to explain variations in the local precipitation response to absorbing aerosol in the tropics. Our work highlights that the spatial pattern of absorbing aerosol and its interactions with circulation are a key determinant of its overall climate impact and must be taken into account when developing our understanding of aerosol–climate interactions.

Abstract

Six years (1989–94) of cloud-to-ground lightning data are used to examine the distribution of lightning across the Florida panhandle and adjacent coastal waters and its relationship to the prevailing low-level flow. Only warm season data between 1 May and 31 October are used. The prevailing flow is determined by subdividing the low-level (1000–700 mb) vector mean wind into categories that are either parallel or perpendicular to various parts of the coastline. Moderate wind speeds (2–5 m s⁻¹) generally are found to be more conducive to producing lightning than stronger speeds. Wind speeds stronger than 5 m s⁻¹ likely inhibit the formation of the sea breeze, the main focus for summertime thunderstorms in the region.

Onshore, offshore, and parallel flows are found to play important roles in determining the patterns of flash locations in each flow regime. The complexity of the coastline also is found to have a major impact on the flash distributions. The prevailing wind direction is shown to be related to the time of peak afternoon lightning occurrence as well as the frequency of nighttime storms.

Abstract

Software build 9.0 for the Weather Surveillance Radar-1988 Doppler (WSR-88D) contains several new or improved algorithms for detecting severe thunderstorms. The WSR-88D Operational Support Facility supports testing and optimization of these algorithms by local National Weather Service offices. This paper presents a new methodology for using Storm Data in these local evaluations. The methodology defines specific conditions a storm cell must meet to be included in the evaluation. These conditions include cell intensity and duration, population density along the cell track, and any previous severe reports in the county where the storm is located. These requirements avoid including storm cells that may have produced severe weather where reports would be very unlikely. The technique provides a more accurate picture of algorithm performance than if Storm Data is used with no special considerations.

This study utilizes the new methodology with data currently available for the Tallahassee, Florida, county warning area (TLH CWA). It describes the performance of two algorithms used for detecting severe hail. The first is the Probability of Severe Hail (POSH), a component of the build 9.0 Hail Detection Algorithm. The second is the algorithm that calculates vertically integrated liquid (VIL).

Early results show that the recommended POSH threshold of 50% appears appropriate for the TLH CWA. This suggests that the height of the freezing level provides a reasonably good estimate of the best severe hail index (SHI). However, early results also indicate that the average wet-bulb temperature from 1000 to 700 mb (low-level wet-bulb temperature) might produce an even better indication of the SHI threshold. Similarly, the threshold for VIL is highly correlated to the low-level wet-bulb temperature. Finally, the VIL algorithm is found to perform as well as the POSH parameter if the best VIL threshold can be determined in advance. Since the database used in these evaluations was relatively small, these findings should be considered tentative.