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James Wilson

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James W. Wilson

Abstract

Five tornadoes occurred within a 40 min period on 18 May 1984 in eastern Colorado. The evolution of these tornadoes was documented by a single Doppler radar, research aircraft, mesonetwork and chase team. Three of these tornadoes were narrow (≈300 m), rotating dust columns extending from the surface to cloud base more than 5 km from the nearest precipitation. The Doppler-observed parent circulations were <2 km deep and <1 km in diameter. Tornadoes of this type do relatively minor damage and are frequently called gust front tornadoes or gustnadoes. It is believed this is the first Doppler radar documentation of this tornado type. In an operational environment, even at close radar range, it would be difficult to detect the parent circulation associated with these tornadoes. However, by closely monitoring wind shift boundaries and associated localized strong shear regions, preferred tornado areas can be identified.

The other two tornadoes were associated with condensation funnels and occurred near precipitation. The Doppler parent circulations were deeper and wider than the first three tornadoes but were relatively small compared to many of those reported in the literature. All five of these tornadoes occurred along two wind shift lines near the point where the lines intersected. These lines were of synoptic scale origin—a cold front and a trough line.

The low-level echo structure and wind field associated with the parent storm of the two larger tornadoes closely resemble those described in the literature for supercell storms. While the environmental vertical wind shear was less than normally expected for supercell storms, it is believed that the preexisting boundaries created the necessary vorticity and vorticity production mechanisms for tornadogenesis.

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James W. Wilson

Abstract

Radar and raingage data collected during the International Field Year for the Great Lakes were used to determine the effect of Lake Ontario on precipitation patterns. Objective analysis techniques were used to combine the radar and gage data.

During the warm season the relatively cold lake frequently suppressed afternoon shower activity, particularly when the showers were not associated with large-scale well-organized weather systems. When the showers were scattered, the land portion of the watershed received 402% more rain than the lake compared to 14% more for widespread rain. During the cold season, the lake frequently stimulated precipitation when the 850 mb temperature was more than 7°C colder than the lake.

While the lake influenced the precipitation patterns for about half the days, the total effect on precipitation amounts was small. The lake-effect days were generally those with small-area average amounts. The total warm season rainfall for land areas within 30 km of the lake was 10% more than the lake. For the cold season, the land received 2% less than the lake. There was an orographic component to the precipitation over the far eastern end of the lake and land. Removal of the orographic component tends to reduce the warm season land-to-lake difference while increasing the cold season difference.

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James W. Wilson

Abstract

Quantitative data collected with the WSR-57 radar at Atlantic City from five rainstorms and two snow-storms are compared with precipitation data from 60 recording rain gages within 100 mi of the radar. Hourly rainfall amounts of from 0.01–0.02 inches are detected by the radar in at least 95 per cent of the cases at all radar ranges out to 70 mi. Hourly amounts of from 0.04–0.05 inches are detected in at least 95 per cent of the cases at all ranges out to 100 mi.

The relationship between radar echo intensity and rainfall rate varies from storm to storm. Although the radar appears to have excellent potential for determination of area-average rainfall, reflectivity measurements provide only coarse estimates of point rainfall intensity. The radar estimates of hourly rainfall averages, over a 750 sq mi area within 60 mi of the radar, are within the confidence limits of the average of 10 gage measurements, when a best-fitting radar-rainfall relationship is used for each storm. Use of one grand average relationship for all storms provides estimates of the average areal rainfall whose accuracy corresponds to those of a single rain gage located near the area center.

An analysis of errors made in transferring PPI photographs to digitized arrays and in measuring the echo intensity in steps of 6 db indicates that a reduction in the size of these errors would not substantially improve the accuracy of the radar measurements.

An important unresolved problem concerns the development of techniques for quick determination, under field conditions, of the most accurate reflectivity-rainfall relationship for a particular storm.

A chart based on the average relationship developed in this study is presented for converting echo intensities measured with a WSR-57 to rainfall intensities.

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James W. Wilson

Abstract

Oklahoma thunderstorm data were used to determine how the estimation of area rainfall by radar can be improved by using one or several raingages. The radar data were collected between 1964 and 1968 with the WSR-57 radar at the National Severe Storms Laboratory, Norman, Okla. The rainfall data were obtained from the Agriculture Research Service's dense network of raingages near Chickasha, Okla.

The improvement of area rainfall measurements by combining radar measurements with discrete raingage measurements is demonstrated. It is shown, for example, that the rms error of radar measurements of storm rainfall amount, for a 1000 mi2 area, was reduced by 39% after the radar was calibrated with only one rain-gage. At least four uniformly spaced gages are required to measure storm rainfall amounts for the same area as accurately as the radar calibrated with only one gage. The present network of gages over the United States is approximately one gage per 1000 mi2.

The ability of radar to measure rainfall variability accurately has been demonstrated; therefore, it is possible to assess objectively whether a particular gage measurement will be useful for adjusting radar rainfall measurements.

With the recent development of an effective system for automatically digitizing and communicating radar data in a form suitable for computer processing, these findings make possible the development of an operational system for measuring rainfall with an accuracy and timeliness never before achieved.

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James Wilson
,
Dan Megenhardt
, and
James Pinto

Abstract

This paper examines nowcasts of precipitation from the High-Resolution Rapid Refresh (HRRRv2) model from the summer of 2017 along the Colorado Front Range. It was found that model nowcasts (2 h or less) of precipitation amount were less skillful than extrapolation of the KFTG WSR-88-D data at a spatial scale of 120 km. It was also found that local-scale (mesoscale) influences on rainfall intensity and amount have a much greater impact on rainfall intensity than large-scale (synoptic) influences. Thus, large-scale trends are not useful for modifying extrapolation nowcasts on the local scale. Errors in the HRRR nowcasts are attributed to an inability of the model and data assimilation to resolve convergence along outflow boundaries and other terrain-influenced mesogamma-scale flows that contribute to storm formation and evolution. While the HRRRv2 1-h nowcasts were strongly correlated with observed precipitation events, the nowcast precipitation amounts were in error by more than a factor of 2 about 50% of the time, with half of the cases being overestimates and half being underestimates. A large fraction of the HRRRv2 overestimates were associated with stratiform rain events. It is speculated that this was a result of misinterpretation of the radar bright band as more intense precipitation aloft by the data assimilation scheme. A large fraction of the HRRRv2 underestimates occurred when the data assimilation and model were unable to fully resolve the low-level convergence along small-scale, narrow boundaries that led to new storm initiation and/or storm growth.

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Roger M. Wakimoto
and
James W. Wilson

Abstract

Analyses of tornadoes that are not associated with supercells are presented. The database for this study was collected during CINDE (Convention INitiation and Downburst Experiment), a field project operated during the summer of 1987 in Colorado. A total of 27 visual vortices were studied. They appeared to form as shear instabilities along radar detected convergence lines. The circulations initiated at low levels generally in the absence of precipitation echo. Subsequently as these vortices propagated along the convergence line they appeared to strengthen to tornadic intensity when they became colocated with the updraft of a rapidly developing storm. It is hypothesized that vortex stretching is responsible for intensifying the initial rotation. Although these tornadoes were weaker than those accompanied by strong midlevel mesocyclones, estimates of their strength suggest damage capability as high as F2. The implications for operational radars to detect these types of phenomena were identified. The sensitivity of NEXRAD radars to detect motions in clear air, as well as an increased awareness of the radar observations at the lowest elevation angles, were found to be important.

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James W. Wilson
and
Wendy E. Schreiber

Abstract

The origin of 653 convective storms occurring over a 5000 km2 area immediately east of the Colorado Rocky Mountains from 18 May to 15 August 1984 was examined. Seventy-nine percent of the 418 storms that initiated within the study area occurred in close proximity to radar-observed boundary-layer convergence lines. This percentage increased to 95% when only the more intense storms (≥60 dBZ e ) were considered. Colliding convergence lines initiated new storms or intensified existing storms in 71% of the cases. A new storm took a median time of 24 min to grow to 30 dBZ following line collision.

The convergence lines ranged in length between ten and several hundred kilometers. Both radar and mesonet stations indicated that the primary convergence was concentrated in a zone 0.5 to 5 km in width. These lines were characterized on Doppler radar as thin lines of enhanced reflectivity between 0 and 20 dBZ e and as a line of strong radial or azimuthal gradient in Doppler velocity. These lines were observed even in clear air in the absence of any clouds. The origin of many of the convergence lines was unknown and requires further study. The most common identified origin was from convective storm outflows. Other origins were believed to be topography and differential heating.

This study, which utilized radar data, supports the findings of Purdom (1982), who utilized satellite data, which indicated that mesoscale boundary-layer convergence lines play a major role in determining where and when storms will form. These results suggest that what often appears as random thunderstorm formation (air mass thunderstorms) is usually deterministic.

Major advances now appear possible in the 0–2 h time-specific forecasts of thunderstorms. Realization of this potential will require the integration of Doppler radar to detect and monitor convergence lines, high resolution satellite data to monitor cloud growth, and surface and sounding data to estimate atmospheric susceptibility to deep convection.

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Rita D. Roberts
and
James W. Wilson

Abstract

Dual-Doppler radar analyses of three tornadoes associated with a multicellular line of storms are presented. The F2–F3 intensity tornadoes occurred on 15 June 1988 near Denver, Colorado, during the Terminal Doppler Weather Radar (TDWR) Project. These tornadoes developed from misocyclones of no larger than 2 km in diameter that formed along the collision of two surface outflows. The misocyclones were observed to build in height and intensify with time, coincident with rapid storm growth overhead. All three misocyclones were clearly associated with the maximum storm updrafts. Downdrafts and associated outflows did not play a role in the formation of one of the tornadoes, but may have contributed to the genesis of the other two tornadoes. It is clear that a downdraft is not a necessary condition for the formation of a nonsupercell tornado, but when present, likely plays a role in determining the timing and intensity of the tornado. This is achieved by the downdraft and outflow causing an increase in the magnitude of the low-level convergence and updraft.

Vertical vorticity production terms were examined for each tornado. Given the close proximity in time and space of the tornadoes, there was surprising variability in the magnitudes and locations of the stretching, tilting, and advection terms for each tornado. In general, however, the predominant contribution to positive vertical vorticity and tornadogenesis was from vorticity stretching in the 0.2–2.0-km layer resulting from intensification of low-level convergence and storm updrafts. Above 2.0 km, increased vertical vorticity resulted from a redistribution of low-level vorticity vertically. Small areas of positive vorticity tilting were found within the regions of large streamwise vorticity just prior to tornadogenesis but not during the formative stages of the mesocyclones, amplifying the already strong contributions to tornadogenesis from vertical stretching of the vortices.

The spatial resolution of the data presented here is as high as any documented in tornado literature. However, limitations in what features are actually resolvable became strikingly apparent and are discussed in the paper.

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James W. Wilson
and
Rita D. Roberts

Abstract

The data-rich International H2O Project (IHOP_2002) experiment is used to study convective storm initiation and subsequent evolution for all days of the experiment. Initiation episodes were almost evenly divided between those triggered along surface-based convergence lines and elevated initiation episodes that showed no associated surface convergence. The elevated episodes occurred mostly at night, and the surface-based episodes occurred during the afternoon and evening. Surface-based initiations were mostly associated with synoptic fronts and gust fronts and less so with drylines and bores. Elevated initiations were frequently associated with observable convergent or confluent features in the Rapid Update Cycle (RUC) wind analysis fields between 900 and 600 hPa. The RUC10 3-h forecast of the precipitation initiation episodes were correct 44% of the time, allowing a tolerance of 250 km in space and for the forecast being early by one period. However, the accuracy was closely tied to the scale of the initiation mechanism, being highest for synoptic frontal features and lowest for gust fronts.

Gust fronts were a primary feature influencing the evolution of the initiated storms. Almost one-half of the storm complexes associated with initiation episodes did not produce surface gust fronts. Storm systems that did not produce gust fronts most often lived 2–6 h while those that did frequently lived at least 8 h. The largest and longest-lived storm complexes had well-developed intense gust fronts that influenced the propagation of the storm system. The RUC10 was generally not successful in forecasting the evolution and motion of the larger, more intense storm complexes; presumably this was because it did not produce strong gust fronts.

Implications for forecasting convective storm initiation and evolution are discussed.

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