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Ronald M. Reap

Abstract

Relationships between network lightning data and hourly thunderstorm observations were examined for the northeastern United States, Oklahoma, Florida, and the western United States to provide additional information on the possible effects of using lightning data to replace or supplement the hourly observations. Identification of thunderstorms for three of the four regions was found to agree closely with the hourly observations, provided the network reports were accumulated for a radius of 48 km or more about the station. The best agreement was found over Florida where high ground-flash densities resulted in a greater likelihood of both observer and network recording a given thunderstorm, In the immediate vicinity (8 km) of a station, use of lightning data from current national or regional networks would not provide observations comparable to the manual observations of thunderstorms due to the poor agreement between the two sets of observations at this radius. Selection of an 8-km radius would result in a decrease of nearly 75% in the number of thunderstorms detected by the network relative to that reported by the observer.

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Ronald M. Reap

Abstract

Over two million cloud-to-ground lightning strike locations for the period from mid-June through mid-September for the 1983–84 summer seasons were evaluated to determine the large-scale climatological characteristics of summertime lightning activity over mountainous terrain in the western United States. During the evaluation, manually digitized radar data and GOES satellite observations from the Techniques Development Laboratory's data archives were included for comparison. Generalized statistical relationships were established among the lightning data, radar data, and minimum cloud-top infrared temperature and maximum visible brightness values from the satellite VISSR data.

In the climatological analysis, a high correlation between terrain elevation and the hour of maximum frequency of lightning was found along with a pronounced increase in the magnitude of lightning activity over high terrain. Maps of daily lightning frequency were in general agreement with conventional thunderstorm climatologies but contained considerably more detail, especially in the higher elevations. Due to inadequate radar coverage in the West, approximately 41% of the lightning strikes occurred when no radar echoes were reported. In addition, 87% of the strikes occurred with radar intensity levels less than VIP3, the threshold normally used for delineating thunderstorms in the eastern United States. During daylight hours, lightning frequencies were 10–20% higher for the GOES infrared observations compared to frequencies associated with the visible brightness values, reflecting a tendency for the lightning strikes to be clustered under or near the coldest cloud tops. The effect of thunderstorm cells imbedded in large cloud masses on the relationship between lightning frequency and GOES infrared data was found to be significant. For a given infrared cloud-top temperature, it was not unusual to find a 40–50% increase in lightning frequency for subsets of grid blocks with VIP4 or greater echoes compared to blocks where no echoes were reported.

A fairly stable seasonal and geographical distribution of lightning activity was found in the West, resulting from the strong control exerted by the underlying topography on the timing, location, and magnitude of the lightning activity. This is especially significant because it will allow the lightning frequencies to be used as climatic predictors in the development of new objective thunderstorm probability forecasts for this region, thereby improving the temporal and spatial resolution of the forecasts by the implicit introduction of small-scale topographic effects into the forecasts.

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Ronald M. Reap

Abstract

Archived lightning data from the Bureau of Land Management automated network of direction-finding stations in Alaska were examined to determine the seasonal, diurnal, and spatial distributions of cloud-to-ground lightning, including the effects of the underlying topography on the timing, location, and magnitude of the lightning activity. The interior of Alaska was found to exhibit a continental type climate with a pronounced afternoon maximum in lightning activity and no significant nocturnal activity. Over 90% of the strikes were found to occur during June and July, the time of maximum solar insulation. A relatively high incidence of positive flashes was found during all phases of thunderstorm occurrence. High flash accumulations over mountainous regions clearly revealed the affinity of lightning activity for elevated terrain. Maximum activity was found to occur earliest in the higher elevations, moving to lower elevations later in the day.

Linear screening regression analysis and the Model Output Statistics approach were used to statistically relate lightning data to Nested Grid Model forecast fields. Operational equations were derived to provide objective thunderstorm probabilities for use as guidance by forecasters in estimating the potential hazard of wildfire initiation in Alaska. The primary prerequisite for the formation of thunderstorms over Alaska was found to be large-scale static instability followed by secondary contributions from local convergence in the wind and moisture fields. Low-level moisture convergence was also found to be very important to the production of high flash accumulations on any given day.

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Ronald M. Reap

Abstract

The temporal and spatial distributions of lightning activity associated with specific synoptic regimes of low-level wind flow were analyzed as part of an experiment to develop improved statistical thunderstorm forecasts for Florida. The synoptic regimes were identified by means of a linear correlation technique that was used to perform pattern classification or “map typing” of 18- and 30-h sea level pressure forecasts from the National Meteorological Center's Nested Grid Model (NGM). Lightning location data for the 1987–90 warm seasons were subsequently analyzed on a 12-km grid to determine the thunderstorm distribution for each of the predetermined map types. The analysis revealed organized coastal maxima in lightning activity related to land-sea-breeze convergence zones that form in direct response to the low-level wind flow. Surface effects were also indicated by the persistent minima in lightning activity over Lake Okeechobee and by the lightning maxima found in regions with shoreline curvature favoring localized convergence. Experimental thunderstorm probability equations for Florida were subsequently developed from climatological lightning frequencies and NGM forecast fields. The lightning frequencies were combined with the K stability index to form interactive predictors that take into account the temporal and spatial variations in lightning occurrence for each map type but modulate the climatology in response to the daily large-scale synoptic situation. The statistical forecast equations were developed for each map type in an attempt to simulate the effects of small-scale processes, such as land-sea-breeze convergence zones, on the subsequent development of peninsular-scale convection.

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Ronald M. Reap

Abstract

A numerical model is developed to compute three-dimensional trajectories from operational wind forecasts generated by the six-layer primitive equation model of the National Meteorological Center. Detailed 24-hr forecasts of temperature and dew point, designed for application to severe storm prediction, are derived by computing the 6-hr variations of potential temperature and mixing ratio for air parcels assumed to follow paths defined by the trajectories. Initial values at the trajectory origin points are provided by an objective analysis technique which reproduces detailed patterns and gradients with only light smoothing of the observations. The influence of a relatively detailed terrain is also included in the program. Verification statistics indicate a significant improvement over the primitive equation model forecasts in the lowest 150 mb, where temperature and moisture distributions are crucial to severe storm development. Output from the model for the surface, 850 and 700 mb is currently transmitted via National Weather Service facsimile and teletype circuits for use as guidance in severe storm forecasting.

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Ronald M. Reap and Donald S. Foster

Abstract

No abstract available.

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Ronald M. Reap and Donald R. MacGorman

Abstract

Data for nearly 2 million lightning flashes recorded during the 1985–86 warm seasons by the National Severe Storm Laboratory's (NSSL's) lightning strike locating network were evaluated to determine some of the climatological characteristics of cloud-to-ground lightning. Among the characteristics studied were the seasonal, diurnal, and spatial variations Of Positive and negative lightning strike activity, including flush rates, signal strength, and flash multiplicity. The lightning data were also compared to manually digitized radar data, reports of tornadoes, large hail, and damaging winds, and to analyzed 0000 UTC fields obtained from operational numerical models.

An examination of the diurnal distribution of lightning revealed that peak rates occurred later than in other sections of the country, reflecting the prevalence of nocturnal convection within much of the NSSL network. An analysis of the spatial variations in lightning activity also confirmed the existence of distinct climatological regimes within the network. A study of the diurnal variations in signal strength revealed that first return strokes lowering negative charge have higher signal strengths at night and in the early morning hours, when flash rates are normally decreasing. In addition, positive flashes were found to exhibit three distinct peaks in signal strength, two of which are associated with late afternoon and nocturnal maxima in fish activity.

A good correspondence between lightning frequency and radar echo intensity was found for the region within the effective range of the Oklahoma City WSR-57 radar. Both positive and negative flashes were found to be strongly correlated with the low-level moisture flux and circulation, as characterized by favorable moisture convergence, cyclonic relative vorticity, and strong upward vertical motions in the boundary layer. Contrary to expectations, freezing level height and wind shear were not nearly as important as the boundary layer fields in determining thunderstorm formation and subsequent positive lightning activity. A significant correlation was also found between the occurrence of severe local storm and elevated rates of 30 or more positive flashes per hour within 48 km grid blocks.

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Ronald M. Reap and Richard E. Orville

Abstract

Relationships were established between lightning location data and surface hourly observations of thunderstorms for 132 stations in the northeastern United States. The relationships are based on statistics derived from 2 × 2 contingency tables that were constructed for each station from a two-year sample of data for the 1985–86 warm seasons. To construct the tables, ground strike totals for the valid period of the hourly observations were accumulated for circular regions of varying radius (8–80 km) centered at each station. Separate contingency tables were constructed for each radius.

For the entire sample, the fraction of observer reported thunderstorms that were also recorded by the lightning network was found to increase rapidly from 0.44 within a 16 km radius to 0.82 at 50 km, beyond which the rate of increase was much smaller with a maximum of 0.89 at 80 km. Values over 0.9 at 50 km were, however, typical for stations located well within the interior of the network. The continuing improvement in the relationships beyond the audible range of thunder (20 km) was most likely related to the errors biases, and uncertainty inherent in both sets of observations. The agreement between the network reports and station observations was slightly degraded by the possible reporting of intracloud flashes or weak ground strikes that were detected by the observer but not the network, especially at night. A significant increase was found in the ratio of the number of thunderstorms reported by observers at night to the number detected by the network, reflecting the improved ability of the observer to see lightning at night. In general, the lightning network was able to detect more thunderstorms than the observer beyond 17 and 26 km, respectively, for the daytime and nighttime samples. The improved detection capability for expanded regions about the station should have a significant impact on operational forecasts and warnings, especially with regard to aviation-related activities both en route and near airport terminals. Finally, improvements in the observer's ability to see lightning and hear thunder were found to be related to corresponding increases in flash density and peak signal strength.

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Ronald M. Reap and Donald S. Foster

Abstract

Probability equations were developed for 12–36 h forecasts of thunderstorms, severe local storms, and major or family tornado outbreaks. The equations were derived by applying screening regression techniques to relate manually digitized radar (MDR) data and severe storm reports to large-scale meteorological predictors obtained from numerical forecast models. The probability forecasts are prepared once daily on the NOAA computer system and are transmitted over facsimile and teletypewriter circuits.

One of the more important innovations in the forecast equations was the development of an interactive predictor which takes into account the seasonal variations in thunderstorm occurrence, yet is responsive to the daily synoptic situation. This predictor is formed by combining the K stability index with daily mean thunderstorm relative frequencies estimated from MDR data. Local variations in thunderstorm occurrence over the MDR grid were also included in the generalized equation by incorporating probability estimates for each MDR grid block, based on statistical relationships between the interactive predictor and thunderstorm occurrence.

Verification of the thunderstorm probability forecasts against independent MDR data and observed thunderstorms during the 1977–78 convective seasons have shown the probability forecasts to be highly reliable. During the spring months, severe local storm probabilities near 40% were forecast with good reliability, even though the climatological probability for the same period was only 7%. Verification scores show that categorical forecasts based on the conditional severe local storm probabilities compare favorably with operational convective outlooks.

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