Abstract

Cloud-to-ground lightning data from the National Lightning Detection Network are used to create a warm season (May–September) lightning climatology for the northern Gulf of Mexico coast for the 14-yr period 1989–2002. Each day is placed into one of five flow regimes based on the orientation of the low-level flow with respect to the coastline. This determination is made using the vector mean 1000–700-hPa wind data at Lake Charles and Slidell, Louisiana. Flash densities are calculated for daily, hourly, and nocturnal periods.

Spatial patterns of composite 24-h and nocturnal flash density indicate that lightning decreases in an east-to-west direction over the region. Flash densities for the 24-h period are greatest over land near the coast, with relative maxima located near Houston, Texas; Lake Charles, Baton Rouge, and New Orleans, Louisiana; Biloxi, Mississippi; and Mobile, Alabama. Flash densities during the nocturnal period are greatest over the coastal waters.

Lightning across the northern Gulf coast is closely related to the prevailing low-level synoptic flow, which controls the sea breeze, the dominant forcing mechanism during the warm season. Southwest flow, the most unstable and humid of the five regimes, exhibits the most flashes. In this case, sea-breeze-induced convection is located slightly inland from the coast. Northeast flow, the driest and most stable of the regimes, exhibits the least amount of lightning. The large-scale flow restricts the sea breeze to near the coastline.

Geographic features and local mesoscale circulations are found to affect lightning across the region. Geographic features include lakes, bays, marshes, swamps, and coastline orientations. Thermal circulations associated with these features interact with the main sea breeze to produce complex lightning patterns over the area.

Abstract

Satellite-derived profiles of temperature and dewpoint (retrievals) are obtained using radiance data from the Visible-infrared Spin Scan Radiometer Atmospheric Sounder. Individual fields of view that are input to the retrieval algorithm must be horizontally averaged to provide suitable signal-to-noise ratios. This paper investigates three methods for performing this averaging: 1)a blocking approach that is employed operationally, 2) a manual procedure that seeks to maximize atmospheric gradients, and 3) an objective procedure called clustering that takes advantage of similarities in satellite measurements to avoid smearing the gradient information. The three techniques are examined on 10–11 July 1989 when intense gradients of humidity were present over the Florida peninsula.

Results show that the clustering scheme produced retrievals that were very similar to those obtained manually. Both schemes indicated strong humidity gradients in the lower troposphere. The blocking procedure produced less intense gradients. The retrieval information is used to examine conditions leading to fair weather on 10 July but intense thunderstorm development on 11 July.

Abstract

Pulse severe storms are single-cell thunderstorms that produce severe wind and/or severe hail for a brief period of time. These storms pose a major warm season forecasting problem since forecasters presently do not have sufficient guidance to know which, if any, of the cells that are observed will become severe. The empirical Probability of Severe (ProbSevere) model, developed by the Cooperative Institute for Meteorological Satellite Studies (CIMSS), fuses real-time data to produce short-term (0–60 min), statistically derived probabilistic forecasts of thunderstorm intensity. This study evaluates the ability of ProbSevere to predict pulse severe storms in the southeast United States. ProbSevere objects fitting the usual definition of a pulse severe environment were matched with severe events from Storm Data to create a dataset of ProbSevere objects that corresponded to pulse severe thunderstorms. A null dataset consisted of objects in pulse severe environments that did not match with a severe event. Results reveal that ProbSevere’s probabilities are small to moderate at the times corresponding to pulse severe events. While probabilities of nonsevere storms are generally smaller, there are a large number of outliers. Lightning flash rate is the only predictor relevant to this study that correlates strongly with increasingly favorable pulse storm probabilities. We conclude that ProbSevere provides forecasters only limited guidance as to whether a pulse severe event will soon occur. Developing a version of ProbSevere specifically for pulse severe storms would likely lead to better predictability for this mode of convection.

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

Radiative transfer simulations are performed to determine how water vapor and nonprecipitating cloud liquid water and ice particles within typical midlatitude atmospheres affect brightness temperatures T _B 's of moisture sounding channels used in the Advanced Microwave Sounding Unit (AMSU) and AMSU-like instruments. The purpose is to promote a general understanding of passive top-of-atmosphere T _B 's for window frequencies at 23.8, 89.0, and 157.0 GHz, and water vapor frequencies at 176.31, 180.3 1, and 182.31 GHz by documenting specific examples. This is accomplished through detailed analyses of T _B 's for idealized atmospheres, mostly representing temperate conditions over land. Cloud effects are considered in terms of five basic properties: droplet size distribution, phase, liquid or ice water content, altitude, and thickness. Effects on T _B of changing surface emissivity also are addressed. The brightness temperature contribution functions are presented as an aid to physically interpreting AMSU T _B 's.

Both liquid and ice clouds impact the T _B 's in a variety of ways. The T _B 's at 23.8 and 89 GHZ are more strongly affected by altostratus liquid clouds than by cirrus clouds for equivalent water paths. In contrast, channels near 157 and 183 GHz are more strongly affected by ice clouds. Higher clouds have a water impact on 157- and 183-GHz T _B 's than do lower clouds. Clouds depress T _B 's of the higher-frequency channels by suppressing, but not necessarily obscuring, radiance contributions from below. Thus, T _B 's are less closely associated with cloud-top temperatures than are IR radiometric temperatures. Water vapor alone accounts for up to 89% of the total attenuation by a midtropospheric liquid cloud for channels near 183 GHz. The Rayleigh approximation is found to be adequate for typical droplet size distributions; however, Mie scattering effects from liquid droplets become important for droplet size distribution functions with modal radii greater than 20 µm near 157 and 183 GHz, and greater than 30–40 µm at 89 GHz. This is due mainly to the relatively small concentrations of droplets much larger than the mode radius. Orographic clouds and tropical cumuli have been observed to contain droplet size distributions with mode radii in the 30–40-µm range. Thus, as new instruments bridge the gap between microwave and infrared to frequencies even higher than 183 GHz, radiative transfer modelers are cautioned to explicitly address scattering characteristics of such clouds.

Abstract

The National Hurricane Center (NHC) has stated that guidance on tropical cyclone (TC) genesis is an operational forecast improvement need, particularly since numerical weather prediction models produce TC-like features and operationally required forecast lead times recently have increased. Using previously defined criteria for TC genesis in global models, this study bias corrects TC genesis forecasts from global models using multiple logistic regression. The derived regression equations provide 48- and 120-h probabilistic genesis forecasts for each TC genesis event that occurs in the Environment Canada Global Environmental Multiscale Model (CMC), the NCEP Global Forecast System (GFS), and the Met Office's global model (UKMET). Results show select global model output variables are good discriminators between successful and unsuccessful TC genesis forecasts. Independent verification of the regression-based probabilistic genesis forecasts during 2014 and 2015 are presented. Brier scores and reliability diagrams indicate that the forecasts generally are well calibrated and can be used as guidance for NHC’s Tropical Weather Outlook product. The regression-based TC genesis forecasts are available in real time online.

Abstract

Accurately forecasting tropical cyclone (TC) genesis is an important operational need, especially since the National Hurricane Center’s Tropical Weather Outlook product has been extended from 2 to 5 days. A previous study by the coauthors verified North Atlantic TC genesis forecasts from five global models out to 4 days during 2004–11. This study expands on the previous research by 1) verifying TC genesis forecasts over both the Atlantic and eastern North Pacific basins, 2) extending the forecast window to 5 days, and 3) updating the analysis period through 2014. Verification statistics are presented and compared between the two basins. Probability of detection and critical success indices generally are greater over the eastern North Pacific basin compared to the North Atlantic. There is a trade-off between models that exhibit a greater probability of detection and a greater false alarm ratio, and models that exhibit a smaller false alarm ratio and a smaller probability of detection. Results also reveal that the models preferentially miss TCs over the North Atlantic (eastern North Pacific) that have a relatively small radius of the outer closed isobar (radius of maximum wind) at the forecast genesis time. Overall, global models have become a more reliable source of TC genesis guidance during the past few years compared to the early years in the dataset.

Abstract

Compared to conventional rain gauge networks, the Weather Surveillance Radar-1988 Doppler provides precipitation estimates at enhanced spatial and temporal resolution that River Forecast Centers can use to improve streamflow forecasts. This study documents differences between radar-derived (stage III) mean areal precipitation (MAPX) and rain gauge–derived mean areal precipitation (MAP). The area of study is the headwaters of the Flint River basin, specifically the Culloden basin located in central Georgia south of Atlanta, with a drainage area of 1853 mi². The timing of radar installations in the southeast United States provided overlapping data for only 2 yr (Jun 1996–Jul 1998). The MAP and MAPX products being examined were prepared using procedures identical to those employed operationally at the National Weather Service’s Southeast River Forecast Center.

Results show that the radar (MAPX) underestimates gauge-derived rainfall (MAP) by ∼38% at the end of the 2-yr period. This underestimate is most pronounced during the winter months of November–April when MAPX underestimates MAP by ∼50%. Comparisons during the summer (May–Oct) indicate that MAPX is similar to MAP. The underestimation of winter rainfall likely is due to several factors: the inappropriate combination of radar values in areas of overlapping coverage, the radar beam overshooting the tops of stratiform rainfall, an inappropriate Z–R relationship, faulty radar calibration, and too few hourly rain gauges to prepare an accurate stage II bias adjustment factor and quality control the stage III product.

Abstract

This study develops conceptual models of how a land–water interface affects the strength and structure of squall lines. Two-dimensional numerical simulations using the Advanced Regional Prediction System model are employed. Five sets of simulations are performed, each testing eight wind shear profiles of varying strength and depth. The first set of simulations contains a squall line but no surface or radiation physics. The second and third sets do not contain a squall line but include surface and radiation physics with a land surface on the right and a water surface on the left of the domain. The land is either warmer or cooler than the sea surface. These three simulations provide a control for later simulations. Finally, the remaining two simulation sets examine squall-line interaction with a relatively cool or warm land surface. The simulations document the thermodynamic and shear characteristics of squall lines interacting with the coastline. Results show that the inclusion of a land surface did not sufficiently affect the thermodynamic properties ahead of the squall line to change its overall structure. Investigation of ambient shear ahead of the squall line revealed that the addition of either warm or cool land reduced the strength of the net circulation in the inflow layer as measured by ambient shear. The amount of reduction in shear was found to be directly proportional to the depth and strength of the original shear layer. For stronger and deeper shears, the reduction in shear is sufficiently great that the buoyancy gradient circulation at the leading edge of the cold pool is no longer in balance with the shear circulation leading to changes in squall-line updraft structure. The authors hypothesize two ways by which a squall line might respond to passing from water to land. The weaker and more shallow the ambient shear, the greater likelihood that the squall-line structure remains unaffected by this transition. Conversely, the stronger and deeper the shear, the greater likelihood that the squall line changes updraft structure from upright/downshear to upshear tilted.

Abstract

Cloud-to-ground lightning data from the National Lightning Detection Network are examined over the Florida peninsula during the warm seasons of 1989 through 1998. The lightning data are stratified according to the location of the subtropical ridge (i.e., north of Florida, south of Florida, and within Florida) as well as other common flow types. Each day is placed into a flow regime based on radiosonde-derived low-level winds at three stations within the study area. Maps of lightning flash density are generated for each flow regime over hourly, daily (24 h), and nocturnal periods.

Results for the 24-h period indicate that complexities in the Florida coastline produce four areas of relatively large flash densities: near Tampa, Fort Myers, West Palm Beach, and Cape Canaveral. Nocturnal lightning is found to occur mostly offshore—related to the Gulf Stream, coastline orientations, the prevailing flow, and land breezes.

The location of the subtropical ridge with respect to the Florida peninsula is found to play an important role in the spatial and temporal distribution of lightning. For example, when the large-scale flow is from the southeast, the east coast sea breeze and its associated lightning are relatively weak. However, the west coast sea breeze is strong and remains near the coastline, producing the most lightning near Tampa. Conversely, when the large-scale flow is from the southwest, there is relatively little convection along the west coast, but major lightning activity occurs along the east coast.