Showing papers in "Weather and Forecasting in 2001"

Some Common Ingredients for Heavy Orographic Rainfall

Abstract: The purpose of this paper is to synthesize some common synoptic and mesoscale environments conducive to heavy orographic rainfall. Previous studies of U.S. and Alpine cases and new analyses of some Alpine and east Asian cases have shown the following common synoptic and mesoscale environments are conducive to heavy orographic rainfall: 1) a conditionally or potentially unstable airstream impinging on the mountains, 2) a very moist low-level jet (LLJ), 3) a steep mountain, and 4) a quasi-stationary synoptic system to slow the convective system over the threat area. A deep short-wave trough is found to approach the threat area in the U.S. and European cases, but is not found in the east Asian cases. On the other hand, a high convective available potential energy (CAPE) value is observed in east Asian cases, but is not consistently observed in the U.S. and European cases. The enhancement of low-level upward motion and the increase of instability below the trough by the approaching deep short-wave tr...

A Tropical Cyclone Genesis Parameter for the Tropical Atlantic

Abstract: A parameter to evaluate the potential for tropical cyclone formation (genesis) in the North Atlantic between Africa and the Caribbean islands is developed. Climatologically, this region is the source of about 40% of the Atlantic basin tropical cyclones but roughly 60% of the major hurricanes. The genesis parameter is the product of appropriately scaled 5-day running mean vertical shear, vertical instability, and midlevel moisture variables. The instability and shear variables are calculated from operational NCEP analyses, and the midlevel moisture variable is determined from cloud-cleared GOES water vapor imagery. The average shear and instability variables from 1991 to 1999 and moisture variable from 1995 to 1999 indicate that tropical cyclone formation in the early part of the season is limited by the vertical instability and midlevel moisture. Formation at the end of the season is limited by the vertical shear. On average, there is only a short period from mid-July to mid-October when all thre...

Examination of Derecho Environments Using Proximity Soundings

Abstract: Observed upper air soundings that occurred within 2 h and 167 km of derechos were collected and analyzed to document atmospheric stability and wind shear conditions associated with long-lived convective windstorms. Sixty-seven derechos, accompanied by 113 proximity soundings, were identified during the years 1983–93. Owing to the large variability of the synoptic-scale environments associated with derechos, each derecho was further divided into categories based on the strength of synoptic-scale forcing associated with each event. Derechos are shown to develop and persist in a wide range of shear and instability conditions. Although this range of shear and instability narrows when derechos are grouped by synoptic-scale forcing strength, considerable variation of values remains, primarily with the shear. These results suggest that ambient shear and instability values alone are not sufficient to distinguish derecho environments from those associated with nonsevere mesoscale convective system (MCS) e...

Cold pools in the Columbia Basin

Abstract: Persistent midwinter cold air pools produce multiday periods of cold, dreary weather in basins and valleys. Persistent stable stratification leads to the buildup of pollutants and moisture in the pool. Because the pool sometimes has temperatures below freezing while the air above is warmer, freezing precipitation often occurs, with consequent effects on transportation and safety. Forecasting the buildup and breakdown of these cold pools is difficult because the interacting physical mechanisms leading to their formation, maintenance, and destruction have received little study. In this paper, persistent wintertime cold pools in the Columbia River basin of eastern Washington are studied. First a succinct meteorological definition of a cold pool is provided and then a 10-yr database is used to develop a cold pool climatology. This is followed by a detailed examination of two cold pool episodes that were accompanied by fog and stratus using remote and in situ temperature and wind sounding data. The two episodes illustrate many of the physical mechanisms that affect cold pool evolution. In one case, the cold pool was formed by warm air advection above the basin and was destroyed by downslope winds that descended into the southern edge of the basin and progressively displaced the cold air in the basin. In the second case, the cold pool began with a basin temperature inversion on a clear night and strengthened when warm air was advected above the basin by a westerly flow that descended from the Cascade Mountains. The cold pool was nearly destroyed one afternoon by cold air advection aloft and by the growth of a convective boundary layer (CBL) following the partial breakup of the basin stratus. The cold pool restrengthened, however, with nighttime cooling and was destroyed the next afternoon by a growing CBL.

Real-Time Low-Level Wind and Temperature Analysis Using Single WSR-88D Data

Abstract: A four-dimensional variational Doppler radar analysis system (VDRAS) has been developed and implemented at a weather forecast office to produce real-time boundary layer wind and temperature analyses using WSR88D radar data. This paper describes significant changes made to convert VDRAS from a research tool to a real-time analysis system and presents results of low-level wind and temperature analysis using operational radar data. In order to produce continuous analyses with time, VDRAS was implemented with a cycling procedure, in which the analysis from the previous cycle is used as a first guess and background for the next cycle. Other enhancements in this real-time system include direct assimilation of data on constant elevation angle levels, addition of mesonet observations, inclusion of an analysis background term, and continuous updating of lateral boundary conditions. An observed case of a line of storms and strong outflow is used to examine the performance of the real-time analysis system and its sensitivity to various system changes. The quality of the analysis for this case is examined by comparing the subsequent 90-min forecast with the observed radial velocity. It is shown that the forecast initialized using the VDRAS analysis outperforms persistence and a forecast using a mesoscale analysis. The accuracy of the retrieved wind in six convective cases is also verified against automated weather reports from commercial aircraft data. The verification shows an average difference of 3.3 m s21 over these six cases.

Normalized Damage from Major Tornadoes in the United States: 1890 1999

Abstract: Historical records of damage from major tornadoes in the United States are taken and adjusted for inflation and wealth Such adjustments provide a more reliable method to compare losses over time in the context of significant societal change From 1890 to 1999, the costliest tornado on the record, adjusted for inflation, is the 3 May 1999 Oklahoma City tornado, with an adjusted $963 million in damage (constant 1997 dollars) Including an adjustment for growth in wealth, on the other hand, clearly shows the 27 May 1896 Saint Louis–East Saint Louis tornado to be the costliest on record An extremely conservative adjustment for the 1896 tornado gives a value of $22 billion A more realistic adjustment yields a figure of $29 billion A comparison of the ratio of deaths to wealth-adjusted damage shows a clear break in 1953, at the beginning of the watch/warning/awareness program of the National Weather Service

Improvements in the Seasonal Forecasting of Tropical Cyclone Activity over the Western North Pacific

Abstract: A recent scheme to predict tropical cyclone (TC) activity over the western North Pacific partially failed in 1997 and 1998, during which a warm and a cold event of the El Nino-Southern Oscillation (ENSO) occurred, respectively. This paper presents results of two approaches to improve on such predictions. The first is to include new predictors that are related to ENSO based on some recent research, and the second is to provide an updated prediction by incorporating monthly values of predictors in April and May of the current year. The results suggest that new predictors related to ENSO can indeed be identified, which include temporal changes in the Southern Oscillation index, strength of the Australian monsoon, and intensity of the subtropical high in the South Pacific. These predictors, together with those selected from the original prediction scheme, are combined to form a modified scheme that in general gives better forecasts of TC activity. The updated scheme that includes April and May predictors further improves the accuracy of the predictions. Real-time predictions from both schemes for the year 2000, which were made in April and June, are found to be largely accurate. Both schemes show better skill compared with the original one.

Standardized Anomalies Applied to Significant Cold Season Weather Events: Preliminary Findings

Abstract: Forecasting significant weather events, such as floods, heat waves, arctic outbreaks, ice storms, large severe weather outbreaks, and major winter storms, is a critical function for all weather services. However, conventional pressure level geopotential and temperature fields often are insufficient to determine whether an event represents a large departure from normal. This is largely due to the variability that exists throughout the year and regionally throughout the world. What represents an unusual departure from average conditions in fall may not be as unusual in winter. What is an unusual departure from average conditions in California may be normal in New England. This paper presents a method, normalized field departures from local climatology, that gives forecasters guidance on the relative rarity of events. Thus, in this paper a method is presented to help forecasters identify potentially significant weather events. The focus of this paper is on significant winter storms. However, a recor...

Tropical Cyclone–Like Vortices in the Extratropics: Observational Evidence and Synoptic Analysis

Abstract: In this article two subsynoptic-scale cyclones that developed between 3 and 10 October 1996 over the western-central Mediterranean, causing floods, strong winds, and severe damage, are analyzed. Surface observations reveal that the accumulated rainfall at Santuario di Polsi (southern Calabria, Italy) is more than 480 mm for the first event (cyclone 9610A). The second cyclone (9610B) was characterized by a storm track predominantly over the sea, thus causing less recorded precipitation, but stronger wind. Satellite imagery shows two intensely convective vortices with a scale of 200–400 km and a spiral structure, with the cyclone 9610B displaying a well-defined eyelike feature. The corresponding National Centers for Environmental Prediction analyses, although limited by 1° resolution, confirm the cyclones’ positions and intensities, as they can be inferred from satellite imagery, SSM/I data, and observations, but display also the “signature” of two tropical cyclone–like vortices, including a perfec...

A Bayesian Neural Network for Severe-Hail Size Prediction

Abstract: The National Severe Storms Laboratory has developed algorithms that compute a number of Doppler radar and environmental attributes known to be relevant for the detection/prediction of severe hail. Based on these attributes, two neural networks have been developed for the estimation of severe-hail size: one for predicting the severe-hail size in a physical dimension, and another for assigning a probability of belonging to one of three hail size classes. Performance is assessed in terms of multidimensional (i.e., nonscalar) measures. It is shown that the network designed to predict severe-hail size outperforms the existing method for predicting severe-hail size. Although the network designed for classifying severe-hail size produces highly reliable and discriminatory probabilities for two of the three hail-size classes (the smallest and the largest), forecasts of midsize hail, though highly reliable, are mostly nondiscriminatory.

The Use of Ensembles to Identify Forecasts with Small and Large Uncertainty

Abstract: In the past decade ensemble forecasting has developed into an integral part of numerical weather prediction. Flow-dependent forecast probability distributions can be readily generated from an ensemble, allowing for the identification of forecast cases with high and low uncertainty. The ability of the NCEP ensemble to distinguish between high and low uncertainty forecast cases is studied here quantitatively. Ensemble mode forecasts, along with traditional higher-resolution control forecasts, are verified in terms of predicting the probability of the true state being in 1 of 10 climatologically equally likely 500-hPa height intervals. A stratification of the forecast cases by the degree of overall agreement among the ensemble members reveals great differences in forecast performance between the cases identified by the ensemble as the least and most uncertain. A new ensemble-based forecast product, the “relative measure of predictability,” is introduced to identify forecasts with below and above ave...

Impact of Improved Initialization of Mesoscale Features on Convective System Rainfall in 10-km Eta Simulations

Abstract: A 10-km version of the NCEP Eta Model has been run over a roughly 1000 km 3 1000 km domain centered over the upper Midwest for 20 cases where heavy warm season rainfall occurred from mesoscale convective systems to investigate the response of the precipitation forecasts to improvements in the depiction of mesoscale features at initialization time. Modifications to the initial conditions included (i) use of a cold pool initialization scheme, (ii) inclusion of mesonetwork surface observations using the model’s own vertical diffusion formulation to allow the surface data to be assimilated into a deeper layer through a simulated initialization period, and (iii) addition of water vapor at points covered by radar echo to ensure relative humidities greater than 80%. All of these modifications were implemented in runs using both the operational Betts‐Miller‐Janjic (BMJ) and Kain‐ Fritsch (KF) convective parameterizations. In addition, simulations were also run with a doubling of the convective time step, alternation of the two convective schemes within one run, and exclusion of a convective scheme in another run. For all 20 cases, 14 variants in the model initilization/moist physics were used, creating a high grid resolution (10-km grid spacing) ensemble. Although techniques (i) and (ii) both resulted in initial surface fields agreeing better with available observations, average skill scores for precipitation forecasts did not change appreciably when (i) was used, with (ii) resulting in a modest improvement in equitable threat score (ETS), with an increase in the bias that already exceeded 1.0 for most precipitation thresholds in the BMJ runs. Skill scores among the cases varied widely; no single adjustment consistently improved the scores. Interestingly, the simplest modification, the addition of water vapor in relatively dry atmospheric regions at points where radar echo was present, had the greatest positive impact on ETSs for most precipitation thresholds. Although the impacts were greatest in the first 6 h of the forecasts, some improvements occurred through the full 24-h integration period. Variations among the runs for a given case were far greater when different convective schemes were used than when initialization modifications were made, further supporting other recent research suggesting that high grid resolution short-range ensembles may benefit from the use of a variety of models or physical parameterizations.

An Examination of Radar and Rain Gauge-Derived Mean Areal Precipitation over Georgia Watersheds

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 m...

An Operational Ingredients-Based Methodology for Forecasting Midlatitude Winter Season Precipitation

Abstract: An ingredients-based methodology (IM) for the operational analysis and prediction of midlatitude winter season precipitation is developed. Diagnostics for five fundamental physical ingredients involved in the production of precipitation—forcing for ascent, moisture, instability, precipitation efficiency, and temperature—are incorporated into the IM. The forcing ingredient is combined with the instability ingredient to form a new parameter, PVQ, that serves as an indicator of heavy precipitation potential by identifying regions where these two ingredients coexist. The diagnostics and PVQ are incorporated into ingredients maps that facilitate a systematic approach to forecasting the duration, intensity, and type of winter precipitation.

Estimating Downburst-Related Maximum Surface Wind Speeds by Means of Proximity Soundings in New South Wales, Australia

Abstract: A regional climatology of strong wind gusts associated with thunderstorms is presented, and the ability to estimate gust strength from ambient conditions is tested. Strong wind events were selected for 10 stations in New South Wales, Australia, from anemograph records and coincident thunderstorm reports. Most events took place between midafternoon and late evening and during the warmer months of the year, which is broadly consistent with the occurrence of severe thunderstorms in general. One sounding-based index, designed to predict the strength of microbursts, proves to be of limited value in predicting the magnitude of strong convective gusts, even of short-lived gusts. A modified index that combines the microburst index with upper-level wind speed is more useful.

Airflow Configurations of Warm Season Southerly Low-Level Wind Maxima in the Great Plains. Part I: Spatial and Temporal Characteristics and Relationship to Convection

Abstract: A synoptic climatological approach was used to investigate 1) the airflow characteristics of southerly low-level wind maxima in the Great Plains, 2) the typical thermodynamic environments in which low-level wind maxima form, 3) the convergence fields accompanying the wind maxima, and 4) the associated patterns of cloud-to-ground lightning activity. A total of 260 low-level jet events that occurred at either 0000 or 1200 UTC during the warm seasons (Apr–Sep) of 1991–92 were employed in the analysis. The spatial configuration of southerly low-level jets was shown to be considerably more complex than previously portrayed. A subjective classification of the jet events—based on streamline curvature, latitudinal extent, and the orientation of confluence and deformation zones—resulted in 12 distinct configuration types. Only 37% of the events were broadly classified as anticyclonically curving jets, even though the majority of previous case studies focused on this type of wind maximum. Another unanticip...

A Dynamical Approach to Seasonal Prediction of Atlantic Tropical Cyclone Activity

Abstract: Analysis of ECMWF reanalyses and operational analyses covering the period between 1979–98 has confirmed that seasonal Atlantic tropical cyclone activity is strongly and negatively correlated with the observed vertical wind shear present in the main development region (MDR) between July and September. In 1983 and 1995, the least active and most active tropical cyclone years, respectively, anomalous shear was shown to be present in spring and to persist throughout each of the tropical cyclone seasons. While monitoring of MDR shear is recommended for highlighting the risk of such extreme events, the springtime MDR shear is not generally a good indicator of shear in the summer months. Seasonal forecasts of MDR shear made with the U.K. Met Office (UKMO) atmospheric GCM (AGCM) and observed SSTs for the years 1979–97 have been analyzed. The model possesses potential skill for predicting the MDR shear as determined by a consideration of the ensemble mean shear variability and an evaluation of the relativ...

Beta Test of the Systematic Approach Expert System Prototype as a Tropical Cyclone Track Forecasting Aid

Abstract: The authors have developed error mechanism conceptual models with characteristic track departures and anomalous wind or sea level pressure patterns for dynamical tropical cyclone track predictions primarily occurring in tropical regions or those associated with midlatitude circulation patterns. These conceptual models were based on a retrospective study in which it was known that the 72-h track error exceeded 300 n mi (555 km). A knowledge-based expert system module named the Systematic Approach Forecast Aid (SAFA) has been developed to assist the forecaster in the information management, visualization, and proactive investigation of the frequently occurring error mechanisms. A beta test of the SAFA module was carried out for all available track forecasts for the western North Pacific cyclones 19W–30W during 1999. The objective was to determine if the SAFA module could guide the team to apply the conceptual models in a real-time scenario to detect dynamical model tracks likely to have 72-h errors...

Evaluation of the Timing and Strength of MM5 and Eta Surface Trough Passages over the Eastern Pacific

Abstract: The timing and strength of surface troughs forecast by the Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5) and the National Centers for Environmental Prediction's (NCEP's) Eta Model are evaluated over the eastern Pacific during the 1997–2000 cool seasons (Sep–Mar). The troughs are verified by comparing observations from eight offshore buoys with 3–48-h model forecasts. An automated approach is used to identify trough events by setting thresholds for 3-hourly cyclonic wind shifts and pressure rises. The mean timing errors (biases) for surface troughs in the 36-km resolution MM5 domain are 1–2 h early (negative) to the north of 42°N and near the coast, and 0.5–1.0 h late (positive) over the offshore waters to the south of 42°N. The mean absolute timing errors range from 2–3 h for the southern offshore sites to 4–5 h over northern offshore waters as well as near steep coastal terrain. For stronger trough events, the MM5 trough mean errors...

Weekly Precipitation Cycles along the Northeast Corridor

Abstract: Twenty years of precipitation data from seven cities along or near the east coast of the United States from the northern mid-Atlantic region to northern New England have been analyzed to determine if there are any weekly cycles in either daily precipitation frequency or intensity. Any such weekly cycle could be considered evidence of anthropogenic influence on the climate of that region. Data were examined for each individual site and for all sites combined. The data were subjected to various statistical procedures, including one-way analysis of variance, Student's t-test, and the chi-square goodness-of-fit test. Overall, results were not significant at the 95% confidence level. Thus, this study is unable to detect any weekly cycle in daily precipitation intensity or frequency.

Winter Lightning and Heavy Frozen Precipitation in the Southeast United States

Abstract: This study addresses winter season lightning by examining synoptic-scale circulations, cloud-to-ground (CG) lightning patterns, and frozen precipitation. Specifically, locations, frequencies, and polarities of CG flashes are related to the location, intensity, and type of heavy frozen precipitation (snow, freezing rain, or ice pellets) for seven winter storms affecting the southeast United States from 1994 through 1997. The results suggest two distinct phases of winter storm development, each producing different patterns of CG lightning and frozen precipitation. These phases are termed the arctic front (AF) and migratory cyclone (MC) types. Analysis was performed on 27 periods within the seven cases. In several periods, there were significant numbers of CG flashes within or near a subfreezing surface air mass and frozen precipitation when a quasistationary arctic front existed. These periods were classified as AF phases. This flash pattern indicates a connection between the intensity of convection (associated with CG flashes) and downwind frozen precipitation. In these situations there was strong southwesterly flow aloft, which may have advected ice particles from these convective clouds into stratiform clouds near the frontal surface. This process resembles the ‘‘seeder‐feeder’’ mechanism of precipitation growth. The AF phases eventually developed into MC phases, and the latter were more common in this study. The MC phases in general exhibit a different spatial relationship between CG lightning and heavy frozen precipitation; that is, CG flashes retreat toward the warm sector of the cyclone and thus are not proximal to the 0 8C surface isotherm. There appears to be little connection between convection and frozen precipitation in most of these situations. The distinction between AF and MC phases, in conjunction with CG lightning monitoring, may aid forecasts of the duration and amount of frozen precipitation during winter storms.

Synoptic Structure and Evolution of a Kona Low

Abstract: A subtropical cyclone or kona low affected the island of Hawaii on 24‐28 February 1997 and brought with it record winds at Hilo, large hail, blizzard conditions at higher elevations, and high surf. Damage estimates for the storm due to crop loss, property damage, and utility line destruction exceed $4 million. A detailed case study of the storm was conducted using all available operational data and data from the National Centers for Environmental Prediction‐National Center for Atmospheric Research reanalysis dataset. The kona low formed on 23 February 1997 along a stalled trough northeast of the Hawaiian Islands and is investigated during five evolutionary stages: (i) incipient, (ii) intensifying, (iii) mature, (iv) weakening, and (v) dissipating. The system’s initial development is linked to dynamics at the 250-mb level. The maximum circulation, absolute vorticity, divergence, and height anomalies all occurred at 250 mb during the period of most rapid deepening. Cold anomalies occurred in a deep layer between 850 and 250 mb that tilted eastward with height. Quasigeostrophic analysis showed enhanced vorticity to the west of a thickness trough, a configuration that maintained an area of positive vorticity advection to the west of the surface low and over new convection east and southeast of the low. The vorticity tendency is dominated by the advection of vorticity aloft in this case, especially during the incipient and intensifying stages. The vorticity tendency is dominated by the generation of vorticity by divergence in the lower troposphere. Cloud bands with embedded convective cells formed on the low’s eastern side and propagated eastward, eventually leaving the area of synoptic-scale ascent and losing their convective properties. Areas where the bestlifted index values were less than zero and areas of positive low-level advection of equivalent potential temperature coincided with regions of deep convection, as inferred from satellite imagery.

Ensemble Forecast of a Typhoon Flood Event

Abstract: A high-resolution nested regional spectral model and an ensemble prediction system are combined to forecast the track, intensity, and flooding precipitation arising from Typhoon Winnie of August 1997, which eventually reached supertyphoon status. The prediction of floods is operationally challenging since rainfall distributions can have a high degree of spatial and temporal variability. Rare event probabilities, however, can be estimated more readily via ensemble forecasting. This technique is used to evaluate a typhoon flood event in which rainfall amounts greater than 200 mm led to landslides and major flooding of crops. Seven-member ensembles were generated using an EOF-based technique. An experiment was conducted with a regional model resolution of 0.5° latitude. A Mercator transform grid with a grid mesh size of approximately 55 km in the east–west and 48 km in the north–south was employed. The results indicated very accurate track and intensity forecasts for both the control and ensemble me...

Further Investigation of a Physically Based, Nondimensional Parameter for Discriminating between Locations of Freezing Rain and Ice Pellets

Abstract: The general applicability of an isonomogram developed by Czys and coauthors to diagnose the position of the geographic boundary between freezing precipitation (freezing rain or freezing drizzle) and ice pellets (sleet or snow grains) was tested using a 25-yr sounding database consisting of 1051 soundings, 581 where stations were reporting freezing drizzle, 391 reporting freezing rain, and 79 reporting ice pellets. Of the 1051 soundings, only 306 clearly had an environmental temperature and moisture profile corresponding to that assumed for the isonomogram. This profile consisted of a three-layer atmosphere with 1) a cold cloud layer aloft that is a source of ice particles, 2) a midlevel layer where the temperature exceeds 0°C and ice particles melt, and 3) a surface layer where T < 0°C. The remaining soundings did not conform to the profile either because 1) the freezing precipitation was associated with the warm rain process or 2) the ice pellets formed due to riming rather than melting and refr...

Experimental Test of the Effects of Z–R Law Variations on Comparison of WSR-88D Rainfall Amounts with Surface Rain Gauge and Disdrometer Data

Abstract: Reflectivity factors and rainfall rates found from Level II WSR-88D data for the National Weather Service (NWS) radar in Greer, South Carolina (KGSP), are compared with similar parameters found from disdrometer data collected at the Clemson Atmospheric Research Laboratory. These comparisons are used to determine experimentally the sensitivity of rainfall amounts found from the WSR-88D data to variations in the parameters A and b of the Z–R law (Z = ARb) that is used in analysis of the data. Analyses of data for nine storms in upstate South Carolina are described. These nine cases encompass a variety of rainfall types including stratiform rain, airmass thunderstorms, and strong cold front convective activity. It is found, after correction of the radar reflectivity factors for obvious calibration offset, that the rainfall depths found by radar are in good agreement with those found from the disdrometer when the NWS default values of A and b (A = 300, b = 1.4) are used. If the values of A and b foun...

An Automated System for the Analysis of Gravity Waves and Other Mesoscale Phenomena

Abstract: An automated near-real-time system for the surface analysis of gravity waves and other mesoscale phenomena is developed, tested, and applied to several cases. Five-minute observations from the Automated Surface Observing System (ASOS) network provide the primary source of data for the mesoanalysis system. ASOS time series data are downloaded, subjected to considerable quality control, bandpass filtered, and objectively analyzed using a time-to-space conversion (TSC) adaptation of the traditional Barnes scheme. The resultant analyses, which can resolve features in the ASOS network with wavelengths as short as 150 km and at 15-min intervals, are made available as animated contoured fields. Even though this mesoanalysis system was designed primarily for gravity wave detection, it is capable of resolving other kinds of mesoscale phenomena and allowing the analyst to monitor their changing structure. The effectiveness of the system is demonstrated with two recent events selected from several cases that have been analyzed. The first case consisted of a gravity wave train that propagated through the Ohio River valley and produced multiple precipitation bands. The second event involved a complex family of mesohighs and wake lows associated with a convective system over the southeastern United States. Variations in the surface wind field and precipitation distribution are related to the mesoscale pressure field in both cases. The ability of this mesoanalysis system to monitor mesoscale phenomena resides in the successful application of TSC principles to high temporal resolution surface data. Although the TSC assumption may not be strictly valid in more complex situations, for many applications this mesoanalysis system offers critical information needed for making accurate nowcasts, with the caveat that the means by which ASOS 5-min data are made available can be improved.

The 21 June 1997 Flood: Storm-Scale Simulations and Implications for Operational Forecasting

Abstract: On 20–21 June 1997, a convective outbreak in Nebraska, Iowa, Illinois, and Wisconsin resulted in two fatalities, eight injuries, and approximately $104 million in damage. The majority of the damage ($92 million) was the result of flooding in southeastern Wisconsin owing to nearly 250 mm of rain produced by training convection and to a lesser extent the passage of a persistent, elongated convective system. The flood event is analyzed and storm-scale (5- and 1.67-km grid spacing) resolution model simulations at 0–24-, 12–36-, and 24–48-h ranges are produced to study the evolution and predictability of the rainfall. Synoptic conditions corresponded closely to the mesohigh pattern frequently associated with heavy rainfall events. Despite the recognition by National Weather Service forecasters of the potential for heavy rainfall, uncertainty concerning event magnitude and affected areas, exacerbated by poor operational model guidance, resulted in a failure to issue flash flood watches prior to the ons...

Topographic Distortion of a Cold Front over the Snake River Plain and Central Idaho Mountains

Abstract: The topographic distortion of a cold front over the Snake River Plain (SRP) and central Idaho Mountains on 3 December 1998 is described using high-density surface observations from MesoWest, a collection of meteorological networks over the western United States. Although relatively unperturbed upstream of central Idaho, the cold front became distorted as it was deflected and accelerated up the low-elevation SRP, where a pronounced frontal bulge developed. The speed of the cold front over the SRP was comparable to the magnitude of the postfrontal winds that, due to terrain channeling, were oriented normal to the front. Meanwhile, the front advanced more slowly over the central Idaho mountains and southwest Montana, becoming increasingly diffuse over the former. Eventually, cold air surrounded the central Idaho Mountains and the two portions of the cold front merged over eastern Idaho. The cold front intensified as it moved from the eastern to central SRP, with rapid changes in temperature and pres...

Verification of Supercooled Cloud Water Forecasts with In Situ Aircraft Measurements

Abstract: In situ measurements of temperature (Ta), horizontal wind speed (V), dewpoint (Td), total water content (TWC), and cloud and supercooled cloud water (SCW) events, made during 50 flights from three research field programs, have been compared to forecasts made with the High Resolution Model Application Project version of the Global Environmental Multiscale model. The main purpose of the comparisons was to test the accuracy of the forecasts of cloud and SCW fields. The forecast accuracy for Ta,V, and Td agreed closely with the results from radiosonde‐model validation experiments, implying that the aircraft‐model validation methodology was equally feasible and, therefore, potentially applicable to SCW forecast verifications (which the radiosondes could not validate). The hit rate (HR), false alarm rate (FAR), and true skill statistic (TSS) for cloud forecasts were found to be 0.52, 0.30, and 0.22, respectively, when the model data were inferred at a horizontal resolution of 1.5 km (averaging scale of the aircraft data). The corresponding values for SCW forecasts were 0.37, 0.22, and 0.15, respectively. The HRs (FARs) for cloud and SCW events are sensitive to horizontal resolution and increase to 0.76 (0.50) and 0.66 (0.53), respectively, when a horizontal resolution of 100 km is used. The model TWC was found to agree poorly with aircraft measurements, with the model generally underestimating TWC. For cases when the forecasts and observations of cloud agreed, the SCW-forecast HR, FAR, and TSS were 0.63, 0.22, and 0.41, respectively, which implies that improvement in the model cloud fields would substantially improve the SCW forecast accuracy. The demonstrated comparison methodology will allow a quantitative comparison between different SCW and cloud algorithms. Such a comparison will provide insight into the strengths and weaknesses of these algorithms and will allow the development of more accurate cloud and SCW forecasts.

Correlation of real and model wind speeds in different terrains

Abstract: Wind speeds over a 6-month period from 21 surface stations, 3 upper-wind stations, and 2 different models are compared. Similar data are used for three different topographic regions of New Zealand broadly classed as having low, moderate, and high terrain. Mesoscale model winds are obtained in the three regions over 300 km by 300 km squares. The dataset is at 40-km grid spacing but some 20-km winds are also used. Correlation coefficients among the surface wind speeds decrease more quickly with separation the more mountainous the terrain. Correlation coefficients of observed surface speeds are lower than for model winds but composites of the observed station winds in each region have similar spatial variations to the model winds. An extrapolation technique for estimating the quality of model wind speed fields is inferred.