Steven G. Decker

Temporal–Spatial Scales of Observed and Simulated Precipitation in Central U.S. Climate

Abstract Precipitation intensity spectra for a central U.S. region in a 10-yr regional climate simulation are compared to corresponding observed spectra for precipitation accumulation periods ranging from 6 h to 10 days. Model agreement with observations depends on the length of the precipitation accumulation period, with similar results for both warm and cold halves of the year. For 6- and 12-h accumulation periods, simulated and observed spectra show little overlap. For daily and longer accumulation periods, the spectra are similar for moderate precipitation rates, though the model produces too many low-intensity precipitation events and too few high-intensity precipitation events for all accumulation periods. The spatial correlation of simulated and observed precipitation events indicates that the models 50-km grid spacing is too coarse to simulate well high-intensity events. Spatial correlations with and without very light precipitation indicate that coarse resolution is not a direct cause of excessi...

A Comparison of CMIP3 Simulations of Precipitation over North America with Observations: Daily Statistics and Circulation Features Accompanying Extreme Events

AbstractClimate model simulations of daily precipitation statistics from the third phase of the Coupled Model Intercomparison Project (CMIP3) were evaluated against precipitation observations from North America over the period 1979–99. The evaluation revealed that the models underestimate the intensity of heavy and extreme precipitation along the Pacific coast, southeastern United States, and southern Mexico, and these biases are robust among the models. The models also overestimate the intensity of light precipitation events over much of North America, resulting in fairly realistic mean precipitation in many places. In contrast, heavy precipitation is simulated realistically over northern and eastern Canada, as is the seasonal cycle of heavy precipitation over a majority of North America. An evaluation of the simulated atmospheric dynamics and thermodynamics associated with extreme precipitation events was also conducted using the North American Regional Reanalysis (NARR). The models were found to captur...

A Local Energetics Analysis of the Life Cycle Differences between Consecutive, Explosively Deepening, Continental Cyclones

Local energetics diagnostics of the life cycles of consecutive, explosively deepening, extratropical cyclones that migrated across central North America in April 2001 are presented. Both storms developed rapidly and followed nearly identical tracks through the region. Despite similar mature-stage intensities, the two storms underwent vastly different evolutions during cyclolysis; the first decayed as rapidly as it had developed, and the second decayed very slowly. Examination of the volume-integrated eddy kinetic energy (EKE) budget for each storm reveals that the sea level pressure minimum associated with the first cyclone developed well after its associated EKE center had reached its maximum intensity. In contrast, the second cyclone’s sea level pressure minimum developed much more in concert with the development of its associated EKE center. As a consequence, the first cyclone began losing EKE through downstream energy fluxes even as it was developing at the surface, whereas the second cyclone did not disperse EKE downstream until later in its life cycle. Consideration of the EKE budget results in terms of baroclinic wave packets demonstrates that the first cyclone developed and decayed on the upstream edge of a wave packet, whereas the second cyclone developed in the midst of a wave packet, only decaying once it had reached the upstream edge. Thus, it is suggested that postmature phase decay is dynamically linked to a cyclone’s position in a given wave packet.

THE LOCAL WINTER STORM SCALE A Measure of the Intrinsic Ability of Winter Storms to Disrupt Society

A local winter storm scale (LWSS) is developed to categorize the disruption caused by winter storms using archived surface weather observations from a single location along the U.S. East Coast. Development of LWSS is motivated by the recognition that the observed societal impact from a given winter storm (called realized disruption here) arises from the convolution of two factors, the meteorological conditions that lead to disruption (i.e., intrinsic disruption) and societys susceptibility to winter weather. LWSS is designed to measure the first factor, intrinsic disruption. The scale uses maximum sustained winds, wind gusts, storm-total snowfall and icing accumulations, and minimum visibility to arrive at a categorical value between 0 and 5 inclusive. An alternate method is used to quantify the realized disruption that each storm produced and helps calibrate aspects of LWSS. All winter storms observed at Newark Liberty International Airport over the 15 cold seasons between 1995/96 and 2009/10 were categ...

Impact of Coupling an Ocean Model to WRF Nor’easter Simulations

AbstractThe impact of ocean–atmosphere coupling and its possible seasonal dependence upon Weather Research and Forecasting (WRF) Model simulations of seven, wintertime cyclone events was investigated. Model simulations were identical aside from the degree of ocean model coupling (static SSTs, 1D mixed layer model, full-physics 3D ocean model). Both 1D and 3D ocean model coupling simulations show that SSTs following the passage of a nor’easter did tend to cool more strongly during the early season (October–December) and were more likely to warm late in the season (February–April). Model simulations produce SST differences of up to 1.14 K, but this change did not lead to significant changes in storm track ( 1) and have low-to-moderate threat scores (0.31–0.59). Analysis of the storm enviro...

Unexpected High Winds in Northern New Jersey: A Downslope Windstorm in Modest Topography

AbstractThis study presents the first evidence for the occurrence of a downslope windstorm in New Jersey. During the early morning hours of 4 January 2009, an unanticipated strong wind event was observed. Despite a zone forecast calling for winds less than 4 m s−1 issued 4 h prior to the event, winds up to 23 m s−1 were reported at High Point, New Jersey (elevation 550 m), with gusts to 30 m s−1 in its immediate lee (elevation 311 m). These winds were highly localized; a nearby Automated Surface Observing System (ASOS) station (Sussex, New Jersey, 12 km distant) reported calm winds between 0700 and 1000 UTC, just as the winds were peaking near High Point. High Point is the highest point in New Jersey, and is part of the quasi-two-dimensional Kittatinny Mountain extending from Pennsylvania into New York. This study tests the hypothesis that the topography of High Point, upon interacting with the local atmospheric stability and wind profiles, was sufficient to produce a downslope windstorm, thus causing the...

Influence of Bulk Microphysics Schemes upon Weather Research and Forecasting (WRF) Version 3.6.1 Nor'easter Simulations

This study evaluated the impact of five, single- or double- moment bulk microphysics schemes (BMPSs) on Weather Research and Forecasting model (WRF) simulations of seven, intense winter time cyclones impacting the Mid-Atlantic United States. Five-day long WRF simulations were initialized roughly 24 hours prior to the onset of coastal cyclogenesis off the North Carolina coastline. In all, 35 model simulations (5 BMPSs and seven cases) were run and their associated microphysics-related storm properties (hydrometer mixing ratios, precipitation, and radar reflectivity) were evaluated against model analysis and available gridded radar and ground-based precipitation products. Inter-BMPS comparisons of column-integrated mixing ratios and mixing ratio profiles reveal little variability in non-frozen hydrometeor species due to their shared programming heritage, yet their assumptions concerning snow and graupel intercepts, ice supersaturation, snow and graupel density maps, and terminal velocities lead to considerable variability in both simulated frozen hydrometeor species and radar reflectivity. WRF-simulated precipitation fields exhibit minor spatio-temporal variability amongst BMPSs, yet their spatial extent is largely conserved. Compared to ground-based precipitation data, WRF-simulations demonstrate low-to-moderate (0.217-0.414) threat scores and a rainfall distribution shifted toward higher values. Finally, an analysis of WRF and gridded radar reflectivity data via contoured frequency with altitude (CFAD) diagrams reveals notable variability amongst BMPSs, where better performing schemes favored lower graupel mixing ratios and better underlying aggregation assumptions.

Development and Analysis of a Probabilistic Forecasting Game for Meteorology Students

Calls for moving from a deterministic to a probabilistic view of weather forecasting have become increasingly urgent over recent decades, yet the primary national forecasting competition and many in-class forecasting games are wholly deterministic in nature. To counter these conflicting trends, a long-running forecasting game at Rutgers University has recently been modified to become probabilistic in nature. Students forecast high- and low-temperature intervals and probabilities of precipitation for two locations: one fixed at the Rutgers cooperative observing station, the other chosen for each forecast window to maximize difficulty. Precipitation errors are tabulated with a Brier score, while temperature errors contain a sharpness component dependent on the width of the forecast interval and an interval miss component dependent on the degree to which the verification falls within the interval. The inclusion of a probabilistic forecasting game allows for the creation of a substantial database of forecasts...

Nonlinear Balance in Terrain-Following Coordinates

Abstract Potential vorticity (PV) is a powerful concept in geophysical fluid dynamics. One property of PV that makes it so powerful is that it may be inverted under certain conditions, one of which is the imposition of a balance constraint. Previous studies have made use of a particular nonlinear balance constraint suited to isobaric coordinates as part of their inversion procedures. The present study constructs and tests a new nonlinear balance constraint that may be applied directly to the output of the Weather Research and Forecasting (WRF) model on its native terrain-following vertical coordinate. Output from the nonlinear balance operator is examined in the context of idealized and real-data WRF forecasts, and the results indicate that the simplifications necessary to derive the nonlinear balance operator are justified on the synoptic and meso-α scales. On the other hand, once the scales resolved by the model are small enough, neglected terms reach magnitudes on the order of the retained terms, even ...