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Bradley R. Colman

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Bradley R. Colman

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The second of two papers describing thunderstorms that occur above frontal surfaces, frequently in environments without positive convective available potential energy (CAPE), focuses on an impressive outbreak of elevated thunderstorms during AVE-SESAME I. It is shown that the thunderstorms occurred in three convective impulses, each of which developed in the warm sector before propagating onto the frontal surface; subsequent thunderstorms developed over the frontal surface. While in the warm sector, the convection was supported by an extremely unstable boundary layer. However, this convective energy quickly diminished above the frontal surface and thunderstorms continued and developed for many hours in an essentially stable hydrostatic environment. During the lifetime of these impulses, mesoscale updrafts developed and moved with the convective areas, maintaining nearly steady-state systems with strong low-level inflow. The environment was found to be symmetrically neutral in the region of the inflow. Numerous pressure waves were observed in association with the elevated thunderstorms, yet thew features were evidently not important in triggering of the storms. An investigation of a convective band that formed above the frontal surface revealed that the development probably took place in two steps. Initially, high θe air overlying the frontal inversion was stable to vertical displacements, but inertially unstable. Then, along the instantaneous path of the unstable parcel, the thermodynamic structure changed, the parcel became gravitationally unstable, and upright convection resulted.

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Bradley R. Colman

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The first of two papers describing thunderstorms that occur above frontal surfaces, frequently in environments without positive convective available potential energy (CAPE), focuses on the climatology of such storms for the conterminous United States. The dataset used consists of 1093 observations made over a 4-year period. The events were selected using conventional network data and a set of criteria that eliminated thunderstorms rooted in the boundary layer. A composite of the dataset shows that the typical “elevated” thunderstorm occurs northeast of an associated surface low-pressure center, and north of a surface warm front in a region with northeasterly surface winds. The planetary boundary layer is generally very stable as determined by comparisons with both the 50-kPa and 85-kPa air. The thunderstorms are usually found in the left exit region of a low-level wind maximum (an area of horizontal deformation). The large-scale environment is strongly baroclinic with large vertical wind shear and warm advection. Several of the identified characteristics suggest that frequently elevated thunderstorms are the result of physical mechanisms different from those fundamental to surface-based thunderstorms. The most striking of these is that for elevated thunderstorms there is generally very little, if any, positive CAPE in the environment, as the atmosphere is slightly more stable than moist adiabatic above the frontal inversion. The annual frequency distribution of elevated thunderstorms is bimodal, with a primary peak in April and a secondary peak in September. The events are concentrated in an area extending northward from the central Gulf Coast along the Mississippi River valley. The data further show that nearly all winter-season (December through February) thunderstorms east of the Rocky Mountains are of the elevated type. The primary exception involves those over the Florida Peninsula, where surface-based convection persists throughout the year. Most of the winter-season elevated thunderstorms occur near the Gulf Coast downstream from migrating cyclones.

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Stanley L. Barnes and Bradley R. Colman

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A major snowstorm in Colorado is considered in order to demonstrate the utility of a quasigeostrophic (QG) diagnostic scheme that is capable of separating from the total QG forcing field the cross-isentrope, ageostrophic circulations associated with jet-streak dynamics. The storm did not develop as a consequence of typical baroclinic wave development but instead developed in association with a previously cutoff cyclone. It posed a perplexing forecast problem to Denver area forecasters. It is discovered that at least 12 h before the onset of cyclogenesis there existed QG signatures (computed from rawinsonde data) of the thermally direct-indirect circulations associated with a jet-level wind maximum. These circulations are known to be associated with tropopause folding and the descent of stratospheric potential vorticity into the midtroposphere. It is verified that indeed such a process took place by tracking maxima of potential vorticity on an isentropic surface (295 K) that extended into the midtroposphere. Using analyses of lapse rate and mixing ratio near a “dry slot” in satellite water vapor imagery, our interpretation of the QG signatures are confirmed. The diagnostic scheme can be of value to forecasters who daily must adapt their knowledge of conceptual cyclone models to ascertain the dynamic potential of threatening storms.

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Louisa B. Nance and Bradley R. Colman

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Severe downslope windstorms are a mesoscale, primarily wintertime, phenomenon that affect regions in the lee of large mountain ranges. The resolution of current weather prediction models is too coarse to explicitly predict downslope windstorms. Hence, additional operational tools are needed for making downslope windstorm forecasts. Current windstorm forecast techniques commonly utilize a tool referred to as a “decision tree.” Although decision trees provide valuable guidance, operational forecasters have not found this type of tool to be highly reliable. With recent advances in computer technology, a new type of operational tool is available for forecasting downslope windstorms: two-dimensional, nonlinear, mesoscale numerical models. This study investigates whether this type of model, initialized with upstream profiles taken from operational Eta Model forecasts, can produce accurate downslope windstorm forecasts.

Numerical simulations for high-wind events that affected seven regions in the United States between January 1993 and April 1997 indicate this tool is able to produce lee-slope wind speeds that meet the local peak gust threshold for a High Wind Warning for a majority of those cases where observed winds met this threshold. These simulations were initialized with upstream soundings taken from the 12- and 18-h Eta forecasts valid at the time of each high-wind event. A comparison for one region between the number of events for which High Wind Watches were posted and the number of events for which the two-dimensional model prediction met the peak gust threshold suggests this new tool would be a definite improvement over the current forecast technique. On the other hand, a preliminary test of the model’s ability to differentiate between windstorm and nonwindstorm events suggests the false warning rate for this tool may be high. Further testing of this tool is ongoing and will continue through the winter months of 2000/01.

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Stanley L. Barnes and Bradley R. Colman

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A quasigeostrophic (QG) diagnostic model is used to evaluate the nested grid model's (NGM) predictions for a December cyclone whose impact on northeastern Colorado was underpredicted. Although the NGM predicted deepening of the associated 500-mb low, the model was 12 h slow in the onset of deepening and moved the storm too far east too quickly. Synthetic soundings, generated from 12-h predicted data initialized 24 h before cyclogenesis became apparent, were submitted to the same QG diagnostic algorithms used to analyze verifying rawinsonde data. Comparisons reveal that the NGM apparently 1) transported too much potential vorticity, westerly momentum, and cold air into the lower troposphere along the axis of the jet stream; 2) moved the first of two short-wavelength jet streaks too far northeastward and with too much strength; 3) failed to predict the strength of the following jet maximum; and 4) failed to develop an apparent tropopause fold. It is established that these errors were not caused by obvious discrepancies in the model's initialization. Through inference, the errors could have been caused by rapid growth of subtle, undetected initialization errors or by the model's inadequate parameterization of some physical process—perhaps of turbulent dissipation over mountainous terrain. Diagnosis of the model's subsequent initialization (12 h after its first erroneous prediction) indicates that the model did not have available crucial Mexican soundings that might have prevented it from making a similar error in predicting the position and strength of the then-intensifying cyclone. The diagnostic results could have alerted forecasters not only to the presence of the complex jet stream but also to the extent and intensity of its associated tropopause fold. Furthermore, QG diagnostics can alert forecasters to model errors that are not made obvious by conventional model comparisons.

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Bradley R. Colman and Carl F. Dierking

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The purpose of this study is to investigate the occurrence of severe winds in southeast Alaska (locally known as Taku winds) based on recent theoretical advances in the understanding of severe downslope windstorms. We found that the Taku wind is a manifestation of an amplified mountain wave. A complicating factor in understanding the Taku is the coincident occurrence of gap flow. Analysis of a number of historical events, in addition to a unique set of wind records from a nearby ridge, shows the separate identity of these concurrent phenomena. A set of criteria is identified that must be fulfilled in order for the downslope winds to occur, which is much more restrictive than the conditions necessary for gap flow. The three necessary criteria are 1) an inversion at or just above ridgetop, somewhere between 1500 and 2000 m MSL, 2) strong cross-barrier flow near ridgetop, typically 15–20 m s−1 in geostrophic wind speed, and 3) cross-barrier flow decreasing with height to a critical level somewhere between 3000 and 5500 m MSL. The similarities to other local downslope windstorms are also discussed.

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William M. Whitney, Robert L. Doherty, and Bradley R. Colman

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The Puget Sound Convergence Zone (PSCZ) is a terrain-induced mesoscale phenomenon that occurs in western Washington and has a dramatic impact on local weather. This paper presents the operational forecasting techniques that are used at the National Weather Service Forecast Office in Seattle to forecast the weather associated with the PSCZ. A case study is used to demonstrate both the medium-range and short-range techniques. Future considerations are also discussed in light of the planned modernization of the National Weather Service.

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Josephine R. Brown, Scott B. Power, Francois P. Delage, Robert A. Colman, Aurel F. Moise, and Bradley F. Murphy

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Understanding how the South Pacific convergence zone (SPCZ) may change in the future requires the use of global coupled atmosphere–ocean models. It is therefore important to evaluate the ability of such models to realistically simulate the SPCZ. The simulation of the SPCZ in 24 coupled model simulations of the twentieth century is examined. The models and simulations are those used for the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC). The seasonal climatology and interannual variability of the SPCZ is evaluated using observed and model precipitation. Twenty models simulate a distinct SPCZ, while four models merge intertropical convergence zone and SPCZ precipitation. The majority of models simulate an SPCZ with an overly zonal orientation, rather than extending in a diagonal band into the southeast Pacific as observed. Two-thirds of models capture the observed meridional displacement of the SPCZ during El Niño and La Niña events. The four models that use ocean heat flux adjustments simulate a better tropical SPCZ pattern because of a better representation of the Pacific sea surface temperature pattern and absence of cold sea surface temperature biases on the equator. However, the flux-adjusted models do not show greater skill in simulating the interannual variability of the SPCZ. While a small subset of models does not adequately reproduce the climatology or variability of the SPCZ, the majority of models are able to capture the main features of SPCZ climatology and variability, and they can therefore be used with some confidence for future climate projections.

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EXECUTIVE COMMITTEE, Eugene M. Rasmusson, George L. Frederick Jr., Ronald D. McPherson, Paul D. Try, Susan K. Avery, Bradley R. Colman, Richard E. Hallgren, and Kenneth C. Spengler
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