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William A. Gallus Jr.

Abstract

A two-dimensional cloud model is used to investigate whether microphysical processes alone within the stratiform rain regions of mesoscale convection systems can induce strong descent and intense surface wake lows accompanying such systems. Idealized simulations are run with a domain that captures the back edge of the stratiform rain region. A simplified microphysical field, representing snow alone, is prescribed within the stratiform cloud to produce radar reflectivities similar to observations. When the prescribed snow field is assumed time-independent, strong subsidence develops but does not induce an intense wake low since microphysical cooling strongly opposes adiabatic warming. Simply increasing snow quantities, although resulting in heavier rain rates and stronger subsidence, does not produce significant pressure falls. However, when precipitation rates are prescribed to decrease with time as might occur with collapsing precipitation cores, subsidence induces greater pressure falls, and a tighter pressure gradient near the wake low, in better agreement with observations.

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William A. Gallus Jr.

Abstract

A 10-km-grid-spacing version of NCEP's Eta Model was used to simulate 11 warm-season convective systems occurring over the U.S. upper midwest. Quantitative precipitation forecasts (QPFs) from the model valid for 6-h periods were verified using 4-km-grid-spacing stage-IV precipitation estimates. Verification first was performed on the model's 10-km grid by areally averaging the 4-km observations onto the model grid. To investigate and quantify the impact of the verification grid-box size on some standard skill scores, verification was also performed by averaging the 10-km model forecasts onto 30-km grid boxes and then areally averaging the observations onto the same 30-km grid. As a final test of the impact of the verifying grid-box size, the same 11 events were simulated with a 30-km version of the Eta Model, with verification then being performed on this 30-km grid. For all cases in both the 10- and 30-km versions of the model, 12 variations of the model were used, with variations involving either (i) modifications to the initial conditions to better represent mesoscale features present at the initialization time or (ii) changes in moist physics. Equitable threat scores (ETSs) increased when verification occurred on a coarser grid, whether the coarser grid was created by averaging the 10-km model results or was that used in the 30-km model runs. This result suggests that it may be difficult to show improved skill scores as model resolution improves if the verification is performed on the model's own increasingly fine grid. It should be noted, however, that the use of different verification resolutions does not change the general impacts on ETSs of variations in model physics or initial conditions. The sensitivity of ETSs to verifying grid-box size does, however, vary somewhat between model variants using different model moist-physics formulations or initialization procedures.

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William A. Gallus Jr.

Abstract

Simulations were performed using the Eta Model with its eta vertical coordinate and stepwise treatment of terrain, and with a substitution of the terrain-following sigma vertical coordinate to investigate the impact of step orography on flow near high mountains. Two different cases were simulated: (i) a downslope windstorm along the Front Range of the Rocky Mountains, and (ii) stably stratified flow blocked by high mountains in Taiwan. Flow separation on the lee side of the mountains, previously shown to occur in idealized two-dimensional Eta simulations, was also apparent in these real data cases, even for the downslope wind event. The step orography resulted in a substantial underestimate of wind speeds to the lee of the Rockies during the windstorm. Near the surface, both the eta and sigma simulations of the Taiwan blocking event were comparable. For both events, the use of step orography resulted in much weaker mountain waves than occurred when the sigma vertical coordinate was used. Localized vertical velocity perturbations associated directly with the step corners were minor for these cases.

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William A. Gallus Jr.

Abstract

A versatile workstation version of the NCEP Eta Model is used to simulate three excessive precipitation episodes in the central United States. These events all resulted in damaging flash flooding and include 16–17 June 1996 in the upper Midwest, 17 July 1996 in western Iowa, and 27 May 1997 in Texas. The episodes reflect a wide range of meteorological situations: (i) a warm core cyclone in June 1996 generated a meso-β-scale region of excessive rainfall from echo training in its warm sector while producing excessive overrunning rainfall to the north of its warm front, (ii) a mesoscale convective complex in July 1996 produced excessive rainfall, and (iii) tornadic thunderstorms in May 1997 resulted in small-scale excessive rains.

Model sensitivity to horizontal resolution is investigated using a range of horizontal resolutions comparable to those used in operational and quasi-operational forecasting models. Sensitivity tests are also performed using both the Betts–Miller–Janjic convective scheme (operational at NCEP in 1998) and the Kain–Fritsch scheme. Variations in predicted peak precipitation as resolution is refined are found to be highly case dependent, suggesting forecaster interpretation of increasingly higher resolution model quantitative precipitation forecast (QPF) information will not be straightforward. In addition, precipitation forecasts and QPF response to changing resolution are both found to vary significantly with choice of convective parameterization.

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William A. Gallus Jr.

Abstract

Both the Method for Object-based Diagnostic Evaluation (MODE) and contiguous rain area (CRA) object-based verification techniques have been used to analyze precipitation forecasts from two sets of ensembles to determine if spread-skill behavior observed using traditional measures can be seen in the object parameters. One set consisted of two eight-member Weather Research and Forecasting (WRF) model ensembles: one having mixed physics and dynamics with unperturbed initial and lateral boundary conditions (Phys) and another using common physics and a dynamic core but with perturbed initial and lateral boundary conditions (IC/LBC). Traditional measures found that spread grows much faster in IC/LBC than in Phys so that after roughly 24 h, better skill and spread are found in IC/LBC. These measures also reflected a strong diurnal signal of precipitation. The other set of ensembles included five members of a 4-km grid-spacing WRF ensemble (ENS4) and five members of a 20-km WRF ensemble (ENS20). Traditional measures suggested that the diurnal signal was better in ENS4 and spread increased more rapidly than in ENS20.

Standard deviations (SDs) of four object parameters computed for the first set of ensembles using MODE and CRA showed the trend of enhanced spread growth in IC/LBC compared to Phys that had been observed in traditional measures, with the areal coverage of precipitation exhibiting the greatest growth in spread with time. The two techniques did not produce identical results; although, they did show the same general trends. A diurnal signal could be seen in the SDs of all parameters, especially rain rate, volume, and areal coverage. MODE results also found evidence of a diurnal signal and faster growth of spread in object parameters in ENS4 than in ENS20.

Some forecasting approaches based on MODE and CRA output are also demonstrated. Forecasts based on averages of object parameters from each ensemble member were more skillful than forecasts based on MODE or CRA applied to an ensemble mean computed using the probability matching technique for areal coverage and volume, but differences in the two techniques were less pronounced for rain rate and displacement. The use of a probability threshold to define objects was also shown to be a valid forecasting approach with MODE.

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John Lawson and William A. Gallus Jr.

Abstract

Bow echo structures, a subset of mesoscale convective systems (MCSs), are often poorly forecast within deterministic numerical weather prediction model simulations. Among other things, this may be due to the inherent low predictability associated with bow echoes, deficient initial conditions (ICs), and inadequate parameterization schemes. Four different ensemble configurations assessed the sensitivity of the MCSs’ simulated reflectivity and radius of curvature to the following: perturbations in initial and lateral boundary conditions using a global dataset, different microphysical schemes, a stochastic kinetic energy backscatter (SKEB) scheme, and a mix of the previous two. One case is poorly simulated no matter which IC dataset or microphysical parameterization is used. In the other case, almost all simulations reproduce a bow echo. When the IC dataset and microphysical parameterization is fixed within a SKEB ensemble, ensemble uncertainty is smaller. However, while differences in the location and timing of the MCS are reduced, variations in convective mode remain substantial. Results suggest the MCS’s positioning is influenced primarily by ICs, but its mode is most sensitive to the model error uncertainty. Hence, correct estimation of model error uncertainty on the storm scale is crucial for adequate spread and the probabilistic forecast of convective events.

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Brian J. Squitieri and William A. Gallus Jr.

Abstract

An error was discovered in the code used to calculate neighborhood equitable threat scores (nETSs) in Squitieri and Gallus. Replicating results with the error corrected revealed that most of the conclusions from Squitieri and Gallus remained the same, but with one significant new finding and one notable change in results. In the original manuscript, very few correlations between MCS QPF skill and LLJ forecast accuracy could be denoted among weakly forced cases, with none of them being statistically significant. Applying the aforementioned correction, it was found that QPF skill during the mature stage of MCSs significantly correlated with moisture forecast accuracy within developing LLJs for weakly forced events. It was also found that correlations between MCS QPF skill and LLJ potential temperature forecast accuracy occurred earlier in the evening.

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William A. Gallus Jr. and Richard H. Johnson

Abstract

A dynamic version of the two-dimensional kinematic cloud model of Rutledge and Houze is used to study the individual roles of hydrometeor and heat advection from the convective line to the stratiform region of the 10–11 June 1985 PRE-STORM squall line. The design of the model allows for specified inputs of hydrometeors, heat, and water vapor from the convective line.

Convective heating alone generates significant ascent and condensate in the anvil; however, surface rainfall is scarce without the advection of hydrometeors from the convective line. Hydrometeor advection alone does not produce strong ascent in the anvil cloud, implying the important additive effects of both heat and hydrometeor advection in generating broad regions of significant stratiform rainfall.

The roles of individual microphysical processes within the stratiform region are examined, along with the sensitivity of stratiform region dynamics to postconvective region environmental conditions. Of the processes evaporation, melting, and sublimation—evaporation in mesoscale downdrafts is the most important affecting the intensity of circulations such as the rear-inflow jet and precipitation—followed by melting and then sublimation. Stability strongly controls vertical motion in the stratiform region. Greater instability increases in situ production of condensate, surface rainfall, and low-level drying within the mesoscale downdraft. The intensity of hydrometeor advection from convective elements significantly influences surface rain rates in the stratiform region. These findings suggest that natural variability arising from three-dimensional convective line structures and inhomogeneities in the environment can induce significant hydrometeor and now perturbations (asymmtries) in the trailing stratiform regions of squall lines.

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William A. Gallus Jr. and Moti Segal

Abstract

The impact of diabatic heating on late winter frontogenesis is evaluated both through conceptual scaling and the use of high-resolution Eta Model simulations of a strong but relatively dry cold surface front that occurred during the Storm-scale Operational Research Meteorology Fronts Experiment Systems Test (STORMFEST) project. Although skies were clear ahead of the front, it was trailed by an extensive area of cloud cover that influenced frontal strength during the daylight hours by reducing solar insolation and sensible heat flux.

An Eta control simulation of the event agreed reasonably well with observations and indicated intensification of the frontal temperature gradient during the daytime with a weakening at night. Additional simulations have been done to investigate sensitivity to several diabatic processes. These tests include the role of cloud shading on surface sensible heat flux, the role of soil moisture in the warm sector, and the role of evaporative cooling of precipitation in the light precipitation area behind the cold front. All of these diabatic processes have a measurable impact on the front, although soil moisture and cloud shading appear to play the most important roles. The moisture and static stability of the frontal environment were unfavorable for precipitation along the front, and the increase in frontal strength due to reduced surface sensible heat flux from extensive cloud shading behind the front did not significantly influence near-front precipitation for this event.

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William A. Gallus Jr. and Moti Segal

Abstract

High-resolution Eta Model simulations of a strong but relatively dry late winter surface cold front that occurred during the STORM-FEST project depicted a pronounced acceleration of the front during the afternoon hours over the southern end of Lake Michigan. In this note, the impact of the lake on the front is examined. Reduced lower atmosphere turbulence due to both thermal stabilization and diminished surface roughness acting on postfrontal northerly winds increased frontogenesis strongly over the lake. The enhanced frontal circulation increased the front speed so that a noticeable frontal bulge occurred over the southern end of Lake Michigan. Some observational evidence is available to support the simulated frontal acceleration.

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