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Moti Segal
and
Graham Feingold

Abstract

The potential impact of daytime local summer convective cloud systems on shelter air temperature is illustrated by numerical modeling and observations. Prolonged reductions in surface solar irradiance due to cloudiness result in a noticeable decrease in shelter temperature over drylands and a moderate temperature fall over wet surfaces. When cloudiness is abruptly diminished, shelter temperature increases rapidly. Numerical modeling of downdrafts associated with rainfall in a dry convective atmosphere indicates a pronounced drop in shelter temperature (as high as 12°C). The modeling results are consistent with observations. The significance of the results and their potential applications are outlined.

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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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Zaitao Pan
,
Moti Segal
, and
Charles Graves

Abstract

Characteristics of surface water vapor deposition (WVD) over the continental United States under the present climate and a future climate scenario reflecting the mid-twenty-first-century increased greenhouse gas concentrations were evaluated by using a regional climate model forced by initial and lateral boundary conditions generated by a GCM. Simulated seasonal WVD frequency and daily amounts are presented and elaboration on their relation to potential surface dew/frost is also provided. The climate scenario showed in winter a noticeable decline in WVD frequency over snow-covered areas in the Midwest and over most of the elevated terrain in the western United States, contrasted by an overall increase in the eastern United States. In summer, a decline in frequency was simulated for most of the United States, particularly over the mountains in the west. A spatially mixed trend of change in the frequency was indicated in spring and fall. The trend of change in WVD amount resembled that of the frequency in summer, whereas a largely reversed relation was shown in winter. Quantitatively, changes in frequency and amount of WVD in the range of −30% to +30% generally were indicated for all locations and seasons, except for the western half of the United States, where the change was larger in summer. While areas passing a local statistical test on WVD changes ranged from 11% to 36% of land domain, the WVD differences as a whole field between present climate and future scenarios are significant.

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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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William A. Gallus Jr.
and
Moti Segal

Abstract

A 10-km version of the NCEP Eta Model has been run over a roughly 1000 km × 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.

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William A. Gallus Jr.
and
Moti Segal

Abstract

The impact of soil moisture on the forecast of a small-scale convective system, and sensitivity of results to the convective parameterization used, are investigated through Eta Model simulations (run in an operational-like setting) of a convective system occurring on 27 May 1997 in Texas. The event was influenced by a southwestward-propagating gravity wave from early morning convection in Arkansas that intersected a slow-moving cold front, releasing extreme conditional static instability. Isolated heavy rainfall, over 100 mm, occurred in some regions.

A control simulation with 22-km horizontal resolution reasonably simulated the event, even though mesoscale influences such as the gravity wave important to this event are often poorly captured by numerical models. A series of sensitivity tests were performed to examine the impact of soil moisture on the simulations. Two different convective parameterizations were used for the tests. Although domain average precipitation is found to generally vary in a straightforward way with soil moisture, peak precipitation in the regions of intense convection shows more complex behavior. Sensitivity of precipitation amounts to soil moisture differs significantly among runs having different convective parameterizations. For instance, with the Kain–Fritsch convective scheme, relatively dry soil is found to result in stronger convective outflows that converge with stronger ambient flow to greatly enhance the precipitation in the region where heaviest rainfall occurs. With the Betts–Miller–Janjic scheme, drier soil generally results in less precipitation than in the control run, although some enhancement in peak amount does occur within a narrow range of drying. The differences between the peak quantitative precipitation forecasts in the runs is primarily due to the inclusion of a convective downdraft in the Kain–Fritsch parameterization, and its impact on secondary convective development.

Additional sensitivity tests find limited impact from prescribed vegetation coverage. A final sensitivity test shows that precipitation amounts are even more strongly affected by the vertical resolution of the data used to initialize the shallow but moist boundary layer than by variations in the soil moisture or vegetation fraction.

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William A. Gallus Jr.
and
Moti Segal

Abstract

The likelihood of simulated rainfall above a specified threshold being observed is evaluated as a function of the amounts predicted by two mesoscale models. Evaluations are performed for 20 warm-season mesoscale convective system events over the upper Midwest of the United States. Simulations were performed using 10-km versions of the National Centers for Environmental Prediction Eta Model and the Weather Research and Forecasting (WRF) model, with two different convective parameterizations tested in both models. It was found that, despite large differences in the biases of these different models and configurations, a robust relationship was present whereby a substantial increase in the likelihood of observed rainfall exceeding a specified threshold occurred in areas where the model runs forecast higher rainfall amounts. Rainfall was found to be less likely to occur in those areas where the models indicated no rainfall than it was elsewhere in the domain; it was more likely to occur in those regions where rainfall was predicted, especially where the predicted rainfall amounts were largest. The probability of rainfall relative-operating-characteristic and relative-operating-level curves showed that probabilistic forecasts determined from quantitative precipitation forecast values had skill comparable to the skill obtained using more traditional methods in which probabilities are based on the fraction of ensemble members experiencing rainfall. When the entire sample of cases was broken into training and test sets, the probability forecasts of the test sets evidenced good reliability. The relationship noted should provide some additional guidelines to operational forecasters. The results imply that the tested models are better able to indicate the regions where atmospheric processes are most favorable for convective rainfall (where the models generate enhanced amounts) than they are able to predict accurately the rainfall amounts that will be observed.

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Zaitao Pan
,
Moti Segal
, and
Raymond W. Arritt

Abstract

Regional model sensitivity simulations in which the height of elevated terrain was reduced to explore simulated changes in features of the low-level jet (LLJ) are presented. Such an approach has not been reported, and it provides complementary insight to the previous LLJ studies. The simulations were carried out for a 45-day period during the 1993 summer flood in the central United States, when strong LLJs were frequent. The simulations illustrate directly the significance of topographical blocking, leeside cyclogenesis, and terrain thermal effects exerted by the Rocky Mountains in support of LLJ formation. In particular, it is shown that in the absence of topography the ridging from the Bermuda high extended considerably westward with weaker southerly flow over the High Plains, thus diminishing the potential for LLJ development. The slope-induced nocturnal horizontal thermal gradient was indicated to have a significant role in the formation of the LLJ.

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Moti Segal
,
Zaitao Pan
, and
Raymond W. Arritt

Abstract

Impacts of diurnal radiative forcing on flow and rainfall patterns during summer flood and drought conditions (1993 and 1988, respectively) in the central United States were investigated using a regional climate model. The modeling approach, which included evaluation of sensitivity to modification in the solar hour, enabled evaluation of the impact on an event-by-event basis. The effect of the solar hour forward shift of 12 h on boundary layer wind speed over north-central Texas, which is often related to rainfall in the central United States through northward moisture advection, followed almost exactly the shift in solar hour. Domain-averaged daily rainfall in the central United States simulated with 12-h solar shift frequently showed in the flood year a backward or forward time shift of ∼12 h in the timing of its peak, an increase or decrease of rainfall rate, and on a few occasions noticeable formation of short-lived rainfall events. This pattern suggests relatively high sensitivity to the timing of the diurnal radiative forcing with respect to the large-scale perturbations. In contrast, in the drought year 12-h solar shifted simulations these modifications were weaker. The climatological domain-average diurnal cycle of rainfall showed for the flood year a well-defined 12-h shift when comparing the control and perturbed simulations. In contrast, in the drought year such a shift was not well defined.

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Zaitao Pan
,
Eugene Takle
,
Moti Segal
, and
Richard Turner

Abstract

The sensitivities of soil moisture impacts on summer rainfall in the central United States to different commonly used cumulus parameterization and surface flux schemes are examined using the PSU-NCAR MMS under different atmospheric and soil moisture conditions. The cumulus convection schemes used are the Kuo and Grell parameterization schemes, while the surface-moisture flux schemes used are the aerodynamic formulation and the Simple Biosphere (SiB) Model. Results show that a transient increase in soil moisture enhanced total rainfall over the simulation domain. The increase in soil moisture enhanced local rainfall when the lower atmosphere was thermally unstable and relatively dry, but it decreased the rainfall when the atmosphere was humid and lacked sufficient thermal forcing to initiate deep convection. Soil moisture impacts were noticeably stronger for the Kuo scheme, which simulated lighter peak rainfall, than those for the Grell scheme, which simulated heavier peak rainfall. The greater sensitivity to soil moisture exhibited by the Kuo scheme than either the Grell or explicit scheme implies that it exaggerated the role of soil moisture. This difference was related to how each scheme partitioned rainfall between convective and stable forms, and possibly to each scheme's closure assumptions. Adding details to the surface-moisture flux schemes had a secondary influence on soil moisture impacts on rainfall within a 24-h period.

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