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Raymond W. Arritt

Abstract

A two-dimensional nonlinear numerical model has been used to examine the effects of the ambient wind on the development of characteristic features of the sea breeze. The specific features that were examined were the maximum shoreward velocity component (both total and as a perturbation from the large-scale flow) anywhere in the simulation domain, the inland penetration of the sea breeze, the depth of the inflow layer at the coastline, the maximum vertical velocity anywhere in the domain, the maximum potential-temperature gradient anywhere in the domain, and the surface heat flux. Thirty-one simulators were performed, with large-scale geostrophic winds ranging from onshore (i.e., supporting) flow of 15 m s−1 to offshore (i.e., opposing) flow of 15 m s−1, at 1 m s−1 intervals.

The results indicated that the sea-breeze perturbation was suppressed for onshore large-scale flow of a few meters per second or more. In contrast, a sea breeze was produced for opposing large-scale flow as strong as 11 m s−1. The sea-breeze cell was located offshore for strong opposing flow, with the sea-breeze velocities (both horizontal and vertical) being weaker than when the sea breeze penetrated inland. Both the stable stratification over the water and the convergence term in the froutogenesis equation are important factors in the sensitivity of the sea breeze to the large-scale flow.

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Zhiwei Yang and Raymond W. Arritt

Abstract

Ensemble simulations were performed with the regional climate model RegCM2 using ranges of plausible values for two parameters in the deep convection scheme. The timescale for release of convective instability was varied through a range of five values from 600 to 7200 s, and the maximum stable-layer depth between updraft origin and the level of free convection was varied over five values from 50 to 150 hPa, resulting in 25-member ensembles. Simulations were integrated for two 60-day periods covering extremes of regional climate for the central United States, the severe drought in late spring/early summer of 1988 and the record flood of summer 1993.

The mean-square error for precipitation was dominated by the term resulting from spatial position correlation. The relative operating characteristic showed that for the flood case the ensemble had skill for 60-day excess precipitation up to one interannual standard deviation above climatology, and for the drought case the ensemble had skill to simulate 60-day accumulated precipitation less than climatology. In both cases, the ensemble mean had superior skill to the reference forecast (i.e., the forecast using the default values of the closure parameters). For the flood case, improved skill was obtained through improvement in probability of detection, while for the drought case the improvement resulted from a decrease in the false alarm rate.

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Raymond W. Arritt and Markku Rummukainen

Abstract

No abstract available.

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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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Christopher J. Anderson and Raymond W. Arritt

Abstract

Reanalysis datasets that are produced by assimilating observations into numerical forecast models may contain unrealistic features owing to the influence of the underlying model. The authors have evaluated the potential for such errors to affect the depiction of summertime low-level jets (LLJs) in the NCEP–NCAR reanalysis by comparing the incidence of LLJs over 7 yr (1992–98) in the reanalysis to hourly observations obtained from the NOAA Wind Profiler Network. The profiler observations are not included in the reanalysis, thereby providing an independent evaluation of the ability of the reanalysis to represent LLJs.

LLJs in the NCEP–NCAR reanalysis exhibit realistic spatial structure, but strong LLJs are infrequent in the lee of the Rocky Mountains, causing substantial bias in LLJ frequency. In this region the forecast by the reanalysis model diminishes the ageostrophic wind, forcing the analysis scheme to restore the ageostrophic wind. The authors recommend sensitivity tests of LLJ simulations by GCMs in which terrain resolution and horizontal grid spacing are varied independently.

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Raymond W. Arritt and William M. Frank

Abstract

Modifications to current model output statistics procedures for quantitative precipitation forecasting were explored. Probability of precipitation amount equations were developed for warm and cool seasons in a region in the eastern United States. Twelve-term equations, which were simultaneously regressed for four precipitation categories, were compared to equations that were regressed independently for each of the categories. The effect of varying the number of terms in the independently regressed equations was also considered. The utilities of linear predictors not presently considered and of multiplicative predictors selected with the aid of a one parameter multiplicative model were investigated.

All forecast equations were evaluated using threat scores and biases achieved upon verification for one year of independent data. The independently regressed equations generally achieved threat scores similar to the twelve-term simultaneously regressed equations, and usually required fewer terms to do so. These more compact equations could be more readily interpreted by individual forecasters than could the twelve-term equations, making it easier to develop techniques for local adjustments to the objective forecasts. The prediction of the higher precipitation amount categories may benefit from the inclusion of predictor variables not presently considered.

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James Correia Jr. and Raymond W. Arritt

Abstract

Dropsonde observations from the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) are used to document the spatiotemporal variability of temperature, moisture, and wind within mesoscale convective systems (MCSs). Onion-type sounding structures are found throughout the stratiform region of MCSs, but the temperature and moisture variability is large. Composite soundings were constructed and statistics of thermodynamic variability were generated within each subregion of the MCS. The calculated air vertical velocity helped identify subsaturated downdrafts. It was found that lapse rates within the cold pool varied markedly throughout the MCS. Layered wet-bulb potential temperature profiles seem to indicate that air within the lowest several kilometers comes from a variety of source regions. It was also found that lapse-rate transitions across the 0°C level were more common than isothermal, melting layers. The authors discuss the implications these findings have and how they can be used to validate future high-resolution numerical simulations of MCSs.

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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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Christopher J. Anderson and Raymond W. Arritt

Abstract

Large, long-lived mesoscale convective systems (MCSs) over the United States during the 1997–98 El Niño are documented. Two periods of abnormal MCS activity are identified in 1998: from March to mid-April an unusually large number of quasi-linear MCSs were observed in the Midwest; while quasi-circular MCSs in June–August of 1998 were concentrated near 37°N rather than following a seasonal shift similar to that observed in the climatological distribution. Episodic surges of northerly low-level flow were infrequent in March 1998, thereby leading to an unusually high incidence of quasi-linear MCSs and to precipitation anomalies in the central United States.

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Christopher J. Anderson and Raymond W. Arritt

Abstract

Large, long-lived convective systems over the United States in 1992 and 1993 have been classified according to physical characteristics observed in satellite imagery as quasi-circular [mesoscale convective complex (MCC)] or elongated [persistent elongated convective system (PECS)] and cataloged. The catalog includes the time of initiation, maximum extent, termination, duration, area of the −52°C cloud shield at the time of maximum extent, significant weather associated with each occurrence, and tracks of the −52°C cloud-shield centroid.

Both MCC and PECS favored nocturnal development and on average lasted about 12 h. In both 1992 and 1993, PECS produced −52°C cloud-shield areas of greater extent and occurred more frequently compared with MCCs. The mean position of initiation for PECS in 1992 and 1993 followed a seasonal shift similar to the climatological seasonal shift for MCC occurrences but was displaced eastward of the mean position of MCC initiation in 1992 and 1993. The spatial distribution of MCC and PECS occurrences contain a period of persistent development near 40°N in July 1992 and July 1993 that contributed to the extreme wetness experienced in the Midwest during these two months.

Both MCC and PECS initiated in environments characterized by deep, synoptic-scale ascent associated with continental-scale baroclinic waves. PECS occurrences initiated more often as vigorous waves exited the intermountain region, whereas MCCs initiated more often within a high-amplitude wave with a trough positioned over the northwestern United States and a ridge positioned over the Great Plains. The low-level jet transported moisture into the region of initiation for both MCC and PECS occurrences. The areal extent of convective initiation was limited by the orientation of low-level features for MCC occurrences.

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