Preliminary Regional Experiment For STORM-Central Research Articles

Subgrid‐scale rainfall variability and its effects on atmospheric and surface variable predictions

A new approach, which combines the Penn State/National Center for Atmospheric Research mesoscale model MM5 with a recently developed statistical downscaling scheme, has been investigated for the prediction of rainfall over scales (grid sizes) ranging from the atmospheric model scale (>10 km) to subgrid scale (around 1 km). The innovation of the proposed dynamical/statistical hybrid approach lies on having unraveled a link between larger‐scale dynamics of the atmosphere and smaller‐scale statistics of the rainfall fields [Perica and Foufoula‐Georgiou, 1996a], which then permits the coupling of a mesoscale dynamical model with a small‐scale statistical parameterization of rainfall. This coupling is two‐way interactive and offers the capability of investigating the feedback effects of subgrid‐scale rainfall spatial variability on the further development of a rainfall system and on the surface energy balance and water partitioning over the MM5 model grids. The results of simulating rainfall in a strong convection system observed during the Oklahoma‐Kansas Preliminary Regional Experiment for STORM‐Central (PRESTORM) on June 10–11, 1985 show that (1) the dynamical/statistical hybrid approach is a useful and cost‐effective scheme to predict rainfall at subgrid scales (around 1 km) based on larger‐scale atmospheric model predictions, and (2) the inclusion of the subgrid‐scale rainfall spatial variability can significantly affect the surface temperature distribution and the short‐term (<24 hour) prediction of rainfall intensity.

Numerical simulation of a heavy rainfall event during the PRE‐STORM Experiment

Numerical simulation of a storm that occurred along the Kansas‐Oklahoma border June 9–10, 1985, during the Preliminary Regional Experiment for STORM‐Central (PRE‐STORM) was carried out to examine the effect of atmospheric moisture on extreme rainstorms. This storm developed in a weak synoptic forcing environment and produced more than 200 mm of rainfall in a few hours. Heavy rainfall was confined to a small area and storm structure has been described as that of a chaotic convective system. Rainfall events like the June 9–10 storm which produce large rainfall amounts in small areas over short time periods and which have weak synoptic scale forcing are a particular challenge for numerical modeling. These events are also of particular importance for problems of engineering hydrometeorology, which motivates the analyses presented in this paper. The model used in this study is the nonhydrostatic version of the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) mesoscale model (MM5). Three nested domains were used for modeling the June 9–10 storm. The grid spacing is 90 km for the outer, coarse grid, which covers most of the continental United States; 30 km for the intermediate grid (covering the central and southern plains); and 10 km for the local grid covering the PRE‐STORM region. Using operationaland PRE‐STORM surface and upper air observations, the model accurately reproduces storm total rainfall and storm location. To examine the role of atmospheric water vapor for heavy rainfall events, a series of model runs are carried out in which initial moisture is varied from 75% to 125% of the observed value. Storm total rainfall, storm structure, and storm timing, as well as all elements of the atmospheric water budget, exhibit a striking sensitivity to changes in atmospheric water vapor. These results have significant implications for engineering hydrometeorology procedures, especially procedures used for probable maximum precipitation (PMP) analyses.

Model for multiscale disaggregation of spatial rainfall based on coupling meteorological and scaling descriptions

The precipitation output of a mesoscale atmospheric numerical model is usually interpreted as the average rainfall intensity over the grid cell of the model (typically 30 × 30 km to 60×60 km). However, rainfall exhibits considerable heterogeneity over subgrid scales (i.e., scales smaller than the grid cell), so it is necessary for hydrologic applications to recreate or simulate the small‐scale rainfall variability given its large‐scale average. Rainfall disaggregation is usually done statistically. In this paper, a new subgrid scale rainfall disaggregation model is developed. It has the ability to statistically reproduce the rainfall variability at scales unresolved by mesoscale models while being conditioned on large‐scale rainfall averages and physical properties of the prestorm environment. The model is based on two extensively tested hypotheses for midlatitude mesoscale convective systems [Perica and Foufoula‐Georgiou, 1996]: (1) standardized rainfall fluctuations (defined via a wavelet transform) exhibit simple scaling over the mesoscale, and (2) statistical scaling parameters of rainfall fluctuations relate to the convective available potential energy (CAPE), a measure of the convective instability of the prestorm environment. Preliminary evaluation of the model showed that the model is capable of reconstructing the small‐scale statistical variability of rainfall as well as the fraction of area covered with rain at all analyzed subgrid scales. The performance evaluation was based on comparison of summary statistics and spatial pattern measures of simulated fields with those of known fields observed during the Oklahoma‐Kansas Preliminary Regional Experiment for Storm‐Central (PRE‐STORM).

A Dual-Doppler Radar Study of an OK PRE-STORM Heat Burst Event

Abstract Doppler and conventional radar, surface mesonetwork, and sounding data are used to investigate heat bursts that accompanied a mesoscale convective system (MCS) that traversed the OK PRE-STORM (Oklahoma–Kansas Preliminary Regional Experiment for STORM-Central) mesonetwork on 23–24 June 1985. The MCS formed along a dryline in western Kansas. As the system matured, an area of stratiform precipitation developed behind a line of convective towers that moved toward the southeast. Southwesterly flow at upper levels caused the stratiform precipitation to extend northeastward, allowing it to become isolated from the convective line. A broad surface mesohigh was observed beneath the core of the stratiform region. This feature was flanked by sharp pressure gradients and mesoscale low pressure areas to the northwest, north, and northeast. It was within these mesolows that the heat bursts occurred. Time series of surface mesonetwork data show that heat bursts were characterized by sudden dramatic rises in tem...

Warm Upper-Level Downdrafts Associated with a Squall Line

Abstract Thermodynamic retrieval analysis applied to a composite of dual-Doppler radar data obtained in the 10–11 June 1985 PRE-STORM (Preliminary Regional Experiment for STORM-Central) squall line and a model simulation of a similar squall line show that the upper-level downdrafts located ahead of and behind the main convective updraft zone were generally positively buoyant. As a result, the upper-level downdrafts contributed negatively to the system heat flux.

Effects of Cumulus Ensemble and Mesoscale Stratiform Clouds in Midlatitude Convective Systems

Abstract Diagnostic and semiprognostic analyses are performed using OK PRE-STORM (Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central) data to examine the cumulus-environment interaction in midlatitude convective systems. The similarities and differences of the interaction processes between midlatitude and tropical convective systems are also discussed: Analyses of PRE-STORM and GATE (GARP Atlantic Tropical Experiment) data show generally larger vertical wind shear, large-scale forcing, and moist convective instability in midlatitude MCCs (mesoscale convective complexes) and squall lines than in tropical cloud clusters. It is found that the interaction mechanism based on the cumulus-induced subsidence and detrainment is capable of explaining most of the observed heating and drying under widely different environment conditions. Convective- wale downdrafts act to cool and moisten the lower troposphere in the midlatitudes as in the tropics. The quasi- equilibrium assumption between stabilizatio...

Evaluation of Sampling Errors of Precipitation from Spaceborne and Ground Sensors

The spatial and temporal characteristics of rainfall over Oklahoma and Kansas are analyzed using the raingage data collected during the Preliminary Regional Experiment for STORM-Central (PRE-STORM). The spectra obtained are compared with those obtained from the oceanic precipitation in the GARP and with that obtained from analyzing raingage records in east Texas. In addition, the spectra are used to evaluate the sampling errors that are due to the spatial gaps in measurements. It was found that the temporal spectra from PRE-STORM had a spectral shape similar to that obtained in GARP, except that the tail of the PRE-STORM spectra has a lower slope than in GARP spectra, suggesting that the largest difference between tropical oceanic rainfall and convective land precipitation is at the highest frequencies.

Multiscale Analysis of a Mature Mesoscale Convective Complex

Abstract A multiscale analysis reveals diverse atmospheric structure and processes within a mesoscale convective complex (MCC) observed during the Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central (PRE-STORM) experiment. This midlatitude system was the second in a series of four MCCs that developed and traveled along a quasi-stationary frontal zone over the central United States on 3–4 June 1985. Objectively analyzed mesoscale upper-air soundings encompassing the MCC are interpreted in tandem with more detailed dual-Doppler radar measurements that disclose the storm's internal airflow and precipitation structure. The mature MCC is found to include a variety of local environments and associated weather, ranging from tornadic thunderstorms to more linear convective bands and widespread chilling rains. A corresponding spectrum of mesoscale ver6cW-motion profiles is documented. These findings are related to previous composite-based portrayals of MCCs, as well as detailed case studies of simple...

Evolution of Environmental Conditions Preceding the Development of a Nocturnal Mesoscale Convective Complex

Abstract A large nocturnal mesoscale convective complex (MCC) developed on 4 June 1985 during the Oklaboma-Kansas Preliminary Regional Experiment for STORM-Central (PRE-STORM) field phase. It occurred near the climatological center of the nocturnal maximum in warm-season precipitation situated over the central United States. In this study special rawinsonde and surface mesonet data have been used to examine how the environmental conditions, which supported MCC development, evolved at night over this region. The MCC of interest was the fourth in a series of MCCS, three of which propagated east-northeastward, 100–300 km north of a quasi-stationary surface front. The region where the MCC experienced its most intensive growth was initially characterized by dry and hydrostatically stable conditions (associated with the passage of the previous MCC) above the shallow, wedge-shaped cold air mass. In less than 3 h, interaction between the diurnally varying low-level jet and the frontal boundary led to a local incr...

The Formation of a Cooling-induced Mesovortex in the Trailing Stratiform Region of a Midlatitude Squall Line

Abstract There have been some ambiguities in recent observational studies as to whether midlevel mesovortices are induced by latent heating or cooling, and develop in the descending or ascending portion of mesoscale convective systems (MCS's). In this study, a comprehensive examination of a cooling-induced mesovortex in the trailing stratiform region of a midlatitude squall line that occurred on 10–11 June 1985 during the Preliminary Regional Experiment for STORM-Central (PRE-STORM) is presented using a 20-h high-resolution simulation of the squall system. This cooling-induced midlevel vortex originates from the preexisting cyclonic vorticity associated with a traveling meso-α-scale short wave. The vortex is intensified in the descending rear-to-front (RTF) inflow as a result of continued sublimative melting and evaporative cooling in the stratiform region. It decouples from the front-to-rear (FTR) ascending and anticyclonic flow in the upper troposphere during the formative stage. The vortex tilts northw...

The Development of Negative Moist Potential Vorticity in the Stratiform Region of a Simulated Squall Line

Abstract This paper presents evidence on the development of negative moist potential vorticity (MPV) or moist symmetric instability (MSI) in the stratiform region of a midlatitude squall line, based on a three-dimensional (3D) numerical simulation of a case that occurred on 10–11 June 1985 during the Preliminary Regional Experiment for STORM-Central (PRE-STORM). The results show that the stratiform region, though convectively stable to pure vertical displacement, is considerably unstable to slantwise displacement along the system's broad front-to-rear (FTR) saturated ascending flow. It is found that this instability evolves from boundary-layer convective instability that has previously been removed by upright convection over the leading portion of the squall system. The negative MPV in the stratiform region is mainly the result of upward and rearward transport of the low-level convectively unstable air along the sloping FTR ascending flow and of processes that reverse the signs of both the convective stab...

Comparisons of Rawinsonde-deduced Kinematic and Thermodynamic Quantities with Those Deduced from VHF Profiler Observations

The testing and evaluation of new observing systems (i.e., 50-MHz wind profilers) were listed among the goals of the 1985 Oklahoma–Kansas Preliminary Regional Experiment for STORM-Central (PRE-STORM). Using a PRE-STORM dataset from 12–13 May 1985, kinematic quantities computed from profiler winds are quantitatively compared with those computed from a dense network of rawinsonde wind observations. Both the generalized thermal wind equation and the geostrophic thermal wind equation are used to retrieve horizontal gradients of virtual temperature from the profiler wind observations. The resultant gradients are compared with those computed from rawinsonde temperature and moisture measurements. Comparisons of the kinematic quantities computed from the two data sources show some agreement in temporal trends. Comparisons of the detailed structure reveal significant differences. The temporal trends of the vertical motion fields obtained from the profilers and rawinsondes do not correspond. The virtual temperature gradients retrieved using profiler winds and the generalized thermal wind equation do not agree with those computed from rawinsonde temperature and humidity data. Overall, the use of the thermal wind equation and profiler wind observations to estimate thermal gradients was not successful. An example of detailed thermal structure from rawinsonde temperature and humidity data that was successfully detected using profiler winds and the generalized thermal wind equation is presented. The sampling characteristics and other sources of errors that hinder comprehensive comparisons of the observations from the various PRE-STORM sensors are discussed.

Mesoscale Convective Patterns of the Southern High Plains

Mesoscale convective systems observed in the southern High Plains during the Oklahoma-Kansas Preliminary Regional Experiment for STORM-central (PRE-STORM) field program were analyzed using radar and rawinsonde data. Although radar data indicate that no two systems are identical, basic recurring mesoscale structures are evident. Based on these recurrent features, the systems have been classified into three types of mesoscale convective patterns: linear mesoscale systems, occluding mesoscale systems, and chaotic mesoscale systems. Examples of all three types are discussed. High-density rawinsonde data collected in the regions ahead of the mesoscale systems have been averaged to produce composite soundings; the composites exhibit differences in both thermodynamic and wind structure between types.

A Three-Dimensional Numerical Study of an Oklahoma Squall Line Containing Right-Flank Supercells

Abstract A nonhydrostatic numerical mesoscale model has been applied to the study of an Oklahoma squall line with initial conditions taken from the Oklahoma–Kansas Preliminary Regional Experiment for STORM-Central (PRE-STORM) data for 7 May 1985. The model reproduced features typical of organized propagating convection occurring during spring and summer in this region, namely a squall line/mesoscale convective system containing strong right-flank convection resembling many documented cases. The alignment and motion of the system change during its development and are determined by the ambient wind at three levels, the steering level of the mature cells, the level of free convection, and the surface layer. Three persistent right-flank cells had a characteristic rightward propagation relative to the mean wind shear vector. Their propagation occurred through successive mergers of cells that had formed at a downdraft outflow convergence front and were similar to the flanking line often seen to the south of str...

Uncertainty in Estimating Boundary-Layer Transport during Highly Convective Conditions

Abstract Routine and supplemental rawinsonde observations collected during the Preliminary Regional Experiment for Storm-Central (PRE-STORM) were analyzed to assess the uncertainty in boundary-layer trajectory calculations due to imprecise interpolation of the horizontal wind field. This study was designed to complement our earlier analysis of rawinsonde data collected during the Cross Appalachian Tracer Experiment (CAPTEX; Kahl and Samson 1986). The present study is representative of widespread convective conditions, while our previous study was representative of fairly persistent, undisturbed flow. Spatial autocorrelation analysis revealed significant wind field variability on scales less than 100 km. Evaluation of several spatial and temporal interpolation techniques yielded mean absolute errors in estimation of u and v wind components ranging from 3.3–6.5 m −1. Spatial interpolation accuracy improved only slightly when supplemental measurements were included in the interpolation procedure. Estimates o...

STORM Program

The first field experiment of the National STORM (Stormscale Operational and Research Meteorology) Program is scheduled to get underway this spring in southern Oklahoma. Called PRE‐STORM (for Preliminary Regional Experiment for STORM‐Central), the investigation will help scientists to evaluate new meteorological forecasting equipment and techniques and to gather a preliminary data base for STORM‐Central, the first multiscale field experiment proposed for STORM.