Mesoscale Model Output Research Articles

Analysis and Aladin prediction of a heavy precipitation event on the Eastern side of the Alps during Map IOP 5

During the Mesoscale Alpine Program - Intensive Observing Period 5 (MAP IOP 5) heavy precipitation occurred in the early morning of 4 October 1999 along the border between Slovenia and western Croatia. Orographic influence on convectively unstable air advected by the warm and humid current from the Mediterranean caused a large amount of rain. The existing frontal zone was modified by the convergence between the southerly winds associated with the Alpine lee cyclogenesis, and the northerly flow around the eastern flank of the Alps after a splitting process in the north Alpine orographic blocking. For special MAP IOP5 soundings, observations were made at 3- or 6-hourly time intervals, while prognostic temps on the direct ALADIN/LACE mesoscale model output at every 3 hour were taken for Ljubljana, Udine and Zagreb. These datasets were used as the input data for objectively derived vertical time cross-sections indicating both, the observed and the predicted atmospheric structure and its temporal changes at the locations considered. The diagnostic procedure was based on the High Resolution Isentropic Diagnosis (HRID) model. The analysis of the low tropospheric structure offers an explanation of this extraordinary case with a rapid frontal and orogenic development. The basic features are: 1. the strong vertical wind shear and pronounced convective instability ahead of the front; 2. the very high humidity concentration inside the frontal zone; 3. a typical bora layer covered by a temperature inversion after the frontal passage, and 4. a layer characterized by a weakening wind system above the temperature inversion. The latter process inhibits the vertical growth of convective clouds. Consequently, the observations of hourly precipitation amount in Zagreb and its surroundings show gradual decrease and, finally, cessation of rainfall. It is shown that the ALADIN/LACE model and its application by HRID were able to predict the essential features of these phenomena with high reliability as well as their usefulness and applicability in making local forecasts and nowcasting.

The Use of Digital Warping of Microwave Integrated Water Vapor Imagery to Improve Forecasts of Marine Extratropical Cyclones

Abstract A technique is described in which forecasts of the locations of features associated with marine cyclones may be improved through the use of microwave integrated water vapor (IWV) imagery and image warping of forecast mesoscale model fields. Here, image warping is used to optimally match mesoscale model output to observations of IWV measured by microwave sensors. In the mesoscale model simulations presented here (one of the March 1993 “superstorm,” one of a rapidly deepening cyclone observed in the North Atlantic in February 1992, and one of the ERICA IOP 4 cyclone), the Pennsylvania State University–National Center for Atmospheric Research MM5 model is initialized from the standard National Meteorological Center (recently renamed the National Centers for Environmental Prediction) operational analysis. The simulations are then run until a time at which a Special Sensor Microwave/Imager (SSM/I) overpass occurs. For each simulation, the forecast pattern of IWV is then compared to the field shown in ...

Discriminating between Tornadic and Nontornadic Thunderstorms Using Mesoscale Model Output

The ability to discriminate between tornadic and nontornadic thunderstorms is investigated using a mesoscale model. Nine severe weather events are simulated: four events are tornadic supercell thunderstorm outbreaks that occur in conjunction with strong large-scale forcing for upward motion, three events are bow-echo outbreaks that also occur in conjunction with strong large-scale forcing for upward motion, and two are isolated tornadic supercell thunderstorms that occur under much weaker large-scale forcing. Examination of the mesoscale model simulations suggests that it is possible to discriminate between tornadic and nontornadic thunderstorms by using the locations of model-produced convective activity and values of convective available potential energy to highlight regions of likely thunderstorm development, and then using the values of storm-relative environmental helicity (SREH) and bulk Richardson number shear (BRNSHR) to indicate whether or not tornadic supercell thunderstorms are likely. Values of SREH greater than 100 m2 s−2 indicate a likelihood that any storms that develop will have a midlevel mesocyclone, values of BRNSHR between 40 and 100 m2 s−2 suggest that low-level mesocyclogenesis is likely, and values of BRNSHR less than 40 m2 s−2 suggest that the thunderstorms will be dominated by outflow. By combining the storm characteristics suggested by these parameters, it is possible to use mesoscale model output to infer the dominant mode of severe convection.

Surges over the Gulf of California during the Mexican Monsoon

Abstract Northward surges of relatively cool, moist, maritime air from the tropical Pacific into the southwestern United States occur via the Gulf of California every summer during the Mexican monsoon season. These surges advect large amounts of moisture northward and promote increased convective activity in Arizona. Mesoscale model output from 32 successive 24-h simulations are used to examine both the large-scale and mesoscale features associated with surge events. Model data are compared to high-resolution aircraft measurements on two days, showing clearly that the model is capable of reproducing many of the detailed structures found in the observations. Results from a large-scale analysis indicate that strong surges are produced in the model when a midlatitude trough is in the proper phase relationship with a tropical easterly wave. Weak surges occur when this relationship is missing.

A numerical case study of a polar low in the Labrador Sea

A mesoscale (15 km) version of the Canadian regional finite-element model is used to study a polar low that developed in the Labrador Sea on 11 January 1989, in the wake of an intense cold air outbreak associated with a major synoptic-scale system located to the east of Greenland. The rapid evolution of the polar low is well revealed from satellite imagery showing a complex structure with strong surface winds near the vortex and deep convection nearby during the mature stage. The simulated structure of the polar low agrees quite well with observed features. Based on the detailed mesoscale model outputs, the evolution of the Labrador Sea polar low is discussed at the initiation and mature stages. The polar low developed under a combination of baroclinic and convective processes. At an early stage, baroclinic development takes place in conditions of reversed shear flow, marked by low-level baroclinicity near the ice edge and a mobile upper-level short wave. Rapid modification of the Arctic boundary layer by strong surface heat fluxes is similar to that observed in other areas. Sensitivity experiments indicate that the mutual interaction between the upper-level potential vorticity anomaly and the low-level baroclinicity at the Arctic front, favoured by the deep convective boundary layer, appears to trigger the polar low. At the onset of the mature stage, the approach of a cold air dome favours the outbreak of deep convection, in agreement with satellite imagery. As shown by sensitivity experiments, latent heat release from organized convection contributes to the major part of the rapid deepening of the polar low in its mature stage, and sea surface evaporation is the primary feeding mechanism for condensation processes. The structure of the polar low is characterized by a warm core due to the combined effects of warm air seclusion and diabatic heating. Comparisons with previously studied polar lows developing in similar conditions of reversed shear at other locations are discussed. DOI: 10.1034/j.1600-0870.1996.t01-2-00003.x

A numerical case study of a polar low in the Labrador Sea

A mesoscale (15 km) version of the Canadian regional finite-element model is used to study a polar low that developed in the Labrador Sea on 11 January 1989, in the wake of an intense cold air outbreak associated with a major synoptic-scale system located to the east of Greenland. The rapid evolution of the polar low is well revealed from satellite imagery showing a complex structure with strong surface winds near the vortex and deep convection nearby during the mature stage. The simulated structure of the polar low agrees quite well with observed features. Based on the detailed mesoscale model outputs, the evolution of the Labrador Sea polar low is discussed at the initiation and mature stages. The polar low developed under a combination of baroclinic and convective processes. At an early stage, baroclinic development takes place in conditions of reversed shear flow, marked by low-level baroclinicity near the ice edge and a mobile upper-level short wave. Rapid modification of the Arctic boundary layer by strong surface heat fluxes is similar to that observed in other areas. Sensitivity experiments indicate that the mutual interaction between the upper-level potential vorticity anomaly and the low-level baroclinicity at the Arctic front, favoured by the deep convective boundary layer, appears to trigger the polar low. At the onset of the mature stage, the approach of a cold air dome favours the outbreak of deep convection, in agreement with satellite imagery. As shown by sensitivity experiments, latent heat release from organized convection contributes to the major part of the rapid deepening of the polar low in its mature stage, and sea surface evaporation is the primary feeding mechanism for condensation processes. The structure of the polar low is characterized by a warm core due to the combined effects of warm air seclusion and diabatic heating. Comparisons with previously studied polar lows developing in similar conditions of reversed shear at other locations are discussed. DOI: 10.1034/j.1600-0870.1996.t01-2-00003.x

The Importance of Understanding Mesoscale Model Parameterization Schemes for Weather Forecasting

Abstract A severe weather outbreak that occurred on 21–23 November 1992 in the southern United States is used to illustrate how an understanding of model parameterization schemes can help in the evaluation and utilization of mesoscale model output. Results from a mesoscale model simulation show that although the model accurately simulated many of the observed mesoscale features, there are several aspects of the model simulation that are not perfect. Through an understanding of the model parameterization schemes, these model imperfections are analyzed and found to have little effect on the overall skill of the model forecast in this case. Mesoscale model output also is used to provide guidance to evaluate the severe weather threat. By using the model output to produce hourly calculations of convective available potential energy (CAPE) and storm relative environmental helicity (SREH), it is found that regions with positive CAPE, SREH greater than 150 m2 s−2, and model-produced convective rainfall correspond...

Airmass Modification over the Gulf of Mexico: Mesoscale Model and Airmass Transformation Model Forecasts

Abstract Several numerical models are used to examine strong air-sea fluxes and resultant airmass modification following a cold-frontal passage over the Gulf of Mexico. Data from the Gulf of Mexico Experiment (GUFMEX), which was conducted in February-March 1988, are used for model validation. To provide a benchmark by which to evaluate the role of diabatic processes in airmass modification, the mesoscale model was initially run with surface fluxes deleted. Subsequent full physics runs show profound alterations to the boundary layer due to the diabatic processes. A one-dimensional airmass transformation (AMT) boundary-layer model is also tested and compared with the mesoscale model and GUFMEX data. The Lagrangian character of the AMT model is a useful compliment to the mesoscale model output. Further, at least in one forecast, the AMT model yields a better forecast of boundary-layer depth. Strong sensible and latent heat fluxes in the vicinity of the cold front act frontolytically, while a subsidence-induc...