Radar observations of precipitation production in thunderstorms (original) (raw)
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Microphysics of the Rapid Development of Heavy Convective Precipitation
Monthly Weather Review, 2001
Two rapidly growing, hail-producing storms observed in Alabama during the Microburst and Severe Thunderstorm project in 1986 were examined: the well-studied single-cell storm case on 20 July 1986 and a single cell within a multicellular storm on 6 July 1986. Both storms are examples of extremely efficient accretional growth processes that produced hail within 10 min. A simple hydrometeor classification algorithm based on multiparameter radar data was used to identify regions within the rain and snow portions of the storm volumes that included hail, graupel, and supercooled rain. By comparing the results of the simple hydrometeor classification algorithm to previous polarimetric analysis and modeling of the 20 July 1986 storm by other authors, the hydrometeor classification methodology for the 6 July 1986 storm was indirectly validated. The microphysical development of hail and graupel was similar for both the single isolated cell storm and a cell within a multicellular storm. Rapid coalescence within updrafts with high liquid water contents quickly produced precipitation-sized drops that were lofted above the 0ЊC level and subsequently froze. These frozen drops became hail and graupel embryos and continued to grow by accretion. Supercooled rain was present only in the earliest stages of cell evolution lasting 8-12 min and extending 1-2 km above the 0ЊC level. Hail and graupel appeared several minutes after the first appearance of supercooled rain. Graupel was present at higher altitudes and encompassed a larger area of the storm than hail. Completion of the glaciation of the supercooled rain and the start of hail and graupel fallout occurred at nearly the same time. Examination of volumetric statistics of the storms in terms of time-height frequency of hydrometeor type and contoured frequency by altitude diagrams (CFADs) of reflectivity and vertical velocity showed that the evolution of the storm kinematics and microphysics were closely coupled for individual cells. Individual cells can be described in terms of a single particle fountain. Previous studies had shown that in multicellular storms, the ensemble of particle fountains rapidly evolves toward microphysical characteristics indicative of dominant vapor depositional growth, characteristic of stratiform regions, even when strong updrafts are present. This study aided in clarifying that, in contrast to the ensemble of particle fountains, for individual particle fountains the kinematic and microphysical evolution are more closely coupled in time and that vapor depositional growth does not dominate in the individual cell until the updraft associated with the cell has weakened. In the two cases examined, the combined effects of enhancement of the upper levels of the updraft by the latent heat released by glaciation, and the precipitation loading of the heavy falling particles at lower levels, acted to tear the cell apart at the middle. Previous studies have noted midlevel convergence and constriction of the cell associated with these effects. It is postulated that as a result of these factors, cells producing hail and graupel will hasten their own demise and will have on average shorter lifetimes as distinct cells compared to cells producing only rain.
The Changing Character of Precipitation
Bulletin of the American Meteorological Society, 2003
From a societal, weather, and climate perspective, precipitation intensity, duration, frequency, and phase are as much of concern as total amounts, as these factors determine the disposition of precipitation once it hits the ground and how much runs off. At the extremes of precipitation incidence are the events that give rise to floods and droughts, whose changes in occurrence and severity have an enormous impact on the environment and society. Hence, advancing understanding and the ability to model and predict the character of precipitation is vital but requires new approaches to examining data and models. Various mechanisms, storms and so forth, exist to bring about precipitation. Because the rate of precipitation, conditional on when it falls, greatly exceeds the rate of replenishment of moisture by surface evaporation, most precipitation comes from moisture already in the atmosphere at the time the storm begins, and transport of moisture by the storm-scale circulation into the s...
Precipitation is the release of water from the atmosphere to reach the surface of the earth. The term 'precipitation' covers all forms of water being released by the atmosphere, including snow, hail, sleet and rainfall. It is the major input of water to a river catchment area and as such needs careful assessment in any hydrological study. Although rainfall is relatively straightforward to measure (other forms of precipitation are more difficult) it is notoriously difficult to measure accurately and, to compound the problem, is also extremely variable within a catchment area. TYPES OF PRECIPITATION The precipitation may be due to (i) Thermal convection (convectional precipitation) — this type of precipitation is in the form of local whirling thunder storms and is typical of the tropics. The air close to the warm earth gets heated and rises due to its low density, cools adiabatically to form a cauliflower shaped cloud, which finally bursts into a thunder storm. When accompanied by destructive winds, they are called 'tornados'. (ii) Conflict between two air masses (frontal precipitation) — When two air masses due to contrasting temperatures and densities clash with each other, condensation and precipitation occur at the surface of contact, Fig. 2.1. This surface of contact is called a 'front' or 'frontal surface'. If a cold air mass drives out a warm air mass' it is called a 'cold front' and if a warm air mass replaces the retreating cold air mass, it is called a 'warm front'. On the other hand, if the two air masses are drawn simultaneously towards a low pressure area, the front developed is stationary and is called a 'stationary front'. Cold front causes intense precipitation on comparatively small areas, while the precipitation due to warm front is less intense but is spread over a comparatively larger area. Cold fronts move faster than warm fronts and usually overtake them, the frontal surfaces of cold and warm air sliding against each other. This phenomenon is called 'occlusion' and the resulting frontal surface is called an 'occluded front'. (ii) Orographic lifting (orographic precipitation)—The mechanical lifting of moist air over mountain barriers, causes heavy precipitation on the windward side (Fig. 2.2). For example Cherrapunji in the Himalayan range and Agumbe in the western Ghats of south India get very heavy orographic precipitation of 1250 cm and 900 cm (average annual rainfall), respectively. (iv) Cyclonic (cyclonic precipitation)—This type of precipitation is due to lifting of moist air converging into a low pressure belt, i.e., due to pressure differences created by the unequal heating of the earth's surface. Here the winds blow spirally inward counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. There are two main types of cyclones—tropical cyclone (also called hurricane or typhoon) of comparatively small diameter of 300-1500 km causing high wind velocity and heavy precipitation, and the extra-tropical cyclone of large diameter up to 3000 km causing wide spread frontal type precipitation.
2008
Quantitative precipitation forecasting (QPF) in low-mountain regions is a great challenge for the atmospheric sciences community. On the one hand, orographic enhancement of precipitation in these regions can result in severe flash-flood events. On the other hand, the relative importance of forcing mechanisms leading to convection initiation (CI) is neither well understood nor adequately reproduced by weather forecast models. This results in poor QPF skill, both in terms of the spatial distribution of precipitation and its temporal evolution. Two prominent systematic errors of state-of-theart mesoscale models are identified. Figure 1 shows the difference between a 1-month average of 24-h integrated precipitation forecasted with the Consortium for Small-Scale Modeling (COSMO)-EU Model (formerly known as Lokalmodell) of the German Meteorological Service (DWD) and the corresponding observational data. Shown on this figure is the Black Forest low-mountain region in southwestern Germany. Strong systematic errors are found on both the windward and the lee sides. On the windward side, the model strongly overestimates precipitation, whereas on the lee side it is underestimated, which we call the "windward/lee effect." To our knowledge, this error is found in all mesoscale models for both weather prediction and climate simulations, which require convection parameterization, such as in COSMOCH7 of Meteo Swiss, ARPEGE and ALADIN of Meteo France, as well as in the mesoscale models MM5 and ETA. Although we show a summertime example here, Baldauf and Schulz previously demonstrated that this error structure exists during all seasons. Another key problem is the inadequate simulation research campaign
Relationship between surface conditions and subsequent rainfall in convective storms
Journal of Geophysical Research, 1996
This paper describes the relationship between surface conditions (temperature and humidity) and subsequent rainfall. The focus is on convective storms that are forced and maintained locally due to conditional instability in the vertical distribution of atmospheric temperature. These storms are described using two probabilistic measures: (1) the probability of occurrence of storms given surface conditions and (2) the average storm rainfall. The surface conditions are described by a single variable: surface wet-bulb temperature. The proposed theoretical relationships are tested using an hourly data set on rainfall and wet-bulb temperature from the Amazon region. These observations confirm that both measures increase linearly with wet-bulb temperature. However, for the occurrence of any storm the wet-bulb temperature has to exceed a threshold of about 22øC. The sensitivity of the frequency of storms to changes in the climatology of surface wet-bulb temperature is larger than the corresponding sensitivity of the average storm rainfall. These general concepts are applied in discussing the potential impact of changes in land cover on rainfall patterns using two specific examples: deforestation in the Amazon region and development of irrigation projects in the Columbia River basin. 26,237 26,238 ELTAHIR AND PAL: SURFACE CONDITIONS AND SUBSEQUENT RAINFALL This factor enhances the role of surface conditions in the dynamics of these systems. The studies by Zawadzki and Ro [1978] and Zawadzki et al. [1981] explored the relationship of convective storms and the mesoscale thermodynamic variables. Their studies involved analysis of data from a network of meteorological stations located near Montreal during summer. A significant correlation was found between maximum and mean observed rainfall rates and mesoscale thermodynamic variables such as convective energy and static potential energy. Zawadzki et al. [1981] analyze surface observations as well as upper air soundings.
Journal of Applied Meteorology and Climatology, 2014
Profiling airborne radar data and accompanying large-eddy-simulation (LES) modeling are used to examine the impact of ground-based glaciogenic seeding on cloud and precipitation in a shallow stratiform orographic winter storm. This storm occurred on 18 February 2009 over a mountain in Wyoming. The numerical simulations use the Weather Research and Forecasting (WRF) Model in LES mode with horizontal grid spacings of 300 and 100 m in a domain covering the entire mountain range, and a glaciogenic seeding parameterization coupled with the Thompson microphysics scheme. A series of non-LES simulations at 900-m resolution, each with different initial/ boundary conditions, is validated against sounding, cloud, and precipitation data. The LES runs then are driven by the most representative 900-m non-LES simulation. The 100-m LES results compare reasonably well to the vertical-plane radar data. The modeled vertical-motion field reveals a turbulent boundary layer and gravity waves above this layer, as observed. The storm structure also validates well, but the model storm thins and weakens more rapidly than is observed. Radar reflectivity frequency-by-altitude diagrams suggest a positive seeding effect, but time-and space-matched model reflectivity diagrams only confirm this in a relative sense, in comparison with the trend in the control region upwind of seeding generators, and not in an absolute sense. A model sensitivity run shows that in this case natural storm weakening dwarfs the seeding effect, which does enhance snow mass and snowfall. Since the kinematic and microphysical structure of the storm is simulated well, future Part II of this study will examine how glaciogenic seeding impacts clouds and precipitation processes within the LES.
RESEARCH CAMPAIGN: The Convective and Orographically Induced Precipitation Study
Bulletin of the American Meteorological Society, 2008
Quantitative precipitation forecasting (QPF) in low-mountain regions is a great challenge for the atmospheric sciences community. On the one hand, orographic enhancement of precipitation in these regions can result in severe flash-flood events. On the other hand, the relative importance of forcing mechanisms leading to convection initiation (CI) is neither well understood nor adequately reproduced by weather forecast models. This results in poor QPF skill, both in terms of the spatial distribution of precipitation and its temporal evolution. Two prominent systematic errors of state-of-theart mesoscale models are identified. Figure shows the difference between a 1-month average of 24-h integrated precipitation forecasted with the Consortium for Small-Scale Modeling (COSMO)-EU Model (formerly known as Lokalmodell) of the German Meteorological Service (DWD) and the corresponding observational data. Shown on this figure is the Black Forest low-mountain region in southwestern Germany. Strong systematic errors are found on both the windward and the lee sides. On the windward side, the model strongly overestimates precipitation, whereas on the lee side it is underestimated, which we call the "windward/lee effect." To our knowledge, this error is found in all mesoscale models for both weather prediction and climate simulations, which require convection parameterization, such as in COSMOCH7 of Meteo Swiss, ARPEGE and ALADIN of Meteo France, as well as in the mesoscale models MM5 and ETA. Although we show a summertime example here, Baldauf and Schulz previously demonstrated that this error structure exists during all seasons.
Spatial characteristics of thunderstorm rainfall fields and their relation to runoff
The main aim of this study was to assess the ability of simple geometric measures of thunderstorm rainfall in explaining the runoff response from the watershed. For calculation of storm geometric properties (e.g. areal coverage of storm, areal coverage of the high-intensity portion of the storm, position of storm centroid and the movement of storm centroid in time), spatial information of rainfall is needed. However, generally the rainfall data consists of rainfall depth values over an unevenly spaced network of raingauges. For this study, rainfall depth values were available for 91 raingauges in a watershed of about 148 km 2. There was a question about which interpolation method should be used for obtaining uniformly gridded data. Therefore, a small study was undertaken to compare cross-validation statistics and computed geometric parameters using two interpolation methods (kriging and multiquadric). These interpolation methods were used to estimate precipitation over a uniform 100 m £ 100 m grid. The cross-validation results from the two methods were generally similar and neither method consistently performed better than the other did. In view of these results we decided to use multiquadric interpolation method for the rest of the study. Several geometric measures were then computed from interpolated surfaces for about 300 storm events occurring in a 17-year period. The correlation of these computed measures with basin runoff were then observed in an attempt to assess their relative importance in basin runoff response. It was observed that the majority of the storms (observed in the study) covered the entire watershed. Therefore, it was concluded that the areal coverage of storm was not a good indicator of the amount of runoff produced. The areal coverage of the storm core (10-min intensity greater than 25 mm/h), however, was found to be a much better predictor of runoff volume and peak rate. The most important variable in runoff production was found to be the volume of the storm core. It was also observed that the position of the storm core relative to the watershed outlet becomes more important as the catchment size increases, with storms positioned in the central portion of the watershed producing more runoff than those positioned near the outlet or near the head of the watershed. This observation indicates the importance of interaction of catchment size and shape with the spatial storm structure in runoff generation. Antecedent channel wetness was found to be of some importance in explaining runoff for the largest of the three 0022-1694/03/$-see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2-1 6 9 4 (0 2) 0 0 3 1 1-6