The climatology of thunderstorms and flash floods in the Baltimore metropolitan region is examined through analyses of cloud-to-ground (CG) lightning observations from the National Lightning ...Detection Network (NLDN), volume scan reflectivity observations from weather radars and discharge observations from a dense network of stream gaging stations. A space-time point process framework is utilized for analyses of CG lightning strikes. Analyses focus on initiation and evolution of thunderstorm systems over the Baltimore-Washington corridor and the consequences for flash flood frequency in the region. It is shown that the seasonal and diurnal variation of flash flood occurrence in urban streams of Baltimore mimics the seasonal and diurnal variation of lightning. Analyses also show a close link between the occurrence of major thunderstorms systems and flash flooding on a regional scale.
Estimation of the correlation structure of random fields has many applications in earth sciences. Using a simulation framework, the authors address sampling problems in estimating the spatial ...correlation function. The framework consists of generating realizations of homogeneous, isotropic Gaussian and lognormal random fields with a two-parameter power exponential spatial correlation function. The two parameters in the correlation function are the correlation distance that describes the distance at which the correlation is equal to 1/e and the shape factor that describes the behavior of the correlation function at small scales. The correlation function estimated from multiple realizations of the simulated fields is compared with the true correlation function. The results show that the ratio of correlation distance to the domain size and the shape factor play a key role in the estimation process. A sensitivity analysis on the number of realizations required to accurately estimate the correlation structure is also presented.
This study presents a multicomponent rainfall estimation algorithm, based on weather radar and rain gauge network, that can be used as a ground-based reference in the satellite Tropical Rainfall ...Measuring Mission (TRMM). The essential steps are constructing a radar observable, its nonlinear transformation to rainfall, interpolation to rectangular grid, constructing several timescale accumulations, bias adjustment, and merging of the radar rainfall estimates and rain gauge data. Observations from a C-band radar in Darwin, Australia, and a local network of 54 rain gauges were used to calibrate and test the algorithm. A period of 25 days was selected, and the rain gauges were split into two subsamples to apply cross-validation techniques. A Z-R relationship with continuous range dependence and a temporal interpolation scheme that accounts for the advection effects is applied. An innovative methodology was used to estimate the algorithm controlling parameters. The model was globally optimized by using an objective function on the level of the final products. This is equivalent to comparing hundreds of Z-R relationships using a uniform and representative performance criterion. The algorithm performance is fairly insensitive to the parameter variations around the optimum. This suggests that the accuracy limit of the radar rainfall estimation based on power-law Z-R relationships has been reached. No improvement was achieved by using rain regime classification prior to estimation.
The Flash Flood Guidance System (FFGS) is an operational system which assists forecasters to issue flash flood warnings and watches over the conterminous United States. Currently it is a fully ...deterministic system: given a certain precipitation accumulation, flash flood warnings are issued depending on the exceedance of a certain threshold value (flash flood guidance). The aim of this study is to consider the effects of radar-rainfall and flash flood guidance threshold uncertainties. Earlier studies quantified uncertainties due to the estimation of the hydraulic and terrain characteristics and due to the hydrologic model parameters and initial state. To account for the uncertainties in the radar-rainfall input, the authors use an empirically-based ensemble generator of synthetic radar-rainfall fields. The generator accounts for radar range effects, synoptic conditions, different space-time resolutions, and the spatial and temporal dependence of the errors. Thus, the total effect of uncertainty in both the input and in the FFG can be assessed. The authors discuss the potential of transforming the FFGS into a fully probabilistic system. Using Monte Carlo simulation one can obtain precipitation-flash flood guidance joint probability distribution of flash flood occurrence. This joint probability distribution can be used in the fore casters decision making process. The authors consider a hypothetical example of flash flood occurrence in several small basins in Oklahoma, using hourly radar rainfall data from the Oklahoma City WSR-88D radar (KTLX).
A model for the statistical distribution of radar rainfall estimate errors has been developed empirically from WSR 88D and rain gauge data. The model expresses the expected value of actual rainfall ...and the mean and standard deviation of the multiplicative error in the radar estimate as functions of the radar estimate itself and several power law parameters derived from an historic sample of rain gauge/radar pairs. The model enables an end user of the radar estimates to determine the expected value of point rainfall, the probability that rainfall is less than or greater than a given value, and the probability that the true rainfall is within a given interval. Experiments with data from several WSR 88D umbrellas indicate that the basic form of the model is valid at most or all locations within the conterminous United. States, though parameter adjustments for long-term radar bias and rainfall climatology must be made. The error model has several potential applications in radar hydrology, such as determining the probability that rainfall exceeds flash flood-producing thresholds, quality control of real-time rain gauge estimates, and construction of ensembles of rainfall fields. The development methodology and examples of operational applications will be presented.
Forecast Issues in the Urban Zone Dabberdt, Walter F.; Hales, Jeremy; Zubrick, Steven ...
Bulletin of the American Meteorological Society,
09/2000, Letnik:
81, Številka:
9
Journal Article
Recenzirano
Odprti dostop
The 10th Prospectus Development Team (PDT-10) of the U.S. Weather Research Program was charged with identifying research needs and opportunities related to the short-term prediction of weather and ...air quality in urban forecast zones. Weather has special and significant impacts on large numbers of the U.S. population who live in major urban areas. It is recognized that urban users have different weather information needs than do their rural counterparts. Further, large urban areas can impact local weather and hydrologic processes in various ways. The recommendations of the team emphasize that human life and well-being in urban areas can be protected and enjoyed to a significantly greater degree. In particular, PDT-10 supports the need for 1) improved access to real-time weather information, 2) improved tailoring of weather data to the specific needs of individual user groups, and 3) more user-specific forecasts of weather and air quality. Specific recommendations fall within nine thematic areas: 1) development of a user-oriented weather database; 2) focused research on the impacts of visibility and icing on transportation; 3) improved understanding and forecasting of winter storms; 4) improved understanding and forecasting of convective storms; 5) improved forecasting of intense/severe lightning; 6) further research into the impacts of large urban areas on the location and intensity of urban convection; 7) focused research on the application of mesoscale forecasting in support of emergency response and air quality; 8) quantification and reduction of uncertainty in hydrological, meteorological, and air quality modeling; and 9) the need for improved observing systems. An overarching recommendation of PDT-10 is that research into understanding and predicting weather impacts in urban areas should receive increased emphasis by the atmospheric science community at large, and that urban weather should be a focal point of the U.S. Weather Research Program.
Riverine water temperature extremes have typically been analyzed using event-based simulations, for example the 10-year 7-day low flows, combined with record-high air temperatures and other extreme ...conditions relevant to the location (e.g., maximum power generation). Using this combination of extreme conditions, one can estimate the maximum water temperature that may occur on the river. However, this method does not allow for calculation of, for example, the probability of a given temperature exceedence, or the average duration of such an excursion event. Alternatively, long-term continuous simulation using historical and physically representative reconstructed data records provides a large database of realistic events, which can be used to analyze the thermal regime of a river and its variability under current and changing conditions. This study applies such a procedure to the Missouri River between Gavins Point Dam at Yankton, S.D., and Rulo, Nebr. Along this reach, the thermal regime is influenced by six power installations, which release heated condenser cooling water to the main stem. Several scenarios were simulated numerically with the one-dimensional (1D) CHARIMA model to examine the effects of current power generation, as well as changing operational, hydrologic, and climatological conditions on the river thermal regime. Model simulations revealed that climate change and increased power demand may significantly affect the thermal regime of the Missouri River; however, the scenarios simulated in this study did not result in water temperatures that exceed the current temperature standards.
