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An automated, rapidly relocatable nowcasting and prediction system, whose cornerstone is the full-physics, nested-grid, nonhydrostatic fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5), has been under development at the Pennsylvania State University since the late 1990s. In the applications presented in this paper, the Rapidly Relocatable Nowcast-Prediction System (RRNPS) provides a continuous stream of highly detailed nowcasts, defined here as gridded meteorological fields produced by a high-resolution mesoscale model assimilating available observations and staying just ahead of the clock to provide immediately available current meteorological conditions. The RRNPS, configured to use 36-, 12-, and 4-km nested domains, is applied over the Great Plains for 18 case days in August 2001, over the East Coast region for 8 case days in April 2002, and for 12 case days during the winter and summer of 2003. The performance of the RRNPS is evaluated using subjective and statistical methods for runs with and without the use of continuous four-dimensional data assimilation (FDDA). A statistical evaluation of the dependence of RRNPS skill on the length of model integration yields further insight into the value added by FDDA in RRNPS nowcasts. It must be emphasized that unlike typical operational analysis systems, none of the current data are used in the nowcasts since the nowcasts are made available just ahead of the clock for immediate use. Because none of the verification data are assimilated into the RRNPS at the time of verification, this evaluation is a true test of the time-integrated effects of previous FDDA on current model solutions. Furthermore, the statistical evaluations also utilize independent data completely withheld from the system at all times. Evaluation of the RRNPS versus observations on the 4- and 12-km grids shows that there is little difference in statistical skill between the two resolutions for the two application regions. However, subjective case evaluations indicate that mesoscale detail is added to the wind and mass fields on the 4-km domain of the RRNPS as compared to the coarser 12-km domain. Statistics suggest that 4-km resolution provides slightly more accurate meteorology for the domain including complex terrain and coastlines. The statistics also show that the use of continuous FDDA in a high-resolution mesoscale model improves the accuracy of the RRNPS nowcasts, and that this unique nowcast prediction system provides immediately available forecast-analysis products that are comparable or superior to those produced at operational centers, especially for the surface and the boundary layer. Finally, the RRNPS is also designed to run locally and on demand in a highly automated mode on modest computing platforms (e.g., a dual-processor PC) with potentially very limited data resources and nonstandard data communications. [ABSTRACT FROM AUTHOR]

In this paper several configurations of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5), which is implemented at Beijing Institute of Urban Meteorology in China, are used to demonstrate the initial noise problem caused either by interpolating global model fields onto an MM5 grid or by using MM5 objective analysis schemes. An implementation of a digital filter initialization (DFI) package to MM5 is then documented. A heavy rain case study and intermittent data assimilation experiments are used to assess the impact of DFI on MM5 forecasts. It is shown that DFI effectively filters out the noise and produces a balanced initial model state. It is also shown that DFI improves the spinup aspects for precipitation, leading to better scores for short-range precipitation forecasts. The issues related to the initialization of variables that are not observed and/or analyzed, in particular those for nonhydrostatic quantities, are discussed. [ABSTRACT FROM AUTHOR]

Numerically based models are extensively used for many environmental applications, as for example, to assist in the prediction of weather phenomena (numerical weather prediction models), in risk assessment, or in pollutant emission control (air-quality models). These models often produce predictions for grid points over some temporal window, while observations are usually available as a set of spatially scattered values at individual locations (not coincident with the model grid points), so that the model assessment procedure, that is, the statistical evaluation of how well model output compares with observed data, should address the issue of comparing the grid-based model results with interpolated values estimated from observed data at a different spatial resolution. In this paper, a Bayesian inference procedure for the spatiotemporal interpolation of environmental data from irregularly spaced monitoring sites is presented. The spatial interpolation is based on the use of the predictive posterior distribution. It is shown how this procedure can be exploited to obtain statistically consistent interpolated values (at the same spatial resolution as that of model results) and valid standard errors for these estimates. Comparisons of the wind speed fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) results against grid-cell values estimated from observed data are presented. [ABSTRACT FROM AUTHOR]

This paper investigates the impact of barrier width and freezing level on the microphysical processes and pathways within the Reisner2 bulk microphysical parameterization (BMP) using a two-dimensional version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5). As the barrier half-width is decreased incrementally from 50 km (relatively wide mountain) to 10 km (narrow mountain) for a deep orographic cloud and a 750-mb freezing level, the percentage of water vapor loss (WVL) rate over the windward slope leading to snow deposition decreases from 23% to 7%, while condensation increases from 74% to 93% of WVL rate. A narrow (10 km) barrier has less snow aloft, twice as much cloud water over the windward slope, and a shallow region of intense riming over the crest that results in twice as much graupel as the wide (50 km) barrier. It is found that a relatively wide barrier (≥30 km half-width) allows more time for snow growth aloft; therefore, it is more sensitive to snow parameters over the windward slope such as the slope intercept for number concentration and fall speeds. In contrast, a narrower barrier is more sensitive to rain and graupel processes, such as the cloud water autoconversion and graupel fall speeds. The wide barrier has a larger sensitivity to cloud water processes when the freezing level is elevated to 500 mb, while the narrow barrier is more sensitive to snow processes when the freezing level is lowered to 1000 mb. For a 1000-mb freezing level, the lack of riming and accretion reduces the rapid increase in maximum precipitation that is shown to occur when the barrier half-width is reduced from 20 and 10 km for a higher freezing level. [ABSTRACT FROM AUTHOR]

This paper investigates the local circulation associated with a heavy orographic rainfall event during 19–21 September 1999 [Mesoscale Alpine Programme Intensive Observing Period 2B (MAP IOP-2B)]. This event was simulated with a 5-km horizontal grid spacing using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). The MM5 simulation reproduced the basic features such as the timing and location of the deep trough and the associated precipitation evolution, though the total amount of precipitation is slightly higher than that measured by rain gauges (∼30% in 24 h). The near-surface flow was dominated by an easterly jet originally from the Adriatic Sea and a southerly jet from the Gulf of Genoa. A significant westward turning occurred when the southerly flow approached the south side of the Alps. This deflection was caused by boundary layer friction and rotation, as well as mountain blocking effects. Flow was generally from the south above the surface. Precipitation was mainly concentrated on the windward slopes, especially near the Lago Maggiore region. Sensitivity experiments have been conducted to investigate the effects of upstream orography, the western flank of the Alps, boundary layer friction, and horizontal resolution. The results indicate that precipitation distribution and amount over the southern upslope region of the Alps were not directly related to either the coastal Apennine Mountains or the west flank of the Alps. The boundary layer friction reduces the total amount and alters the distribution of rainfall by weakening the wind near the surface. The 1.67-km horizontal grid spacing simulation indicates that heavy rainfall tended to be concentrated in the vicinity of individual mountain peaks. The total amount of rainfall was overpredicted along the windward slopes because of the strong upward motion that occurred on the upslope of the barrier. The results indicate the importance of dynamical forcing associated with upslope-induced and near-surface horizontal velocity convergence-induced vertical motion, which increases rapidly as horizontal resolution increases. [ABSTRACT FROM AUTHOR]