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A dvances in computing have allowed presentday numerical weather-prediction models to forecast and/or simulate tropical cyclones (TCs) with high resolution (horizontal grid spacing ~1 km). While these numerical models of TCs can capture many of their important structural features (eyewall, rainbands, etc.), the effects of small-scale [~0 (100 m)], three-dimensional turbulence must still be parameterized. No objective basis has yet been found for such parameterizations, yet the maximum wind speeds in axisymmetric numerical TC models have been shown to be highly sensitive to the radial turbulent transfer and diffusion at these scales. As there is little observational guidance on the nature of radial turbulent diffusion in a TC, the present study was conceived to indicate the impact of these effects through computation of the small-scale scale turbulence (i.e., a large-eddy simulation, or LES) using a numerical weather prediction model applied to an idealized tropical cyclone.The numerical experiments reported on herein were carried out with the Advanced Research Weather research and forecasting (ARW) model, version 2.2, using six nested grids centered on the TC. As the ARW model is nonhydrostatic, it can and has been used as an LES model, 1 and therefore one can expect the numerical solutions to capture turbulent eddies, 1 Examples using ARW as an LES model to simulate the idealized convective planetary boundary layer and the sea breeze are listed in the bibliography. given sufficient resolution. Here we show that using a horizontal grid size < 100 m in a TC simulation produces three-dimensional turbulent eddies, which have the effect of decreasing the maximum sustained (1-min average) wind and the maximum predicted azimuthally averaged wind. TURBULENCE EFFECTS INTROPICAL-CYCLONE MODELS. Existing tropical-cyclone models need to simulate circulations reaching outward to a distance of ~0 (1,000 km) from the TC center. Modern-day computers have allowed real-time numerical forecasts and sensitivity studies of the TC with horizontal grid spacing as small as 1 km. We note that TC models fall into the general category of mesoscale model wherein, according to a 2004 Journal of the Atmospheric Sciences essay by J. Wyngaard, the grid scale A is much greater than the length scale I of threedimensional turbulent eddies that may be O (100 m) and smaller, and consequently, all three-dimensional turbulence effects must be parameterized. The parameterization of horizontal diffusion in most mesoscale models is such that the horizontal diffusion coefficient is proportional to A so as to arrest frontal collapse; in the typical case, the front is long and straight and the horizontal diffusion serves its purpose without significantly changing the forecast wind intensity. However, in a TC, the eyewall, which is a type of front (as discussed by K. Emanuel in a 1997 Journal of the Atmospheric Sciences article), is circular, and diffusion exerts great control over the radius of the eyewall (i.e., the radius to which fluid parcels will con...

Changes in precipitation have far‐reaching consequences on human society and ecosystems as has been demonstrated by recent severe droughts in California and the Oklahoma region. Droughts are beside tropical cyclones the most costly weather and climate related extreme events in the U.S. We apply a weather type (WT) analysis to reanalysis data from 1979–2014 that characterize typical weather conditions over the contiguous United States. This enables us to assign precipitation trends within 1980–2010 to changes in WT frequencies and changes in precipitation intensities. We show that in the North Atlantic and Midwest region precipitation intensity changes are the major driver of increasing precipitation trends. In the U.S. Southwest, however, WT frequency changes lead to a significant precipitation decrease of up to −25% related to an increase in anticyclonic conditions in the North East Pacific. This trend is partly counteracted by increasing precipitation intensities.

Summer and winter daily heavy precipitation events (events above the 97.5th percentile) are analyzed in regional climate simulations with 36-, 12-, and 4-km horizontal grid spacing over the headwaters of the Colorado River. Multiscale evaluations are useful to understand differences across horizontal scales and to evaluate the effects of upscaling finescale processes to coarser-scale features associated with precipitating systems.Only the 4-km model is able to correctly simulate precipitation totals of heavy summertime events. For winter events, results from the 4-and 12-km grid models are similar and outperform the 36-km simulation. The main advantages of the 4-km simulation are the improved spatial mesoscale patterns of heavy precipitation (below ;100 km). However, the 4-km simulation also slightly improves larger-scale patterns of heavy precipitation.

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