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G. Brant Foote and Carl G. Mohr

Abstract

The surface hail and rain data collected during the randomized seeding experiment of the National Hail Research Experiment (NHRE) are stratified according to storm intensity in a search for seeding effects. The problems encountered in attempting to stratify the data according to whether the storm cells exhibited radar vaults are discussed, but no stratification is attempted on this basis. The stratification by storm intensity is accomplished using a one-dimensional steady-state cloud model in conjunction with measured radar echo top heights to estimate the maximum updraft speed of the storm. It is shown that the technique is quite successful in classifying the storm days according to hailfall intensity. Analysis of hail and rain data on seed and control days in the various strata do not show an effect of seeding as assessed by 90% confidence intervals. The confidence intervals are quite wide, however, and a variety of possible effects of seeding are also not inconsistent with the data.

The frequency with which vaulted cells were observed during the experiment is documented, and they are shown to constitute only ∼1% of the total. However, the hailfall from vaulted cells, which tend to be very large and long-lived, can be extreme. Three vaulted cells were seeded during the experiment. Analysis of the radar reflectivity structure of these cells did not reveal any obvious effects of seeding.

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David B. Parsons, Carl G. Mohr, and Tzvi Gal-Chen

Abstract

Pressure, buoyancy and virtual potential temperature perturbations are calculated from wind fields derived from Doppler radar data taken in a surface cold front. The dynamics of the front are similar to a density current This hypothesis is also suggested by accompanying numerical simulations of cold air outflows. The updraft at the leading edge of the cold air mass is maintained in conjunction with an upward directed pressure force. The average maximum updraft is in excess of 7 m s−1 without any appreciable potential instability present in the “undisturbed” warm-sector sounding.

The buoyancy and virtual potential temperature data reveal a front with a substantial fraction of the cooling taking place within the first 2 km of a frontal zone. Thus, the aspect ratio (width/depth) of the front, even after the filtering associated with the interpolation and retrieval process, is slightly less than one. The frontogenesis for the shear in the along-front wind and the thermal gradient are discussed. The gradient of these quantities in the lower levels is maintained by confluence and eventually destroyed by tilting of the gradients into the horizontal. The thermal fields are locally influenced by diabatic processes in the frontal updraft and behind the front. The cooling taking place in the cold air is apparently related to evaporation and melting of hydrormeteors. The virtual potential temperature reduction with this cooling is in excess of 0.5 K.

Considerable along-front variations in the pressure, wind, and precipitation field occur due to the presence of a 13-km wave. These variations in the wind field are due to the influence of the waves of the rate of frontogenesis experienced by a parcel as it moves through the frontal zone. The primary factor for the changes in frontogenesis in the direction parallel to the surface front is the variation in the confluence term.

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L. Jay Miller, Carl G. Mohr, and Andrew J. Weinheimer

Abstract

Periodic sampling of the Doppler radar return signal at the pulse repetition frequency causes measured velocities to be ambiguous (folded) when true meteorological velocities along the radial direction exceed the Nyquist or folding value. Furthermore, mean radial velocity estimates become more uncertain as the spatial variability of velocity increases or the returned signal strength decreases. These data have conventionally been prepared for such uses as multiple-Doppler radar wind synthesis by unfolding and editing them in the sampling domain (range-azimuth-elevation spherical coordinates).

An alternative method of locally (to the output grid point) unfolding the unedited radial velocities during their linear interpolation to a regular Cartesian grid is presented. The method preserves the spatial discontinuities of order twice the Nyquist value associated with velocity folding. A nondimensional velocity quality parameter is also computed which serves to identify interpolated values that contain too much variance to be reliable. Editing of radar data is thereby postponed until all radar data are mapped to the analysis coordinate system. This allows for iterative global unfolding and multiple-Doppler synthesis of complicated convective storm flow patterns. The resolution of folding in such flow fields may require more information than is usually available from single radar radial velocity fields in spherical coordinates. Further, the amount of data that must be subsequently manipulated is reduced about ten-fold in the process of interpolation.

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Carl G. Mohr, L. Jay Miller, Robin L. Vaughan, and Harold W. Frank

Abstract

During the 1981 summer season within a 70 000 km2 area surrounding Miles City, Montana, researchers from approximately twenty institutions participated in the Cooperative Convective Precipitation Experiment (CCOPE). The measurements collected during this project comprise one of the most comprehensive datasets ever acquired in and around individual convective storms on the high plains of North America. Principal data systems utilized during CCOPE included 8 ground-based radar (7 of which had Doppler capability), 12 instrumented research aircraft, and a network of 123 surface stations.

A major data processing goal has been to combine these independently acquired mesoscale measurements into a numerical description of observed atmospheric conditions at any point in time. Using the CCOPE data archive as an example, this paper describes the procedures used to reduce these high resolution observations to a common spatial and temporal framework. The final product is a digital description of the environment similar to that employed by most modelers—a three-dimensional Cartesian coordinate system containing fields that represent the instantaneous state of the atmosphere at discrete times across the period of interest. A software package designed to facilitate the construction and analysis of these composite data structures will also be discussed.

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