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Dennis Chesters, Anthony Mostek, and Dennis A. Keyser

Abstract

Local forecasts often rely upon the extrapolation of trends seen in images of clouds from the GOES satellite. This work presents correspondingly high resolution images of atmospheric soundings calculated from the VAS radiometer on GOES. These VAS sounding images vividly depict moisture and stability conditions in preconvective regions, as though GOES were observing the United States with “stability detectors” instead of infrared detectors at 1–3 h intervals and 60 km horizontal resolution. False color images are presented for VAS-derived precipitable water and lifted index fields during two midsummer days that contain a wide variety of preconvective and convective conditions. Since each sounding image requires only 5 min to calculate with an automated regression algorithm on a minicomputer, it should be possible to process VAS data operationally for real-time objective analysis of potential convective instabilities.

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Dennis A. Keyser and Donald R. Johnson

Abstract

The interaction between the mass circulations within a mesoscale convective complex (MCC) and the entrance region of an upper tropospheric polar jet streak is examined to investigate mechanisms responsible for linking these two scales of motion. During NASA's fourth Atmospheric Variability Experiment (AVE IV), maximum wind speeds within a jet streak increased nearly 15 m s−1 over three to six hours as the jet streak propagated eastward over the Great Lakes region. Severe convection located on the rear anticyclonic flank of the jet streak within the direct circulation of the entrance region also intensified and increased in areal extent.

The results analyzed within isentropic coordinates establish that latent heating in the MCC modified the direct mass circulation in the jet streak entrance region through the forcing of diabatic components of ageostrophic motion. The net isallobaric ageostrophic component in the entrance region, determined through the gradients of differential heating and mass flux, exceeded 4 m s−1. The mass divergence in the upper troposphere was due to the slight excess of the diabatic isallobaric mode over the opposing adiabatic mode, while mass convergence in the lower troposphere was due to the slight excess of the adiabatic isallobaric mode over the diabatic mode. The intensity of the other diabatically forced ageostrophic component, induced through vertical advection of momentum in a sheared environment, ranged from 5 to 10 m s−1 in the middle and upper troposphere of the jet's entrance region. Over much of the convective region, both the total isallobaric and the inertial diabatic ageostrophic components were directed from the anticyclonic to the cyclonic side of the jet streak at jet streak level in the same sense as pre-existing ageostrophic motion in the upper branch of the jet's direct mass circulation. This diabatically forced ageostrophic motion directed along the pressure gradient of the larger scale resulted in additional generation of kinetic energy which ultimately produced stronger winds in the jet streak downstream.

A comparison between actual and geostrophic momentum forms for ageostrophic motion revealed discrepancies of 20 m s−1 that were mainly due to differences in the horizontal fields of inertial advective ageostrophic motion. This expected result points out that the rapid evolution of ageostrophic motion within the shorter time scales of MCCs limits the applicability of geostrophic momentum theory in prescribing the structure of ageostrophic motion.

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Ralph A. Petersen, Louis W. UccellinI, Anthony Mostek, and Dennis A. Keyser

Abstract

Infrared and visible imagery from VAS are used to delineate mid- and lower-tropospheric moisture fields for a variety of severe storm cases in the southern and central United States. The ability of sequences of images to isolate areas of large negative vertical moisture gradients and apparent convective instability prior to the onset of convective storms is assessed. Midlevel dryness is diagnosed directly from the VAS 6.7 channel observations, while low-level water vapor is either inferred from the presence of clouds in visible and infrared imagery or, in cloud-free areas, calculated from VAS "split window" channels. A variety ofimage combination procedures are used to deduce the stability fields which are then compared with the available radiosonde data. The results for several severe storm cases indicate that VAS can detect mid- and low-level mesoscale water vapor fields as distinct radiometric signals. The VAS imagery shows a strong tendency for thunderstorms to develop along the edges of bands of midlevel dryness as they overtake either pre-existing or developing low-level moisture maxima. Image sequences depict the speed with which deep moist and dry layers can develop and move, often at scales not resolvable using conventional radiosonde data. The images thus demonstrate the ability of VAS radiance data to detect differential moisture advectionsin rapidly changing pre-convective environments.

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Steven E. Koch, William C. Skillman, Paul J. Kocin, Peter J. Wetzel, Keith F. Brill, Dennis A. Keyser, and Michael C. McCumber

Abstract

A large number of predictions from a regional numerical weather prediction model known as the Mesoscale Atmospheric Simulation System (MASS 2.0) am verified against routinely collected observations to determine the model's predictive skill and its most important systematic errors at the synoptic scale. The model's forecast fields are smoothed to obtain synoptic-scale fields that can be compared objectively with the observation. A total of 23 (28) separate 12 h (24 h) forecasts of atmospheric flow patterns over the United States are evaluated from real-time simulations made during the period 2 April-2 July 1982. The model's performance is compared to that of the National Meteorological Centers operational Limited-area Fine Mesh (LFM) model for this period. Temporal variations in normalized forecast skill statistics are synthesized with the mean spatial distribution of daily model forecast errors in order to determine synoptic-scale systematic errors.

The mesoscale model produces synoptic-scale forecasts at an overall level of performance equivalent to that of the LFM model. Lower tropospheric mass fields are, for the most part, predicted significantly better by the MASS 2.0 model, but it is outperformed by the LFM at and above 500 mb. The greatest improvement made by the mesoscale model is a 73% reduction of cold bias in LFM forecasts of the 1000–500 mb thickness field, primarily over the western United States. The LFM bias is the combined result of model overforecasts of surface anticyclone intensity and underforecasts of surface cyclone intensity and nearby 500 mb geopotential heights.

The poorer forecasts by the MASS 2.0 model in the middle and upper troposphere result primarily from a systematic mass loss which occurs only under a certain synoptic flow pattern termed the mass loss regime. Problems with specification of the lateral boundary conditions and, to a lesser extent, erroneous computation of the map factor seemed to contribute most to the systematic mass loss. This error is very significant since MASS 2.0 performance either equaled or surpassed that of the LFM model in forecasts of virtually every meteorological field studied when mass loss regime days were excluded from the sample.

Two other important systematic errors in MASS model forecasts are investigated. Underforecasts of moisture over the Gulf Coast states are found to be due in large part to a negative bias in the moisture initialization. Also, overforecasts of surface cyclone intensity and 1000–500 mb thickness values over the Plains states are traced to excessive latent beating resulting from the absence of a cumulus parameterization scheme in the model. Awareness of these synoptic-scale forecasts errors enables more effective use to be made of the (unfiltered) mesoscale forecast fields, which are evaluated in the companion paper by Koch.

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