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Mohan K. Ramamurthy and I. M. Navon

Abstract

A conjugate-gradient variational blending technique, based on the method of direct minimization, has been developed and applied to the problem of initialization in a limited-area model in the summer monsoon region. The aim is to blend gridded winds from a high-resolution limited-area analysis with a lower-resolution global analysis for use in a limited-area model that uses the, global analyst for boundary conditions. The ability of the variational matching approach in successfully blending meteorological analyses of varying resolutions is demonstrated. Reasonable agreement is found between the blended analyses and the imposed weak constraints, together with an adequate rate of convergence in the unconstrained minimization procedure. The technique is tested on the 1979 onset vortex vortex case using data from the FGGE Summer MONEX campaign. The results indicate that the forecasts made from the variationally matched analyses show positive impact and perform better than those from the unblended analyses.

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Matthew S. Mayernik, Mohan K. Ramamurthy, and Robert M. Rauber
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Matthew S. Mayernik, Mohan K. Ramamurthy, and Robert M. Rauber
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Matthew S. Mayernik, Mohan K. Ramamurthy, and Robert M. Rauber
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Matthew S. Mayernik, Mohan K. Ramamurthy, and Robert M. Rauber
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Michael T. Shields, Robert M. Rauber, and Mohan K. Ramamurthy

Abstract

A winter snowstorm developed on 10–11 February 1988 over the midwestern United States and produced several inches of snowfall locally over east-central Illinois. Analysis of the mesoscale organization of the storm revealed the presence of complex banded structure throughout its 17-h evolution. Three distinct types of mesoscale precipitation bands were identified during the course of the storm using a 10-cm Doppler radar as part of the University of Illinois Winter Precipitation Program. The bands had different orientations, directions of movement, relationships to synoptic-scale frontal zones, and mechanisms for development.

The mesoscale organization of this storm system is reviewed. Mesoscale, synoptic-scale, and Doppler analyses of the storm structure are presented. The role of boundary-layer convergence, conditional symmetric instability, and frontogenetical forcing in the formation and maintenance of the different mesoscale precipitation bands is discussed.

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Matthew S. Mayernik, Mohan K. Ramamurthy, and Robert M. Rauber
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Robert M. Rauber, Mohan K. Ramamurthy, and Ali Tokay

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A severe freezing rainstorm produced as much as 4.5 cm of freezing rain during an 18-h period at Champaign, Illinois, on 14–15 February 1990, resulting in over $12 million in damage, week-long power outages, and a federal disaster declaration. The ice storm occurred during the University of Illinois Winter Precipitation Program based in Champaign. The early mesoscale evolution of this storm was documented for several hours with a 10-cm Doppler radar and Cross-chain Loran Atmospheric Sounding System soundings launched every 3 h. The freezing rain event occurred when convective bands developed over a slow-moving warm front during a period of strong overrunning. The strongest convection developed in a period of about 1 h, with a narrow elongated band northwest of the radar producing very heavy sleet and a band just south of the radar producing heavy freezing rain, along with in-cloud lightning.

An analysis of conditional symmetric instability yielded no evidence that centrifugal accelerations were important to the development of convection in this storm. Frontogenetic forcing was strongest several hours before the development of the bands but apparently was also insufficient to trigger convection until the local atmosphere became marginally unstable to upright convection. The transition from a conditionally stable to an unstable atmosphere in the vicinity of the bands was directly associated with locally strong warm advection above the warm frontal surface.

Forecast guidance, including the nested grid model (NGM) thickness, precipitation, and 850-mb temperature forecasts, and model output statistics of both the limited fine mesh (LFM) model and the NGM all predicted that the warm front would progress northward and that freezing rain would convert to rain before significant glaze accumulations occurred. Forecasts of midtropospheric parameters such as 1000–500-mb thickness and 850-mb temperature indeed verified; however, surface temperature forecasts were significantly in error, with errors ranging from 5° to 10°C during the period of heaviest glaze accumulation. The observed surface temperature never rose above 0°C during the period of ice accumulation or throughout the following day. The isothermal conditions observed during and after the storm appeared to be the result of sublimation and melting of ice that had accumulated on surface objects. The available evidence suggested that ice sublimation and melting, in addition to cooling the boundary layer, maintained a small wedge of cold air at the surface over which warmer air rose as it advected northward. The result of ice sublimation and melting was to retard the movement of the surface warm front, although warm air aloft was free to move over the narrow wedge of cooled surface air. By maintaining the surface temperature near 0°C, diabatic processes extended the duration of time that heavy glaze accumulations remained on trees and wires, allowing more damage to occur.

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Brian F. Jewett, Mohan K. Ramamurthy, and Robert M. Rauber

Abstract

On 14 February 1992, a long-lived moderate-amplitude mesoscale gravity wave formed in Kansas during the Storm-scale Operational and Research Meteorology-Fronts Experiment Systems Test (STORM-FEST). Wave formation was evident in correlated surface pressure and wind fields. The wave of depression, accompanied by a weak rainband, tracked across the state. A wealth of data was collected on the mature wave as it passed over the STORM-FEST dual-Doppler domain. However, the mechanism of genesis remained difficult to ascertain, since wave formation occurred in a region of less comprehensive observations.

The genesis of the STORM-FEST gravity wave is successfully simulated using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (Penn State–NCAR) Mesoscale Model (MM5), which was run at 6-km grid spacing in the innermost domain. The lee cyclone movement, dry airmass development, and gravity wave formation over Kansas were successfully captured by the model. Results presented here indicate that evaporative processes associated with a rainband resulted in subsidence warming and depression of the underlying warm-frontal inversion. The reduced inversion height produced surface pressure falls, the surface manifestation of a developing gravity wave. Numerical experiments with and without evaporative processes have isolated the key importance of evaporatively driven downdrafts in wave genesis. A conceptual model of the development and evolution of the wave is presented that is consistent with both observations and the findings of the numerical experiments.

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Matthew S. Mayernik, Mohan K. Ramamurthy, and Robert M. Rauber
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