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Christopher J. Anderson and Raymond W. Arritt

Abstract

Large, long-lived mesoscale convective systems (MCSs) over the United States during the 1997–98 El Niño are documented. Two periods of abnormal MCS activity are identified in 1998: from March to mid-April an unusually large number of quasi-linear MCSs were observed in the Midwest; while quasi-circular MCSs in June–August of 1998 were concentrated near 37°N rather than following a seasonal shift similar to that observed in the climatological distribution. Episodic surges of northerly low-level flow were infrequent in March 1998, thereby leading to an unusually high incidence of quasi-linear MCSs and to precipitation anomalies in the central United States.

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Christopher J. Anderson and Raymond W. Arritt

Abstract

Large, long-lived convective systems over the United States in 1992 and 1993 have been classified according to physical characteristics observed in satellite imagery as quasi-circular [mesoscale convective complex (MCC)] or elongated [persistent elongated convective system (PECS)] and cataloged. The catalog includes the time of initiation, maximum extent, termination, duration, area of the −52°C cloud shield at the time of maximum extent, significant weather associated with each occurrence, and tracks of the −52°C cloud-shield centroid.

Both MCC and PECS favored nocturnal development and on average lasted about 12 h. In both 1992 and 1993, PECS produced −52°C cloud-shield areas of greater extent and occurred more frequently compared with MCCs. The mean position of initiation for PECS in 1992 and 1993 followed a seasonal shift similar to the climatological seasonal shift for MCC occurrences but was displaced eastward of the mean position of MCC initiation in 1992 and 1993. The spatial distribution of MCC and PECS occurrences contain a period of persistent development near 40°N in July 1992 and July 1993 that contributed to the extreme wetness experienced in the Midwest during these two months.

Both MCC and PECS initiated in environments characterized by deep, synoptic-scale ascent associated with continental-scale baroclinic waves. PECS occurrences initiated more often as vigorous waves exited the intermountain region, whereas MCCs initiated more often within a high-amplitude wave with a trough positioned over the northwestern United States and a ridge positioned over the Great Plains. The low-level jet transported moisture into the region of initiation for both MCC and PECS occurrences. The areal extent of convective initiation was limited by the orientation of low-level features for MCC occurrences.

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V. J. OLIVER, R. K. ANDERSON, and E. W. FERGUSON

Abstract

TIROS photographs of cloud patterns in the vicinity of the jet stream are examined and compared with surface, upper air, and pilot-report data. It is found that with certain conditions of lighting and satellite attitude the northern edge of the cirrus cloud shield, which lies immediately south of the jet, can be easily identified by a shadow cast by the higher cloud deck on the lower underlying surface. This shadow identifies the cloud structure associated with the jet stream. Differences in texture and pattern also help to identify the northern limits of the high-level cirrus and thus aid in positioning the jet stream.

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James Correia Jr., Raymond W. Arritt, and Christopher J. Anderson

Abstract

The development and propagation of mesoscale convective systems (MCSs) was examined within the Weather Research and Forecasting (WRF) model using the Kain–Fritsch (KF) cumulus parameterization scheme and a modified version of this scheme. Mechanisms that led to propagation in the parameterized MCS are evaluated and compared between the versions of the KF scheme. Sensitivity to the convective time step is identified and explored for its role in scheme behavior. The sensitivity of parameterized convection propagation to microphysical feedback and to the shape and magnitude of the convective heating profile is also explored.

Each version of the KF scheme has a favored calling frequency that alters the scheme’s initiation frequency despite using the same convective trigger function. The authors propose that this behavior results in part from interaction with computational damping in WRF. A propagating convective system develops in simulations with both versions, but the typical flow structures are distorted (elevated ascending rear inflow as opposed to a descending rear inflow jet as is typically observed). The shape and magnitude of the heating profile is found to alter the propagation speed appreciably, even more so than the microphysical feedback. Microphysical feedback has a secondary role in producing realistic flow features via the resolvable-scale model microphysics. Deficiencies associated with the schemes are discussed and improvements are proposed.

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M. Segal, C. Anderson, R. W. Arritt, R. M. Rabin, and D. W. Martin

Abstract

Satellite images that illustrate the clearing of cumulus clouds downwind from snow-covered areas are presented. The cloud clearing resembles that occasionally observed with lakes during warm advection, supporting the suggestion that the thermal forcing associated with a uniform snow-covered area is comparable to that of a cold-water lake of similar size. Analysis of snow cover patterns in the central United States suggests that the climatological probability for situations conducive to the cloud clearing is at most once per month.

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M. Segal, R. W. Arritt, J. Shen, C. Anderson, and M. Leuthold

Abstract

In this note the forcing of cumulus cloud clearing over and downwind from lakes during the warm season is evaluated conceptually by modeling and observational approaches. It is suggested that drying by dynamically induced subsidence and suppression of the CBL over the lake mutually contribute to the cloud clearing. The effect of background flow speed and the extent of potential clearing area is illustrated. Various implications of the cloud clearing are discussed.

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