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G. M. Heymsfield, J. B. Halverson, and I. J. Caylor

Abstract

An extensive wintertime squall line on 13 January 1995 occurring along the U.S. Gulf of Mexico coastline is examined using airborne radar observations combined with conventional data analysis. Flight tracks with the ER-2 Doppler radar (EDOP) mounted on the high-altitude (20 km) ER-2 aircraft provided a unique view of the vertical structure of this line. In this paper, the authors document the squall line structure, and compare and contrast this structure with other published cases.

The squall line had several prominent features that differ from previous studies: 1) the stratiform region was wide in comparison to more typical systems that are 50–100 km wide; 2) the trailing stratiform region consisted of two to three separate embedded trailing bands rather than one continuous band; 3) vertical motions in the trailing stratiform region were nearly twice as strong as previously reported values, with mean values approaching 1 m s−1 between 7- and 9-km altitude, and larger values (1.5 m s−1) in the embedded bands; 4) reflectivities were large with mean stratiform values of about 38 dBZ, and maximum convective values of about 55 dBZ; 5) the squall line rear inflow descended to the surface well behind the leading edge (∼200 km); 6) the convective and squall line inflow region exhibited unique microphysics with small graupel or hail falling out of the tilted squall line updraft, and a wavy, elevated melting region associated with the inflow; and 7) the squall-scale transverse circulation was directly coupled with a jet streak thermally direct circulation, and the ascending branch of this direct circulation may have enhanced production of widespread stratiform rainfall. A conceptual model is presented highlighting the features of this squall line and the coupling of the squall line to the larger-scale flow.

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V. N. Bringi, K. Knupp, A. Detwiler, L. Liu, I. J. Caylor, and R. A. Black

Abstract

The relationships among kinematic, microphysical, and electric field properties within a multicell Florida thunderstorm are investigated using observations from three Doppler radars (one with multiple wavelength and polarization diversity capabilities), four instrumented penetrating aircraft, a surface-based electric field mill network, and other observation facilities. The storm was convectively active for about 1 h and at least five primary cells developed within the storm during this time, one of which went through three consecutive development cycles. The updrafts in this storm were 2–4 km wide, exhibited bubble-like evolution, and had lifetimes of 10–20 min. The maximum updraft determined by the multiple Doppler analysis was about 20 m s−1. A differential reflectivity (Z DR) “column,” indicating regions containing millimeter-size raindrops, extending above the freezing level, was associated with each cell during its developing stages. This column reached altitudes exceeding 6 km (−8°C) in the stronger updrafts. As the Z DR columns reached maximum altitude, a “cap” of enhanced linear depolarization ratio (LDR) and enhanced 3-cm wavelength attenuation (A 3) formed, overlapping the upper regions of the Z DR column. These parameters indicate rapid development of mixed-phase conditions initiated by freezing of supercooled raindrops.

Lightning was observed only in the central and strongest convective cell. Electric fields exceeding 10 kV m−1 were noted during aircraft penetrations in this as well as several other cells that did not produce lightning. Fields exceeding 1 kV m−1 were noted by the instrumented aircraft at midcloud levels within a few minutes of development of mixed-phase conditions at these levels or aloft. The first intracloud lightning was detected by the surface field mill network within 5 min of development of mixed-phase conditions aloft in the first cycle of development in the central cell, and the first cloud-to-ground event was noted within 9 min of this development. Lightning continued through two additional cycles of updraft growth in this central region and diminished as the convection subsided after about 30 min. Aircraft-measured electric fields and lightning retrievals from the surface field meter network are consistent with a tendency for negative charge to accumulate above the 6.5 km(−12°C) level within regions of radar reflectivity maxima and for positive charge to accumulate in the anvil region well above 9 km (−30°C).

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