Structure of a Midlatitude Squall Line Formed in Strong Unidirectional Shear

J. C. Fankhauser National Center for Atmospheric Research, Boulder, Colorado

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G. M. Barnes National Center for Atmospheric Research, Boulder, Colorado

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M. A. LeMone National Center for Atmospheric Research, Boulder, Colorado

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Abstract

Data from five Doppler radars, the surface mesonet, aircraft, and rawinsondes from the Cooperative Convective Precipitation Experiment (CCOPE) are used to document the structure and evolution of a squall line with unusually persistent cells and an anvil that spreads downwind in strong upper-level westerlies. The environmental sounding showed linear shear of ∼4 m s−1 km−1 through the troposphere, a convective available potential energy of 600 m2 s−2, and a convective Richardson number of 10, based on the wind in the lowest 6 km.

The orientation of the squall line, comprised of high-reflectivity centers spaced 20–40 km apart, changed with time. Initially, the squall-line axis was normal to the environmental shear, but with time it became parallel to the shear vector, as the northeastern portion of the subcloud cold dome merged with cold air generated by individual storms that had formed ahead of the line. The intensity of the cells within the squads line diminished as its axis became more parallel to the shear.

Trajectory analyses based on the Doppler-derived wind field show that three-dimensional airflow is crucial to the maintenance of the squall line. Boundary-layer air directly ahead of the strongest reflectivity centers fed the associated updrafts while air on their flanks rose slightly, was cooled by evaporation of rain, and then descended to become the primary source of air in the subcloud cold dome. In contrast to typical midlatitude squall lines, there was no evidence of organized rear-to-front system-relative airflow in the subcloud air. This is explained in terms of the initial front-to-rear momentum of the cold-dome source air, with frictional effects also playing a role for air near the surface. Since the ground is traveling rearward relative to the storm, frictional effects oppose the pressure gradient ahead of the cold-dome pressure maximum and keep the near-surface air moving rearward throughout the cold dome. Only a small fraction of the subcloud air originated at midcloud levels, probably because evaporation above cloud base was inhibited by high relative humidities in the environment and because comparatively weak updrafts produced only modest amounts of condensate for water loading.

The persistence of squall-line elements is discussed in light of (a) their resemblance to supercells as represented in numerical simulations, and (b) recent theories involving the balance of vorticity between vertical shear in the low-level environment and the cold dome in the subcloud layer. The squall line is representative of that part of the spectrum of mesoscale convective systems that does not have a rear inflow jet, does not produce a trailing stratiform precipitation region, and does not rely upon penetrative downdrafts to sustain the air mass within the subcloud cold dome.

Abstract

Data from five Doppler radars, the surface mesonet, aircraft, and rawinsondes from the Cooperative Convective Precipitation Experiment (CCOPE) are used to document the structure and evolution of a squall line with unusually persistent cells and an anvil that spreads downwind in strong upper-level westerlies. The environmental sounding showed linear shear of ∼4 m s−1 km−1 through the troposphere, a convective available potential energy of 600 m2 s−2, and a convective Richardson number of 10, based on the wind in the lowest 6 km.

The orientation of the squall line, comprised of high-reflectivity centers spaced 20–40 km apart, changed with time. Initially, the squall-line axis was normal to the environmental shear, but with time it became parallel to the shear vector, as the northeastern portion of the subcloud cold dome merged with cold air generated by individual storms that had formed ahead of the line. The intensity of the cells within the squads line diminished as its axis became more parallel to the shear.

Trajectory analyses based on the Doppler-derived wind field show that three-dimensional airflow is crucial to the maintenance of the squall line. Boundary-layer air directly ahead of the strongest reflectivity centers fed the associated updrafts while air on their flanks rose slightly, was cooled by evaporation of rain, and then descended to become the primary source of air in the subcloud cold dome. In contrast to typical midlatitude squall lines, there was no evidence of organized rear-to-front system-relative airflow in the subcloud air. This is explained in terms of the initial front-to-rear momentum of the cold-dome source air, with frictional effects also playing a role for air near the surface. Since the ground is traveling rearward relative to the storm, frictional effects oppose the pressure gradient ahead of the cold-dome pressure maximum and keep the near-surface air moving rearward throughout the cold dome. Only a small fraction of the subcloud air originated at midcloud levels, probably because evaporation above cloud base was inhibited by high relative humidities in the environment and because comparatively weak updrafts produced only modest amounts of condensate for water loading.

The persistence of squall-line elements is discussed in light of (a) their resemblance to supercells as represented in numerical simulations, and (b) recent theories involving the balance of vorticity between vertical shear in the low-level environment and the cold dome in the subcloud layer. The squall line is representative of that part of the spectrum of mesoscale convective systems that does not have a rear inflow jet, does not produce a trailing stratiform precipitation region, and does not rely upon penetrative downdrafts to sustain the air mass within the subcloud cold dome.

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