Relationship between a Weakening Cold Front, Misocyclones, and Cloud Development on 10 June 2002 during IHOP

Nettie R. Arnott The Pennsylvania State University, University Park, Pennsylvania

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Yvette P. Richardson The Pennsylvania State University, University Park, Pennsylvania

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Joshua M. Wurman Center for Severe Weather Research, Boulder, Colorado

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Erik M. Rasmussen National Severe Storms Laboratory, Norman, Oklahoma

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Abstract

The finescale structure and evolution of a cold front in the presence of small-scale circulations are examined using overdetermined dual-Doppler syntheses of mobile radar data along with thermodynamic data and cloud imagery collected on 10 June 2002 during the International H2O Project (IHOP). Linear clear-air reflectivity maxima and open cellular convection intersect the cold front, causing spatial variations in convergence along the front. Small-scale vertical vorticity maxima (misocyclones) often are coincident with these intersections and are associated with vertical velocity maxima.

Throughout the deployment, trajectory analyses indicate that parcels entering the frontal circulation travel predominately in the along-front direction. During the first hour of the deployment, upward motion is nearly continuous along the front. Consequently, parcels remain in regions of upward motion as they move along the front, and many eventually ascend to cloud base. In response, a line of cumulus clouds develops along the front.

Later in the deployment, however, enhanced warming behind the cold front causes the frontal circulation to weaken. Small-scale features such as misocyclones play a larger role in organizing upward motion along the front than when the front was stronger. Misocyclones contort the weakened cold front and are associated with kinks in radar reflectivity and fractures in upward motion. Parcels moving along the front now experience regions of both upward and downward forcing due to this fracturing. Hence, many of these parcels do not retain upward motion long enough to reach cloud base, and clouds along the cold front dissipate in the dry air above the boundary layer. Parcels that remain coincident with upward motion maxima, such as those maxima associated with misocyclones, however, often still reach the top of the radar domain and presumably cloud base.

Corresponding author address: Yvette P. Richardson, Dept. of Meteorology, The Pennsylvania State University, 503 Walker Building, University Park, PA 16802. Email: yrichardson@psu.edu

Abstract

The finescale structure and evolution of a cold front in the presence of small-scale circulations are examined using overdetermined dual-Doppler syntheses of mobile radar data along with thermodynamic data and cloud imagery collected on 10 June 2002 during the International H2O Project (IHOP). Linear clear-air reflectivity maxima and open cellular convection intersect the cold front, causing spatial variations in convergence along the front. Small-scale vertical vorticity maxima (misocyclones) often are coincident with these intersections and are associated with vertical velocity maxima.

Throughout the deployment, trajectory analyses indicate that parcels entering the frontal circulation travel predominately in the along-front direction. During the first hour of the deployment, upward motion is nearly continuous along the front. Consequently, parcels remain in regions of upward motion as they move along the front, and many eventually ascend to cloud base. In response, a line of cumulus clouds develops along the front.

Later in the deployment, however, enhanced warming behind the cold front causes the frontal circulation to weaken. Small-scale features such as misocyclones play a larger role in organizing upward motion along the front than when the front was stronger. Misocyclones contort the weakened cold front and are associated with kinks in radar reflectivity and fractures in upward motion. Parcels moving along the front now experience regions of both upward and downward forcing due to this fracturing. Hence, many of these parcels do not retain upward motion long enough to reach cloud base, and clouds along the cold front dissipate in the dry air above the boundary layer. Parcels that remain coincident with upward motion maxima, such as those maxima associated with misocyclones, however, often still reach the top of the radar domain and presumably cloud base.

Corresponding author address: Yvette P. Richardson, Dept. of Meteorology, The Pennsylvania State University, 503 Walker Building, University Park, PA 16802. Email: yrichardson@psu.edu

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