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Convective Evolution and Merger in the FACE Experimental Area: Mesoscale Convection and Boundary Layer Interactions

John B. CunningOffice of Weather Research and Modification, NOAA, Boulder, CO 80303

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Ronald L. HolleOffice of Weather Research and Modification, NOAA, Boulder, CO 80303

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Patrick T. GannonOffice of Weather Research and Modification, NOAA, Boulder, CO 80303

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Andrew I. WatsonOffice of Weather Research and Modification, NOAA, Boulder, CO 80303

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Abstract

This paper investigates the interactions between the various scales of motion and, specifically, the inter-actions between convection and the surface boundary layer in the development of a mesoscale convective system within the Florida Area Cumulus Experiment (FACE) experimental area. Data used in the analysis are from a surface mesonetwork covering a 1500 km2 area which consisted of wind measuring stations, raingages, hygrothermographs, microbarographs and temperature, humidity and pressure transducers.

Surface convergence was shown to exist up to 2 h before the development of precipitation over the convergence area within the mesonetwork. Convergence was not being balanced by divergence within the network, which implies mesoscale and/or synoptic-scale forcing.

The subsidence warming and drying in the near environment of the mesoscale convective system appeared to play an important role in its evolution from the mature to the dissipating stage. Soundings taken in the near environment of the convective system showed a 0.8 km lowering in the depth of the moist layer in time. It is hypothesized that the entrainment of this drier air into the convective system, particularly the new convective elements, helps accelerate the mesoscale outflow away from the convective system, which causes the system to evolve into the dissipating stage.

Surface pressure perturbations are shown to be very important in the feedback between developing convection and the boundary layer. Two types of pressure responses are shown, a mesoscale response which appears to affect the entire mesonetwork area and a convective-scale response which affects only a relatively small area beneath the developing cells. The surface pressure perturbations are shown to increase the surface convergence into the area, and as shown in previous papers, this increased surface convergence should then produce a larger and more intense convective system.

The merger between the two convective systems which developed the mesoscale system on this day is described in terms of the radar, visual cloud and raingage characteristics. It is shown that new cloud development and differential motion between convective elements were the two main causes of merger.

Abstract

This paper investigates the interactions between the various scales of motion and, specifically, the inter-actions between convection and the surface boundary layer in the development of a mesoscale convective system within the Florida Area Cumulus Experiment (FACE) experimental area. Data used in the analysis are from a surface mesonetwork covering a 1500 km2 area which consisted of wind measuring stations, raingages, hygrothermographs, microbarographs and temperature, humidity and pressure transducers.

Surface convergence was shown to exist up to 2 h before the development of precipitation over the convergence area within the mesonetwork. Convergence was not being balanced by divergence within the network, which implies mesoscale and/or synoptic-scale forcing.

The subsidence warming and drying in the near environment of the mesoscale convective system appeared to play an important role in its evolution from the mature to the dissipating stage. Soundings taken in the near environment of the convective system showed a 0.8 km lowering in the depth of the moist layer in time. It is hypothesized that the entrainment of this drier air into the convective system, particularly the new convective elements, helps accelerate the mesoscale outflow away from the convective system, which causes the system to evolve into the dissipating stage.

Surface pressure perturbations are shown to be very important in the feedback between developing convection and the boundary layer. Two types of pressure responses are shown, a mesoscale response which appears to affect the entire mesonetwork area and a convective-scale response which affects only a relatively small area beneath the developing cells. The surface pressure perturbations are shown to increase the surface convergence into the area, and as shown in previous papers, this increased surface convergence should then produce a larger and more intense convective system.

The merger between the two convective systems which developed the mesoscale system on this day is described in terms of the radar, visual cloud and raingage characteristics. It is shown that new cloud development and differential motion between convective elements were the two main causes of merger.

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