Mesoscale dynamics of a deepening secondary cyclone in FASTEX IOP16: Three-dimensional structure retrieved from dropsonde data

Abstract

The mesoscale dynamics of a secondary cyclone sampled during the field phase of the Fronts and Atlantic Storm-Track EXperiment (FASTEX) Intensive Observation Period (IOP) 16 are documented using airborne dropsonde data. the method proposed to recover the mesoscale analytical three-dimensional (3-D) dynamic fields from the dropsonde measurements is evaluated using simulated fields, airborne in situ measurements, and airborne Doppler radar measurements. These simulations and comparisons indicate that the method is able to resolve the mesoscale structures within the studied secondary cyclone both in the regions of clear air and precipitation. It appears, however, that the most accurate description of the 3-D dynamic fields in both these regions would be obtained by combining Doppler and dropsonde measurements. The studied secondary cyclone is a fast moving frontal wave that developed on the trailing cold front of a parent low situated over Greenland. Using the dropsonde-derived 3-D fields, the mesoscale flows involved in the organization of the secondary cyclone are described. the potential-vorticity and equivalent-potential-vorticity fields (deduced from the basic analytical fields) indicate that the atmosphere is roughly neutral to moist slantwise instability. This configuration is known to significantly promote frontogenesis. the vertical configuration of the jets and circulations within this secondary cyclone are compared with the coupling-uncoupling scheme of Shapiro. A slight coupling is found between the thermally indirect circulation and the ageostrophic circulation. the obtained coupling is, however, not as developed as in the ideal case described by Shapiro, which may explain the small upward velocities in our case. Airborne Doppler radar and dropsonde measurements are combined so as to infer the multi-scale processes involved in the mesoscale and convective-scale organization of the Atlantic secondary cyclones and to evaluate the new emerging theoretical interpretations of secondary cyclogenesis.