A Synthetic Aperture Radar–Based Climatology of Open-Cell Convection over the Northeast Pacific Ocean

Abstract

This paper presents an 8-yr (1999-2006) climatology of the frequency of open-cell convection over the northeastern Pacific Ocean and the thermodynamic and kinematic environment associated with its development. The climatology is based on synthetic aperture radar-derived wind speed images and reanalysis data. The climatology shows that open-cell convection was a cold-season phenomenon, having occurred in environments in which the difference in temperature between the near-surface air and the sea surface is negative and in environments with positive surface sensible and latent heat fluxes. Within the region between the surface and 500 hPa, the 700-850-hPa layer median static stability was near moist adiabatic while that for the remainder was conditionally unstable. The median magnitude of the vertical wind shear was largest in the 925-hPa-nearsurface and 500-700-hPa layers while that at midlevels was relatively weak. Similarities are highlighted between the organization of open-cell convection over the northeastern Pacific Ocean and tropical deep moist maritime convection in terms of cold-pool dynamics. Avenues for future work are discussed.

Idealized Mesoscale Model Simulations of Open Cellular Convection Over the Sea

“…However, there is some indication that the region of maximum pressure-wind balance and buoyancy terms is skewed towards the leading edge of the cell in the parallel cross-sections, a result that is confirmed by inspection of the horizontal fields of buoyancy and pressure balance. This result mirrors the observational evidence presented by Sikora et al (2011), who used synthetic aperture-radar-derived wind-speed images and visual satellite images over the north-east Pacific Ocean to suggest that OCC often takes on an arc-shape, with the largest cumulus clouds at the leading edge of the cell and smaller cumulus clouds making up the ring of the sides and trailing edge of the cell. The link between these observational results and the modelling results presented here is an interesting area for further study.…”

On polarimetric characteristics in SAR images of mesoscale cellular convection in the marine atmospheric boundary layer

“…Numerous examples of this type of blistered backscatter pattern have been seen in SAR images of sea surface, linking them to the MCC [Atkinson and Zhang, 1996;Babin et al, 2003]. Young et al [2007] and Sikora et al [2011] have described the morphology of open cell convection using RS-1 and MODIS data; the morphology of our RS-2 SAR image signals is similar to their descriptions. The MODIS image (Figure 1e) that is closest to the SAR image time occurred at 22:10:00 UTC on 1 February; it suggests that MCC did occur and that the SAR image area appears to be a transition zone between open and closed MCC.…”

“…Increased wind shear provides enhanced momentum transfer to the sea surface via shear- driven surface layer turbulence. In an alternate view of things, Young et al [2007] and Sikora et al [2011] suggest that the signatures of open MCC are possibly tied to downdrafts fueled by evaporation, sublimation, or melting, which could constitute momentum mix-down in conjunction with precipitation. In any case, enhanced momentum transfer impacts the mean wind in the boundary layer and associated ocean waves.…”

A Climatology of Precipitating Open-Cell Convection over the Northeast Gulf of Alaska

“…Of interest to the research that is presented here is the finding by Sikora et al (2011) that the vertical distribution of environmental vertical wind shear found in their climatography is conducive to the organization of convection via the shear's interaction with precipitation-driven cold pools (e.g., Johnson et al 2005). That interaction is referred to herein as cold-pool dynamics.…”

“…That interaction is referred to herein as cold-pool dynamics. The resulting organization reported by Sikora et al (2011) manifests as an arc of deeper convective clouds that correspond to a squall and trailing shallower convective clouds ringing a nearly cloud-free lull, with the opencell convection typically occurring in fields. See Figs.…”