Quasigeostrophic diagnosis of an explosively developing cyclone along the Northern California coast

Abstract

A strong extratropical cyclone made landfall in Northern California on 19 February 1993, bringing wind gusts in excess of 90 mph to the Mendocino County coastline and rainfall amounts greater than four inches in mountainous areas. NOAA buoy data showed a 13 mb pressure fall in 14 hours, a rate that exceeds the criterion for 'meteorological bombs'. The eye-like feature at the center of circulation that developed prior to landfall was reminiscent of hurricane-like cyclones that develop at high latitudes termed "polar lows". A qualitative assessment of the quasigeostrophic forcing showed that the storm developed in an area in which synoptic scale forcing both from cyclonic differential vorticity advection and warm air advection were favorable for midtropospheric upward vertical motion and surface cyclone development. A quantitative assessment of the terms in the quasigeostrophic Sutcliffe-Petterssen Development Equation (SPDE) showed that the dominant factor in the development of this cyclone was the large change in the 1000-500 mb thickness field. A quantitative assessment of the three contributing terms to thickness changes returned mixed results. The largest contribution to that term was adiabatic heating due to vertical motion in an environment with weak static stability. The author hypothesizes that diabatic heating occurring in the cooler boundary layer air overrunning the warm ocean surface decreased stability in the lower troposphere. The calculations produced an inconsistent result, as the contribution to thickness change due to sensible heating, calculated as a residual, was negative. This is impossible since meteorological data showed that this term should have been positive. Nevertheless, the results were conclusive that the thickness changes consistent with the warming core of the disturbance made the largest contribution to surface development as the explosive growth occurred just off the coast. This conclusion, along with other diagnostic aspects of the storm, including the clear eye, suggests that the disturbance can be classified as a polar low, the first to be identified in California.