Potential vorticity-based interpretation of the evolution of 'The Greenhouse Low', 2-3 February 1991

Abstract

The explosive synoptic-scale 'Greenhouse Low' that hit Iceland on 3 February 1991 has been examined in the potential vorticity (PV) framework. Three positive PV anomalies were investigated in detail: (1) a surface thermal anomaly, (2) a lower-tropospheric, diabatically produced PV anomaly, and (3) a PV anomaly propagating along the tropopause. Through PV piecewise inversions, we have quantified the contributions of these 3 anomalies to the total geopotential field. By using a specific, quasi-linear procedure, the total geopotential field can be retrieved from the sum of the mean field and all the PV anomalies (positive and negative). The piecewise inversions were performed at different times and for different simulations using HIRLAM. This allows us to draw a comprehensive picture of the time evolution of the cyclone, and to quantify the roles played by different dynamical and physical processes. Initially, the surface thermal anomaly on the southeastern flank of the baroclinic region was crucial in spinning up the cyclone. Latent heating in the lower troposphere due to frontal ascent then took over as the most important contributor to cyclone deepening. The associated PV anomaly intensified explosively between 06 UTC 2 February and 00 UTC 3 February. The upper-level PV anomaly also played an important role, especially in the later stages, between 06 and 12 UTC 3 February. Our findings suggest that this anomaly was mainly a separate entity that influenced the low-level flow, but was partly due to vertical propagation of PV from the lower layers.