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

The explosive synoptic-scale “Greenhouse Low” that hit Iceland on 3 February 1991 has beenexamined in the potential vorticity (PV) framework. Three positive PV anomalies were investigatedin detail: (1) a surface thermal anomaly, (2) a lower-tropospheric, diabatically producedPV anomaly, and (3) a PV anomaly propagating along the tropopause. Through PV piecewiseinversions, we have quantified the contributions of these 3 anomalies to the total geopotentialfield. By using a specific, quasi-linear procedure, the total geopotential field can be retrievedfrom the sum of the mean field and all the PV anomalies (positive and negative). The piecewiseinversions were performed at different times and for different simulations using HIRLAM. Thisallows us to draw a comprehensive picture of the time evolution of the cyclone, and to quantifythe rôles played by different dynamical and physical processes. Initially, the surface thermalanomaly on the south eastern 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 themost important contributor to cyclone deepening. The associated PV anomaly intensifiedexplosively between 06 UTC 2 February and 00 UTC 3 February. The upper-level PV anomalyalso 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 lowlevelflow, but was partly due to vertical propagation of PV from the lower layers.