Baroclinic Instability - Atmosphere

1 Cyclogenesis diagnosed with potential vorticity The goal of this assignment is to gain an appreciation of the utility of potential vorticity diagnostics by following the evolution of tropospheric PV anomalies during a cyclogenesis event and by applying the invertibility principle to the observed distribution of Ertel potential vorticity. We focus on the the President Day Storm of February 16-18, 2003, when a deep cyclone developed along the mid-Atlantic and northeast coast of the US, giving record snowfall in the Boston area. The two dynamical features, which led to the development of the surface cyclone, were a sharp upper trough that grew over Central US during February 16th and a lower tropospheric thermal wave which originated in the southeastern US during Feb 17th . 2 Synoptic evolution. To study the relative importance of these dynamical features we compute Ertel PV on isobaric surfaces from NCEP analyses at 2:5o resolution. Produce cross-sections of the observed PV over a few days (see script.gdcross) and identify any relevant PV structure. Use the tropopause temperature 1 maps to study the synoptic evolution of the storm and describe any signi…cant tropopause and 850mb features. 3 Balanced ‡ow associated with total potential vorticity To employ the ”invertibility principle”, the total observed PV distribution is inverted (subject to a non-linear balance condition) to recover the balanced geopotential height and streamfunction - the procedure is described in Davis and Emanuel (1991). Compare the analyzed height …elds with the balanced height …elds at 1000, 500and 300 mb for Feb 17th 00z (see script.balh and script.obsh). Identify any regions where the balanced …eld di¤ers signi…cantly from the analyzed …eld - compare the non-divergent wind with the observed wind (see script.wind_di¤er). Hint: consider the assumptions made in deriving the inversion equation. 4 Balanced ‡ow associated with perturbation potential vorticity. To study the relative importance of features in the PV …eld, we study the ‡ow associated with di¤erent PV perturbations. Here perturbations are de…ned as departures from a 5-day time-mean (from 030214 00z to 030219 12z). As an example plot the 300 mb perturbation PV and the surface perturbation (925 mb potential temperature perturbation) at selected times (see script.qpert). Sketch the wind …eld you might expect to be associated with each perturbation. To study the e¤ects of each perturbations, it is useful to divide the PV perturbations into upper, lower and surface parts: upper - PV at and above 700 mb and 125 mb potential temperature lower - PV below 700 mb surface - 925 mb potential temperature Each PV perturbation is then inverted subject to non-linear balance condition to recover the balanced geopotential height and streamfunction. Plot the …elds that have been obtained by 2 (i) inversion of the upper PV anomaly alone (see script.upper) and (ii) inversion of the lower PV anomaly alone (see script.lower) Use these plots to assess the relative importance of upper and lower anomalies in controlling the evolution of the upper-level and the lower-level ‡ow How much in‡uence did the lower perturbation have on the 300 mb ‡ow? How much in‡uence did the upper perturbation have on the surface ‡ow? Did these remotely induced circulations tend to amplify or weaken the growing cyclone? 3