Examining deep and shallow Adriatic bora events

Kurzfassung

This Note examines the differences observed between two Adriatic bora events, that took place in the winter of 2011 (Case 1) and 2012 (Case 2), using the Weather Research and Forecasting (WRF) model aided by two TerraSAR-X (TSX) images. The former was a deep bora event, predominantly of an anti-cyclonic type, whereas the latter was a shallow bora case, simultaneously pertaining to cyclonic and frontal types. The high resolution of the TS-X ScanSAR scenes offered for the first time the opportunity to examine the finer details of the bora jet and wake patterns. In the Case 1 bora, the bright patterns of the two major jets dominated the TS-X image, with a well-defined broad wake zone in between. In the Case 2 bora, a much richer pattern was registered in the TS-X Normalised Radar Cross Section (NRCS) field, with a notable difference in the intensity of the response. The major jets were broader, each exhibiting features apparently reflecting the impact of smaller ridges in the Dinaric mountain chain; The impact was also observed in the narrower wake zone, which was populated with three additional but shorter jets. In both cases, the TS-X derived surface wind field convincingly conveyed the rich morphology of the jet-and-wake response. The WRF model near-surface wind simulations, in a one way nested four domain setup, closely resembled the ones obtained with the satellite sensor. Additionally, the vertical along coast cross section of the lower troposphere indicated jets within the lowest 1 km corresponding to the ones registered in the TS-X derived surface wind, with the jets in Case 2 exhibiting much higher velocities. Both the satellite observations and the model simulations indicated that these low level jets (LLJs) can maintain their coherence and speed at considerable offshore distances, which is in agreement with certain theoretical studies. The longer offshore extent and higher velocities suggested by the WRF simulations have yet to be verified with field data.