MesoLAPS predictions of low-level convergence lines over northeastern Australia

Abstract

The prediction of low-level convergence lines over northeastern Australia such as those which give rise to the 'Morning Glory' and the North Australian Cloud Line (NACL) are investigated using MesoLAPS, a mesoscale version of the Australian Bureau of Meteorology's operational Limited Area Prediction System. The model is used also to examine aspects of the dynamics of such lines. The predictions were made during the Gulf Lines EXperiment (GLEX) in 2002 and are compared here with data collected during the experiment. The ability of MesoLAPS to forecast the convergence lines is investigated in detail for selected cases. In two cases with well-developed southerly morning-glory disturbances, the model was able to capture the separation of a bore-like disturbance from an airmass change, although the model does not have the resolution to capture the wave-like structures that develop at the leading edge of the bore waves. A climatology for the entire 44 day period between 11 September to 24 October (EST) shows that MesoLAPS has significant skill in forecasting the lines, but it does not capture all of them. About 85% of forecasts of northeasterly morning glories and southerly morning glories, or of their nonoccurrence, were correct, while the corresponding percentage for the NACL was about 65%. However, about 15% of northeasterly morning glories and about 35% of NACL events that occurred were not forecast by the model. Also, only 6 out of 11 southerly morning glories were forecast. detailed analysis of the MesoLAPS calculations indicates that the broad-scale generation mechanisms of northeasterly and southerly morning glories are similar and it enables us to construct a conceptual model for the generation of southerly morning glories.