Diagnostic capabilities of objective analysis schemes: areal mean vertical motion fields of some weather regimes over Australia

The alms of this report are: (I) to demonstrate that the current data base over southeastern Australia is sufficient to provide some useful information on the divergence and vertical motion fields of synoptic scale weather systems, (ii) to show that this information is reproduced in objective wind analyses, and (iii) to illustrate the vertical distribution of divergence, vorticity and vertical motion of four weather systems. To achieve these ends the analysed windfields from two objective analysis schemes are used to diagnose the divergence, vorticity and vertical motion fields associated with a major weather event over southeastern Australia. The two schemes are the Australian region operational analysis and a univariate optimum interpolation analysis. Only moderate large-scale consistency over wind data areas is evident between the two versions of the 700 nib vertical motion fields, and the cloud imagery. Data base limitations and errors in the derived divergences prevent adequate definition of these fields on a grid point to grid point basis, contaminate the computed vertical motion fields and render constant level charts of divergence and vertical motion of very little practical use because of difficulties in isolating the useful information. To alleviate some of these problems, and to extract what useful information there is, it is necessary to look at the vertical distribution of areal mean values of divergence and vertical motion of large-scale weather regimes. This technique has been used to derive and validate mean profiles of divergence, vorticity and vertical motion for a number of regimes. These are, a prefrontal cloud-free area, a frontal cloud region, a cyclonic 'conicity advection (CVA) maximum (comma cloud), and a post-frontal cloud-free area. The two schemes give qualitatively similar results, with the optimum analysis providing greater structural detail and larger vertical motions. The consistency of the profiles derived from the two schemes, together with verification from contemporary satellite imagery and available temperature and moisture traces, suggests that the profiles are realistic. The role of geostrophic winds derived from high resolution TIROS-N satellite temperature retrievals is indicated.