Abstract:
This study investigates the role of mechanical forcing within the boundary layer in enhancing low-level precipitation and initiating/intensifying convective precipitation during cases of high intensity precipitation in the wider Auckland
region, New Zealand. Eight cases, that occurred between 2001 and 2008 have been investigated. All cases were observed to be strongly dynamically forced, resulting from the passage of mid-latitude cyclones. These features were observed to be centred mainly to the north and west of the study area, with surface winds from the northeast quadrant over the wider Auckland region. Radar imagery is characterised by regions of both convective and stratiform precipitation for all the cases investigated; areas of convection are often observed to be embedded in areas of larger scale precipitation. These cases were subdivided into eleven heavy precipitation events. Nine of these events were subject to further investigation. Environmental conditions during these events were characterised by steady low-level winds from the northeast quadrant, weak to moderate convective instability, with 0-3km wind shear indicating a high level of directional shear in the lower atmosphere. To investigate mechanical forcing in the boundary layer, low-level Doppler velocity and reflectivity fields measured by the Mt Tamahunga radar, were examined. These data revealed mesoscale structures of the Doppler velocity field not previously documented in this region. Mechanical forcing was identified by
the presence of mesoscale zones of radar radial shear, resulting from horizonal
convergence and/or zones of horizontal shear. These features were observed to
be semi-permanent on the windward side of Little Barrier and Great Barrier islands, the windward side of the Coromandel ranges, and along the west
coast of the Auckland region. Further, zones of semi-permanent radar radial shear were observed to extend downstream (lee side) of Mt Moehau and Great Barrier, Little Barrier and Taranga islands in the Hauraki Gulf. These features have not been documented previously for this study area. The features, observed downstream of each obstacle, were characterised by a long thin low velocity zone present in PPI images of radar radial velocity and were bounded by the above mentioned shear zones. Further, these features
were aligned parallel to the surface wind direction, with widths approximately equal to the diameter of the obstacle and extended up to 57km downstream of each obstacle. These features are consistent with characteristics of mountain wakes described in the literature. A partitioning algorithm was calibrated to identify the convective and stratiform components of the radar reflectivity field. This algorithm was applied
to reflectivity data for each heavy precipitation event. Local maxima in the frequency of low-level enhanced precipitation were observed in the vicinity of topographic features such as the Coromandel Peninsula and Mt Tamahunga, in addition to the observed location of wakes in the lee of Great Barrier and Little Barrier Island.
Finally, the relationship between mountain wakes observed in the Hauraki Gulf and low-level precipitation enhancement was examined. Investigations showed that when large scale areas of precipitation interacted with these wakes, in
some cases convective precipitation was observed to be initiated or intensified. However, the observed areas of enhancement were observed to be short lived and shallow, reaching heights below the radar bright band at [approximately ]3.5 km.
