The spatial and temporal characteristics of rainfall in south-eastern Queensland

Cluster analysis of five upper-air variables from Brisbane Airport has identified eight weather regimes associated with rainfall in the south-eastern Queensland. These eight weather regimes are found to clearly distinguish the rain-bearing systems; four are associated with essentially dry conditions and four with wet conditions. The low-rainfall regimes were all linked to synoptic patterns with low moisture flux and weak surface pressure gradients. The rain-bearing regimes tend to have complex synoptic patterns, often associated with interactions between tropical and mid-latitude systems. Two of the dry regimes occur throughout the year: one (black) with a low southeasterly moisture flux and the other (pink) with a low south-westerly flux. Together they occur 42% of the time, but only contribute 13% of the total rainfall. The other two dry regimes occur in winter, and they occur 23% of the time. The south-westerly winter (cyan) regime has a very high shear, and it yields 10% of the annual rainfall. The south-westerly winter (red) regime is statically stable in addition to having a high shear, and contributes only 0.4% of the rainfall. As the rainfall has a strong seasonal cycle associated with the sub-tropical climate, the dry regimes tend to occur in winter when the wind shear is high and the total integrated water in the atmosphere is low. The wet regimes tend to occur in complex synoptic situations when there were interactions between tropical and mid-latitude systems. A wet north-westerly (green)regime occurs 4% of the time throughout the year. It is associated with moist unstable conditions and it contributes 17% of the annual rainfall. A north-easterly (yellow)regime occurs 7% of the time in late summer, and it produces 23% of the total rainfall. These two regimes have very widespread and intense rain. A wet south-easterly (blue) regime is also most frequent in late summer. It occurs 11% of the time and contributes 16% of the rainfall. A wet westerly (purple) regime occurs most frequently in early summer, and it contributes 21% of the rainfall while occurring 12% of the time. Overall,the wet regimes contribute 77% of the annual rainfall while occurring only 35% of the time. Modeling case studies allow for in detail investigation of the precipitation initiation mechanisms and convection modes for each of the moist regimes. Meteorological observations of each case study day (rain gauge, atmospheric profiles, infrared (IR) and visible (VIS) satellite images), in conjunction with analysis charts have been used to characterise the environment. For each case radar observations are compared with model simulated reflectivity using CFADs. In general the WRF model does a fair job of simulating the timing and location of precipitation for the case studies. However in many cases WRF overestimates the precipitation intensity. All of the case studies share in common a lack of synoptic forcing indicating that local circulation is an important factor in this region.