Direct Measurements of the Meso-scale Divergence Rate and Vorticity

The large-scale divergence rate, together with the entrainment rate, directly affects the rate at which the depth of a boundary layer changes. As such, most numerical models of the boundary layer require a specification of the divergence rate. From the earliest models, it has been well documented that the behaviour of the models can be highly sensitive to this parameter. Typically, the divergence rate is derived from the regional analysis. Unfortunately this calculation is not robust, as it is the difference of large numbers - the gradient of the horizontal winds. Moreover, this derivation is commonly observed to suffer from noise within the analysis. A recent comparison of the derived divergence rate from four different global numerical weather prediction analysis found poor consistency between them even after averaging in time and space. In 1995, as part of the ACE-1 experiment, the measurement of the divergence rate (and vorticity) was made directly through aircraft observations. As was expected, the direct observations differed considerably from the values derived from the analysis on numerous occasions. As there was only one aircraft making these measurements, there was no way to verify the consistency of these measurements. In February 2000, the Divergence Experiment (DIVEX) took place over the Great Australian Bight. Two research aircraft from Airborne Research Australia were used to measure the boundary layer divergence rate and vorticity. Following on from the ACE-1 work, large horizontal loops of 20 km radius were flown. Flight plans were designed to test the sensitivity of the measurements to time and horizontal scales. These observations are analysed and presented to establish the sensitivity and consistency of this measurement technique. These calculations are then compared with those derived from the analysis.