1. INTRODUCTION
The Tiwi Islands (located just north of Darwin) represent an excellent laboratory for testing models and theories on moist convective development. The islands are relatively isolated, deep convection develops there regularly during the wet season and, as documented by Beringer et al. (1995), the development follows distinct patterns depending on the large-scale conditions.
Beringer et al. (1995) identify two primary flow regimes; a dominant easterly flow regime and a secondary westerly regime. Under easterly flow, the initial convection develops in the early afternoon along an east-west band situated in the western half of the Tiwi Islands. By mid-afternoon convection becomes more organized into a primary cell which generally develops over Aspley strait (a narrow tidal channel separating Bathurst and Melville Islands in the Tiwi group).
In the westerly regime, initial development occurs in a convective band in the eastern half of the Islands. By midafternoon the convection becomes well-defined into a single cell over the center of Melville Island. Explosive development often occurs at this time with cloud top temperatures reaching -70 degrees C.
In this study we use a numerical model to examine the development of convection over the Tiwi Islands for two days during the Maritime Continent Thunderstorm EXperiment (MCTEX). The first day is an example of large-scale easterly flow, the second day an example of westerly conditions. The simulated development will then be compared with the observed convective development on these days.
2. NUMERICAL MODEL
The numerical model used in this study is the Clark nonhydrostatic, anelastic, mesoscale, model, Clark (1977). The model domain is 220 km in the east-west direction, 120 km in the north-south direction with a horizontal resolution of 1 km. The vertical resolution is 200 m in the lowest 2 km stretching to 800 m at the model top (24 km). Sensible and latent heat fluxes over the islands are specified as a percentage of the incoming solar radiation (35% for both sensible and latent heat in the simulations presented herein).
3. RESULTS
The convective development for a day with easterly flow (Nov. 28, 1995) is shown in Fig. 1. Sea breezes develop along both the north and south coastlines and propagate inland. The two sea-breezes converge around midday along an east-west line in the western half of the Tiwi Islands, Fig. 1(a). As can be seen, a line of convective cells has developed along this convergence line, in agreement with the observations of Beringer et al. (1995). Deep convection develops in this region and two hours later significant rainfall has reached the surface, Fig. 1(b). The location of this rainfall maximum is in reasonable agreement with the reflectivity observations shown in Fig. 1(c).
The convective development for a day with westerly flow (Dec. 4, 1995) is shown in Fig. 2. At 1100 LT, sea-breezes (indicated by lines of cloud in Fig. 2(a)) have formed along both the north and south coastlines and propagated inland. At this time, the two sea breezes are approximately 30 km apart. The southern sea-breeze appears to stall over the higher ground in the south of Melville Island while the northern sea-breeze continues propagating south, eventually converging with the southern sea-breeze around midday. Explosive development occurs at this stage, with simulated updraft speeds in excess of 30 ms-1. The simulated rainwater field at 1600 LT, Fig. 2(b), shows that the main development has occurred in a similar location to the observed storm on that day, Fig. 2(c).
4. SUMMARY
The numerical simulations have indicated that under both easterly and westerly flow, the strongest convective development occurs in the region where sea-breezes from the north and south coastlines converge. Under easterly flow the sea breezes converge over the western half of the Tiwi Islands and conversely for westerly flow.
5. REFERENCES
Beringer, J., T. Keenan and N.J. Tapper. 1995. Evolution of maritime continent island thunderstorms under varying meteorological conditions. Proceedings of the 2nd North Australian Remote Sensing and GIS Forum, Darwin, July 1995 (Australasian Urban and Regional Information Systems Association Inc Monograph No.11), 150-158.
Clark, T.L., 1977: A small-scale dynamic model using a terrain-following coordinate transformation. J Comp Phys 24 186-215
WESTERLY FLOW REGIME
DECEMBER 4, 1995
