Tropische Konvergenzlinien und Squall-lines
Final Report Abstract
The project commenced in 2002 as a partnership between the Ludwig-Maximilians-University of Munich, Monash University and the Australian Bureau of Meteorology in Melbourne and involved several field experiments in northeastern Australia. The principal findings from the GLEX II and GLEX III experiments that are currently being written up for publication with our Australian colleagues may be summarized as follows: • During November and December 2005 the initiation of the NACL was documented principally from two sodars located at Weipa and Scherger Air Force base. Aerosondes were used with limited success to take measurements of the wind, pressure, temperature and humidity. These observations were augmented with high-resolutions simulations of the several of the events to build a more complete picture of the cloud line. • The west-coast sea breeze was well defined with a very sharp leading edge, but did not penetrate far inland as was is opposed by the low-level easterly flow. In contrast, the east-coast sea breeze was poorly defined, and was highly modified by daytime convective mixing as it crosses the peninsula. • The structure of the mature NACL in the early morning was documented also and is shown to have many features in common with a weak gravity current. At low levels the NACL is marked by a step in virtual temperature, and appeared to propagate from the leading edge of a shallow cold pool. The zonal (normal) near-surface wind component showed strong easterly flow behind (east of) the NACL with much weaker easterlies ahead of it. These observations supported earlier theoretical predications. The results are significant as they underscore the importance of the asymmetric structure of the two sea breezes in initiating the cloud line. • Asymmetries such as these are absent from the parametrizations of convection used in all general circulation and climate models, and may account, in part, for the real deficiencies in their capacity to model convection properly in the topics. • In September and October 2006, observations of southwestward-moving bore-like convergence lines over the southern part of the Gulf of Carpentaria region of northern Australia were documented at Normanton, Karumba and Mornington Island. The observations supported the theoretical view that the structure of disturbance produced is highly dependent on the strength of the collision between the asymmetric sea breezes, which in turn is dependent on the strength of the background easterlies. In particular, when the background easterlies were weaker than about 5 m s^-1, the sea breezes were relatively symmetric and their collision relatively violent. Eastward propagating and westward propagating disturbances were produced, although the westward propagating disturbances were emphasised as the background wind was easterly. The disturbance evolved into a series of amplitudeordered solitary waves. When the background easterlies were between about 5 and 12 m s^-1 , the seabreezes were relatively asymmetric. In this case, there was little collision when they met, with the east coast sea breeze simply surmounting the west coast sea breeze and running across the top of the cold air. Thereafter a southwestward-propagating bore developed through the resonant mechanism described theoretically by Goler and Reeder (2004). When the easterlies exceeded about 12 m s^-1, no disturbance formed. Once again, the observations supported the theoretical predictions, and are important as they clarify the role of the background flow in the formation (or not) of one of the most common types of propagating convergence line in the region. • Sodar observations show an asymmetry in the strength of the sea breeze circulation which depends on the strength of the easterlies. • NACLs develop only when the background easterlies are sufficiently large. • Morning glories (solitary waves and undular bores) develop only when the background easterlies are sufficiently weak (strong collisions). In this case a pronounced nocturnal forms. • Bores develop when the background winds are moderately strong (weak collisions).
Publications
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2006. Low-level convergence lines over northeastern Australia. I. The North Australian Cloud Line. Mon. Wea. Rev., 134, 3092-3108
Goler, R., M. J. Reeder, R. K. Smith, H. Richter, S. Arnup, T. Keenan, P. May, and J. Hacker
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2006. Low-level convergence lines over northeastern Australia. II. Southerly disturbances. Mon. Wea. Rev., 134, 3109-3124
Smith, R. K., M. J. Reeder, H. Richter, and P. May
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2006. Simulations of low-level convergence lines over northeastern Australia. Q. J. R. Meteorol. Soc., 132, 691-707
Thomsen, G., and R. K. Smith
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2007. MesoLAPS predictions of low-level convergence lines over northeastern Australia. Wea. Forecasting, 22, 910-927
Weinzierl, B., R. K. Smith, M. J. Reeder, and G. Jackson
