Diurnally forced convergence lines in the Australian Tropics

Abstract

AbstractObservations and analyses of low‐level convergence lines over the Gulf of Carpentaria region of northeastern Australia made during the second and third Gulf Lines EXperiments (GLEX II and GLEX III) are reported.During GLEX II, the initiation of the North Australian Cloud Line (NACL) was documented using a small autonomous aircraft and two Doppler sodars. NACLs are long convergence lines that form on the western side of northern Cape York Peninsula and are typically marked by a line of contiguous cumulus or cumulus congestus cloud. The observations show that the degree of asymmetry between west‐coast and east‐coast sea breezes (as characterized by the wind field) depends on the strength of the background easterlies, and that NACLs develop only when the background easterlies are sufficiently large (≥ 5 m s−1).GLEX III focused on morning glories, which are (mostly) southwestward‐moving bore‐like convergence lines that originate over the southern part of the gulf region. The field experiment involved Doppler sodar measurements as well as high‐temporal‐resolution data from operational automatic weather stations. The sodar measurements showed that undular bore‐like morning glory cloud lines develop only when the background easterlies are sufficiently weak (≤ 10 m s−1) and that strong bore‐like morning glories develop when the background winds exceed about 10 m s−1. If the background easterlies are too strong, no morning glories develop. Numerical simulations show that the structure of convergence line produced depends on the strength of the collision between the sea breezes from each side of the Cape York Peninsula, which in turn depends on the strength of the background easterlies. When the easterlies are weak (≤ 10 m s−1), the sea breezes have similar depths and strengths, and their collision is relatively violent, whereas when the background easterlies are strong (> 10 m s−1) the sea breezes have very different depths and strengths and their collision is comparatively benign.