The Generation of the Morning Glory

Abstract

A high-resolution cloud model is used to explore in detail the generation of the morning glory, a low-level nonlinear atmospheric internal wave observed on the southwestern side of Cape York Peninsula (Australia). The model is two-dimensional and nonhydrostatic and simulates an east?west cross section of the southern part of Cape York Peninsula at a horizontal resolution of 200 m. Most of the numerical experiments are initialized at sunrise with a 5 m s?1 easterly flow and a sounding taken upstream from the peninsula. The sea breezes that develop over Cape York Peninsula are highly asymmetric with the east-coast sea breeze being both deeper and warmer than its western counterpart. When the sea breezes meet, the east-coast sea breeze rides over that from the west coast and in the process produces a series of waves that propagate on the west-coast sea breeze. The model calculations show that when the phase speed of these waves matches the westward propagation speed of the east-coast sea breeze, the waves grow to large amplitude, thus forming the morning glory. When the east-coast sea breeze propagates too fast relative to the waves, the waves do not amplify. In this sense the morning glory is generated by a resonant coupling between the east-coast sea breeze and the disturbances that propagate on the shallow stable layer produced by the west-coast sea breeze. The number of waves produced depends on the stability of the west-coast sea breeze and the strength of the east-coast sea breeze. These numerical experiments have for the first time explicitly modeled the generation of morning glory waves through the interaction of two sea breezes. The inclusion of orography representative of Cape York Peninsula does not change the overall result with a morning glory forming in much the same way as in the case without orography. The main difference is that the sea breezes meet earlier when orography is included.