The effects of a river valley on an isolated cumulonimbus cloud development

Abstract

The mechanism of development, propagation and front-side cell regeneration of a three-dimensional isolated cumulonimbus (Cb) cloud is investigated by a cloud-resolving mesoscale model. Interactions of the simulated storm with orography including a river valley were studied. According to observational evidence, the mountainous environment of the Western Morava valley (Serbia) is an important place for the formation of isolated Cb clouds. Once formed, they move down into the valley and continue to propagate along it. The effects of orography on the development, propagation and regeneration of the model Cb cloud are recognized by comparison of its development with that simulated over flat terrain under the same conditions. In our study, two cases are considered: complex terrain case (referred to CT case) and flat terrain case (referred to FT case). It is found that: – Orography effects (CT case) play an important role on Cb cloud life; the cloud propagation and development are inhibited in lateral direction and its form is more compact. Cold air outflow near the ground remains in the valley with an increased depth compared to the FT case. Warm environmental air approaching the cold air nose from the opposite direction is forced aloft more frequently than in the FT case. As a consequence, the simulated cloud propagates faster in this case. – Warm environmental air forcing over cold air nose in the FT case is stronger initially than in the CT one, since the cloud development is not prevented by orography from the lateral direction. Consequently, the cold air outflow is more intense. In contrast, the cloud regenerates more slowly, since the cold air diverges in all directions, which in turn, makes the cold air nose thinner. The alternate reform and collapse of the cold air nose are more expressed in the CT case.