Abstract
Two- and three-dimensional numerical simulations were performed to investigate the scale selection and initiation of both moist and dry convection over gentle western and gentle eastern slopes where the latter represents an idealization of the eastern Colorado region of the Great Plains of North America. This work extends earlier studies of thermally forced convection by considering a model framework that is large enough to resolve both the convective scale dynamics as well as the larger scale dynamics within which the convection is embedded. As a result, the scale interaction problem leading to the selection of the dominant deep modes of the troposphere and consequent convection initiation is more realistically treated. The main physical mechanisms involved in the initiation of convection in these studies are the usual boundary-layer instabilities leading to the development of eddies and/or shear-aligned rolls, the excitation of gravity waves by the boundary-layer motions interacting with the free atmosphere, and the eventual development of coherent vertical structures that link the boundary layer motions and the overlying gravity waves into larger horizontally spaced modes than typically obtained from an isolated boundary layer.
It has previously been shown that the mean wind shear spanning the region between the top of the boundary layer and the overlying stable layer plays an important role in producing energetic deep modes in the presence of thermal forcing. In the present simulation this shear results from a combination of initial baroclinicity associated with the westerlies and production by the differential thermal gradients formed by heating gently sloping terrain. Westerly geostrophic shears of either 3 or 5 m s−1 km−1 over the first 5.5 km above sea level were used as initial conditions. A balance is maintained between shear production through large-scale forcing and shear destruction through boundary-layer mixing that results in significant shear. The experiments showed a broad range of responses as a consequence of the horizontal variability of the shear structures. The preferred region of both dry and moist convection was found to be the eastern slope where the terrain effects result in an enhancement of the low-level shear. In response to the directional structure of the shear spanning the boundary layer and free atmosphere both a banded and a less coherent scattered organization were obtained for the waves and clouds.
Dominant deep modes were found to organize and initiate moist convection. West–east horizontal scales of the deep modes in the dry experiments were found to range from about 11 to 28 km with either a banded or a cellular structure with scales between 4 to 6 km in the south–north direction. The timing of the onset of the moist convection appeared to affect the final horizontal-scale selection in the moist experiments. The moist convection appears to lock onto the scales of the dry modes that initiate the convection for these particular experiments. The largest horizontal scales of dominant modes in the dry experiments were about 28 km and developed rather slowly as compared with the 11 km scale dominant modes. These largest horizontal scales did not develop in the moist experiments where clouds appeared early but did develop in those moist experiments where moist convection took longer to develop.
