A numerical study of coastal stratus cloud in a two-dimensional meso-scale model

Graduation date: 1984A two-dimensional numerical mesoscale model, which ic1udes\ud radiative and turbulent transfers, has been constructed to study the\ud formation, development and dissipation of coastal stratus cloud\ud under an inversion. In the model, the delta-Eddington and emissivity\ud approximations are used for the solar and thermal radiative\ud transfers, respectively. K-theory parameterization is adopted for\ud the turbulent transfer. Ground surface temperature and moisture are\ud predicted using the methods of Deardorff (1977, 1978). This model\ud is applied to a domain which extends 180 km into sea and 240 km\ud inland horizontally and about 2 km from the earth1s surface vertically.\ud A bare, flat soil surface is assumed.\ud As a prelude to the study of the stratus cloud, sea breeze\ud circulations with and without a temperature inversion have been\ud investigated. The model without an inversion yields speeds of the\ud sea breeze front which are close to the values that have been\ud observed under insolation and other meteorological conditions\ud similar to those used in the model. The presence of an inversion\ud causes the depth of sea breeze circulation to be shallower and its\ud inland penetration during the evening hours to be weaker compared to\ud the case without inversion; however, the basic structure of the sea\ud breeze circulation is unaffected by the inversion.\ud The model has been used to study the growth, development and\ud dissipation of stratus cloud under an inversion in the west coast\ud region of the United States. The effects of large scale motions on\ud these processes have also been examined. Cloud parameters such as\ud the times of initial formation and of dissipation inland, the maximum\ud distance of inland penetration, the maximum liquid water path\ud and the cloud-top height are affected in the presence of such large\ud scale motions; for example, both the maximum liquid water path and\ud the cloud-top height are appreciably enhanced - by about a factor of\ud two - when large scale westerly winds (U[subscript g]=5mfs, V[subscript g]=0) are present\ud compared to the case when U[subscript g]=V[subscript g]=0. The cloud parameters\ud predicted by the model are in close correspondence with existing\ud observations in southern California.\ud It is found that the sea breeze circulation is not appreciably\ud affected by the presence of moderate amounts of stratus cloud.\ud While advection plays a dominant role in the horizontal development\ud of the stratus cloud inland, radiative processes (cooling\ud and heating) are observed to govern the vertical growth and dissipation\ud of the cloud layer. Vertical growth is influenced by the rate\ud of radiative cooling at cloud-top. Because of the combined effects\ud of solar and surface heating, the stratus inland is observed to\ud dissipate more rapidly during the morning hours than the cloud over\ud the ocean where surface heating is minimal