The initiation and development of nighttime thunderstorms using a three-dimensional nocturnal low level jet numerical model
A three dimensional multi-layer grid-point model of the winds within the Planetary Boundary Layer (PBL) has been developed. It is a barotropic model which is designed for the case of an incompressible fluid and it incorporates the continuity equation constraints. The model is used to explore the role of the Nocturnal Low Level Jet (NLLJ) and its relationship with the vertical motion field in the pre-storm environment of the PBL. The model is initialized using theory to determine pressure, geostrophic wind fields and PBL heights. The change of the horizontal and vertical wind fields through time is derived by using a fourth-order Runge-Kutta numerical scheme. Four cases were tested in order to find out the vertical wind and its relationship with nocturnal thunderstorm development. The first case is a standard reference of the study using a high pressure in the east and a low pressure in the west from which the wind fields are derived. The second case is similar to the first case except it does not include the advection of the wind fields. Case three is a more realistic one with respect to the roughness length being greater in the west than the east. This is an analogy of the Rockies to the west and the plain to the east of the Mid-west. The fourth case is initialized with a uniform PBL height and wind field. Theoretical results indicate that the Nocturnal Low Level Jet Stream forms at the western part of the high pressure system. The size of the jet region is similar to those observed which is about the size of a state. The magnitude, in terms of geostrophic ratio, is in the same magnitude as the observed one, being 1.3 to 1.7 times of geostrophic wind.