A Theory for the Forcing Mechanism of the Ageostrophic Component of the Pre-Frontal Low-Level Jet

The pre-frontal Low-Level Jet (LLJ) is found ahead of cold fronts associated with extratropical cyclones in regions of very high moisture content, and at a height of about 1km. They are defined by a local maximum in along frontal wind speed (up to 45m/s) and characterized by a narrow width (~100km) relative to their length scales (~1000km) (Browning and Pardoe 1973). LLJs can deliver a large amount of water vapor within atmospheric rivers which can then lead to heavy precipitation especially in the West-Coast of the US (Neiman, Ralph, 2002; Ralph, Neiman, 2005). The LLJs were initially thought to be entirely geostrophic features driven by the local maxima in temperature ahead of the cold front, but later found to have a strong departure from the thermal wind balance, called the Thermal Wind Imbalance (TWI), (Orlanski and Ross 1977; Dudhia 1993; Thorpe and Clough 1991; Wakimoto and Murphey 2008; LaFore et al. 1994). Only (LaFore, 1994) has attempted to explain the TWI, but with no mention of the physical driving influences of the LLJ. The following question remains: What is the forcing mechanism of the ageostrophic component of the pre-frontal LLJ? The theory presented in this study may be summarized as follows: the pre-frontal air is in a regime of hydrodynamic stability which causes an along-frontal deflection rather than a lift as the cold front propagates. Hydrodynamic stability for Rossby number of order 1 is defined in the same sense as in topographical blocking theories by the Burger number (B = N_mh/fL). The relevant quantities are the stability of pre-frontal air (N_m/f) relative to the slope of the cold front (h/L) (Pierrehumbert 1984; Pierrehumbert and Wyman 1985). The theory presented in this study is then supported with observational evidence from dropsondes taken during the CalWater 1 and 2 field seasons. Conclusions will be presented during the AMS conference.