Diagnostics of the Ageostrophic Component of a Pre-Frontal Low-Level Jet Using the WRF Model

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 (observed 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 (AR) which can then lead to heavy precipitation especially in the West-Coast of the US (Neiman, Ralph, 2002; Ralph, Neiman, 2005). LLJs have been found in numerous cases to be out of geostrophic balance, (Orlanski and Ross 1977; Dudhia 1993; Thorpe and Clough 1991; Wakimoto and Murphey 2008; LaFore et al. 1994), but the driving mechanism of the ageostrophic component lay unexplored. Diagnostic studies of the LLJ have used the non-conservation of potential vorticity (PV) from diabatic processes to quantify the effects of the cold-frontal rainband on low-level wind accelerations from latent heat release (Lackmann 2002; Mahoney and Lackmann 2007; Marciano et al. 2014). These studies, however, implicitly assume a geostrophic balance in the circulation.

In an effort to study the LLJ ageostrophic circulation, we first quantify the role of the along frontal ageostrophic circulation using observations taken during the CalWater flight campaigns over the Pacific Ocean from 2014-2016. The effects on water vapor transport in AR associated with the ageostrophic circulations is discussed. Next, we attempt to quantify the forcing mechanisms of the ageostrophic component from a case study using the Weather Research and Forecasting (WRF) model. The case selected is from a landfalling AR on the U.S west coast beginning on February 5^th, 2015 of which we have excellent CalWater field campaign flight observations. The diagnostic study will proceed as follows. First, the momentum equations are rearranged using a geostrophic decomposition such that the ageostrophic acceleration may be cast in terms of i) isallobaric term, ii) momentum advection, and iii) coriolis turning of the ageostrophic winds. Of these terms, it is found that the isallobaric term dominates the along frontal ageostrophic wind. Next, the isallobaric term is decomposed using the full geopotential tendency equation of which, it is thought, from a scaling analysis, that the differential of temperature advection and the diabatic terms dominate. Results will quantify the roles of each term in the geopotential tendency equation to the ageostrophic winds.

Browning, K. a., and C. W. Pardoe, 1973: Structure of low-level jet streams ahead of mid-latitude cold fronts. Q. J. R. Meteorol. Soc., 99, 619–638, doi:10.1256/smsqj.42203. http://onlinelibrary.wiley.com/doi/10.1002/qj.49709942204/abstract.

Dudhia, J., 1993: Dudhia_1993_PennStateMesoscaleModel.pdf. Mon. Weather Rev., 1493–1513.

Lackmann, G., 2002: Cold-Frontal Potential Vorticity Maxima , the Low-Level Jet , and Moisture Transport in Extratropical Cyclones. Mon. Weather Rev., 59–74.

LaFore, J. P., J. L. Redelsperger, C. Cailly, and E. Arbogast, 1994: Nonhydrostatic Simulation of Frontogenesis in a Moist Atmosphere. Part III: Thermal Wind Imbalance and Rainbands.

Mahoney, K., and G. Lackmann, 2007: The Effect of Upstream Convection on Downstream Precipitation. Weather Forecast., 22, 255–277, doi:10.1175/WAF986.1.

MARCIANO, C. G., G. M. LACKMANN, and W. A. ROBINSON, 2014: Changes in U . S . East Coast Cyclone Dynamics with Climate Change. J. Clim., 28, 468–484, doi:10.1175/JCLI-D-14-00418.1.

Neiman, P., F. M. Ralph, A. White, D. Kingsmill, and P. Persson, 2002: The Statistical Relationship between Upslope Flow and Rainfall in California ’ s Coastal Mountains : Observations during CALJET. Mon. Weather Rev., 130, 1468–1492.

Orlanski, I., and B. Ross, 1977: The Circulation Associated with a Cold Front. Part 1: Dry Case. J. Atmos. Sci., 34, 1619–1633.

Ralph, F. M., P. J. Neiman, and R. Rotunno, 2005: Dropsonde Observations in Low-Level Jets over the Northeastern Pacific Ocean from CALJET-1998 and PACJET-2001 : Mean Vertical-Profile and Atmospheric-River Characteristics. Mon. Weather Rev., 133, 889–910.

Thorpe, A., and S. Clough, 1991: Mesoscale dynamics of cold fronts : Structures described by dropsoundings in FRONTS. Q. J. R. Meteorol. Soc., 1, 903–941.

Wakimoto, R., and H. Murphey, 2008: Airborne Doppler Radar and Sounding Analysis of an Oceanic Cold Front. Mon. Weather Rev., 1475–1491, doi:10.1175/2007MWR2241.1.