California has been getting significant precipitation this winter from the slew of atmospheric rivers that have battering the coast. Fox 5 San Diego reported on March 16 that the state has been directly hit by 14 of these since December, only two were considered “relatively weak”. Unfortunately, a cyclone off the coast on Sunday, March 20, 2023, accompanied yet another atmospheric river into the Bay Area (see figure 1). Orographic uplift during the “landfall” of an atmospheric river will be a consistent contributor for precipitation. In this region, when tropical moisture held within the airmass is advected eastwards with the geostrophic wind it makes its way onto land and is intercepted by California’s Coastal range where it is mechanically forced upwards and the moisture precipitates out of the airmass. The difference here is that the synoptic environment exasperated the terrestrial downpour that occurred in Cali through the early part of the week into Wednesday.
The synoptic environment during the onset of precipitation included several text-book qualities which led to strengthening of the low-pressure system as it was seen over the eastern Pacific Ocean at 0600 UTC on March 21, 2023, in figure 2. The second term in the QG-omega equation will be used to summarize a diagnostic tool that meteorologists use to track cyclones such as this one. The QG-omega equation aims to identify regions of airmass uplift (the Laplacian of omega, known as Term A) due to both differential geostrophic absolute vorticity advection by the geostrophic wind (known as term B), and the Laplacian of temperature advection (known as term C). Each of these terms can be evaluated independently to diagnose vertical motions. To focus only term B’s influence on vertical motions and the intensity of this cyclone over time a 500 mb map showing geopotential heights, ascent and cyclonic relative vorticity are used (figure 2) as well as a surface map with Mean Sea Level Pressure (figure 3) to show the atmospheric conditions at 0600 UTC 21 March. From figure 2, the warmer shaded fill at the base of cutoff low exhibits a strong region of positive relative vorticity. Since this cyclone is in the Northern Hemisphere, planetary vorticity is positive which indicates that absolute vorticity is also positive. Because the wind at this level is geostrophic, we can infer that the positive absolute vorticity values will travel parallel to the geopotential height contours as this cyclone moves Eastward. This is positive absolute vorticity advection by the geostrophic wind. From figure 3, the surface center of this cyclone is further to the east than the center is aloft. Because there is a vorticity maximum at the center of the surface low, absolute vorticity advection by the geostrophic wind here is zero. Because the value of absolute vorticity advection by the geostrophic wind increases with increasing height, term B in the QG-omega equation is positive which makes term A positive if we do not consider the effect from term C.
Using the values from term B, we can plot the magnitude of the differential absolute geostrophic vorticity advection by the geostrophic wind. This is shown in figure 4 below where the warm colors represent that positive values of differential absolute vorticity advection by the geostrophic wind will lead to upwards vertical motions within the vicinity of the cyclone. This upward motion of air from the surface helps intensify the cyclonic flow as time progresses and the storm moves eastward, closer to the California coast. This was seen throughout the day as the cyclone’s surface pressure dropped on March 21. Off the coast of Monterey Bay, CA an ocean buoy recorded a pressure drop of 24mb in just 17 hours, bottoming out just after 1900 UTC on March 21 (2:00 pm PDT).
