In this study, the authors have conducted a series of numerical experiments to investigate the flow circulations in the Central Valley of California and the formation mechanisms of the Fresno eddy. The authors have found the following:
Under an adiabatic northwesterly, low–Froude number flow over the Central Valley, two cyclonic vortices form in the basin. One is located on the lee slope of the northern Coastal Range, while the other is located to the south of the San Joaquin Valley. The first may be identified as the Sacramento eddy, while the second may be identified as the Fresno eddy, although the Fresno eddy is located slightly farther to the south. The formation of the Sacramento eddy may be explained by either the generation of potential vorticity (Smith) or the generation of vorticity due to baroclinicity (Smolarkiewicz and Rotunno) on the lee slope in a low–Froude number flow. The Sacramento eddy may also be classified as a lee mesocylone since it is collocated with a lee mesolow (Lin et al.). In addition, a northwesterly jet forms at the gap of the Coastal Range due to the channeling effect.
The Fresno eddy forms when the low–Froude number northwesterly flow meets the return flow from the Tehachapi Mountains in a rotating fluid system and is strengthened and expands farther to the north due to the effects of nocturnal radiative cooling. The northwesterly jet in the Central Valley, the southeasterly wind from the foothills of the Sierra Nevada, and the blocking effect due to the Tehachapi Mountains all play important roles in the formation of the Freano eddy. The Sacramento eddy moves eastward to the foothills of the Sierra Nevada, while the jet at the gap of the Coastal Range is suppressed when nocturnal radiative cooling is present.
The nocturnal drainage flow over the western slope of the Sierra Nevada is weakened by the southerly return flow from the Tehachapi Mountains. The simulations indicate that in the absence of nocturnal radiative cooling the Fresno eddy still forms but is weaker and is located new the southern end of the San Joaquin Valley.
The Fresno eddy will form in an environment characterized by low–Froude number northwesterly wind. Suitable incoming flow speed and direction are among the major factors in determining the formation and strength of the Fresno eddy.
The return flow from the southern boundary of the San Joaquin Valley plays an important role in the formation of the Fresno eddy.
The Fresno eddy does not form in the absence of planetary rotation.
The β effect plays a negligible role in the formation of the Fresno eddy.