Model dynamics of summertime low-level jets over northwestern Mexico

Using a fine-scale nested regional modeling system, the diurnal forcing of summertime low-level winds over the Gulf of California and northwestern Mex ico is investigated. On diurnal timescales, simulated nocturnal low-level j ets develop over the northern portion of the Gulf, the foothills of the Sie rra Madre accidental, and parts of southern Arizona. The southerly componen t of the nocturnal jet is' the result of a geostrophic balance; involving t he Coriolis force and a cross-gulf pressure gradient force associated with nighttime slope cooling over the elevated Sierra Madre Occidental. Addition ally, horizontal temperature gradients over the sloped orography produce ve rtical variations in this cross-gulf pressure gradient force, generating th e jet-like vertical shear in wind components above the nocturnal boundary l ayer; frictional effects are responsible for producing shear in the wind pr ofiles below the boundary layer This balance is distinctly different from t he inertial balance that is believed to be:responsible for the low-level je t over the Great Plains region of the United States. Daytime winds are part of a directly driven wind field forced dy horizontal pressure gradients as sociated with Slope heating (up the Sierra Madre Occidental) and sea-land t emperature gradients (north of the Gulf). During synoptically forced surge events, a similar diurnal cycle in low-level flow is still present; however , the local thermal forcing appears to be superimposed upon the large-scale synoptic forcing, resulting in weaker up-slope flow during the day and str onger along-slope flow at night.