An Analysis of Mesoscale Transport Phenomena during the Evening Transition Period near Sacramento, California

Abstract

An analytical study of transport phenomena during the evening transition period in the vicinity of Sacramento, California is presented. The study is based on a network of double-theodolite wind stations, aircraft soundings and micrometeorological measurements made in the city. The transition period could be described in terms of three stability regimes: 1) moderately unstable (h/L = ?4.19), 2) moderately stable (h/L = 1.06) and, later in the evening 3) very stable (h/L = 295). During the analysis period, the boundary layer depth first increased in the late afternoon unstable period, reaching a maximum depth of about 1 km, then dropped discontinuously to about 200 m in the early evening (moderately stable period). During the remainder of the evening, the boundary-layer depth increased steadily, extending to over 700 m after midnight. Comparison of aircraft soundings made up- and downwind of the city of Sacramento indicated that the city had the effect of increasing the boundary layer depth by an average of 165 m. During the stability transition, the temperature profile rapidly developed a strong, low level inversion, capping an extremely shallow (< = 100 m) unstable layer, presumably associated with the urban heat island. The most prominent feature of the soundings was the rapid development, during the period of moderate stability, of low-level transient jets in the wind field. The jet was associated with low-level maxima in the horizontal transport of moisture and sensible heat. The boundary layer water vapor budget was evaluated for the entire analysis period. All three stability regimes displayed approximate equilibrium. For each period, the depth-integrated balance was between vertical diffusion, as a source, and horizontal and vertical advection, as sinks. It is noteworthy that subsiding dry air was found to be important throughout the entire period.