AN OBSERVATIONAL STUDY OF THE INTERSTATE 5 DUST STORM CASE

On 29 November 1991 a series of collisions involving 164 vehicles occu rred on Interstate 5 in the San Joaquin Valley in California in a dust storm that reduced the visibility to near zero. The accompanying high surface winds are hypothesized to result from intense upper-troposphe ric downward motion that led to the formation of a strong upper front and tropopause fold and that transported high momentum air downward to midlevels where boundary layer processes could then mix it to the sur face. The objectives of the research presented in this paper are to do cument the event, to provide support for the hypothesis that both uppe r-level and boundary layer processes were important, and to determine the structure of the mesoscale circulations in this case for future us e in evaluating the navy's mesoscale data assimilation system. The str ong upper-level descent present in this case is consistent with what o ne would expect for jet streak and frontal circulations in combination with quasigeostrophic processes. During the period examined, upper-le vel data and analyses portray a strong upper-tropospheric jet streak w ith maximum winds initially in excess of 85 m s(-1) (approximate to 17 0 kt) that weakened as it propagated southward around the base of a lo ng-wave trough. The jet streak was accompanied by a strong upper front and tropopause fold, both of which imply intense downward motion. The vertical motion field near the time of the accidents had two maxima-o ne that was associated with a combination of quasigeostrophic forcing and terrain-induced descent in the lee of the Sierra and one that was associated with the descending branch of the secondary circulation in the jet streak exit region and the cold advection by both the geostrop hic wind and the ageostrophic wind in the upper front. The 700-hPa win d speed maximum over and west of the San Joaquin Valley overlapped wit h the latter maximum, supporting the hypothesized role of downward mom entum transport. Given the significant 700-hPa wind speeds over the Sa n Joaquin Valley during daytime hours on the day of the collisions, bo undary layer mixing associated with solar heating of the earth's surfa ce was then able to generate high surface winds. Once the high surface winds began, a dust storm was inevitable, since winter rains had not yet started and soil conditions were drier than usual in this sixth co nsecutive drought year. Surface observations from a variety of sources depict blowing dust and high surface winds at numerous locations in t he San Joaquin Valley, the Mojave and other desert sites, and in the L os Angeles Basin and other south coast sites. High surface winds and l ow visibilities began in the late morning at desert and valley sites a nd lasted until just after sunset, consistent with the hypothesized he ating-induced mixing. The 0000 UTC soundings in California portrayed a n adiabatic layer from the surface to at least 750 hPa, also supportin g the existence of mixing. On the other hand, the high winds in the Lo s Angeles Basin began near sunset in the wake of a propagating mesosca le trough that appeared to have formed in the lee of the mountains tha t separate the Los Angeles Basin from the San Joaquin Valley.