Observational Analysis of the Multi-Day and Multi-Scale Forcing Organizing the 2010 Los Angeles Basin Heat Burst Event

The Los Angeles Basin epitomizes an urban region whose weather is strongly controlled by a variety of surrounding mountain ranges. One type of weather event that is indicative of how larger scale air mass transport interacts with this unique terrain is extreme surface heating. The extraordinary Los Angeles record setting heat burst which occurred early in the afternoon in late September 2010 is an example and is analyzed employing multi-scale observations prior to very high resolution numerical simulation studies. This poorly forecasted event is extraordinary because of its extreme nature (~45°C at the downtown L.A. Civic Center) as well as the time of day (~1300 hours) and time of year (27 September). Multi-scale observations including: 1) reanalyses datasets, 2) rawinsonde data, 3) profiler data, 4) RASS, and 5) surface observations indicate complex interactions among the larger scale atmosphere, distant terrain, and nearby complex terrain. Three progressively lower-level distinct heat plumes affect the regional boundary layer along the Southern California coast over an 84 hour period leading up to the event. The first plume arrives from the southeast Pacific over Southern California and remains nearly stationary for ~84 hours in advance of, and leading up to, the record heat in response to interactions between the offshore polar and subtropical jets. The second and third heat plumes, which arrive sequentially over the next 72 hours, have origins in a sequence of adjustments involving massive Rossby Wave Breaking over the Intermountain West. Both the second plume, originating and propagating coastward from Sierra Nevada more than 36 hours earlier, and the third plume,transported across the high deserts from southern New Mexico into Arizona and California, arrive over L.A. nearly coincident with the record heat. This third plume is associated with a feature that in many ways resembles a propagating tropical wave structure organized by the complex terrain in between the Great Basin and the equatorward Sierra Madre Mountains. HYSPLIT trajectories indicate these three different transport regimes. Variability among three local profiler and RASS indicate that meso-β/γ scale topographic forcing likely enhanced these larger scale dynamical processes focusing the extreme heating over the L.A. Basin.