ABSTRACT

We have also begun our investigation into several scientific questions suggested in our proposal.

• The first question is how terrain induced circulations pattern (i.e., where, why, and when) mixing and vertical transport occurs in a stable, urban basin? In regard to this question, the data from our site during VTMX indicates that the air flow in the vicinity of the Jordan Narrows was often characterized by the arrival of a northerly winds with a lake breeze from the Great Salt Lake during the afternoon and transitioning during the evening to southerly winds that developed in association with flow through the Jordan Narrows From these orographic flows, we conclude the southern portion of the Salt Lake City valley lies in a different air mass than the more urban areas to the north for most of the day and flow through the narrows greatly ventilates the valley at night. Prior to these measurements, the nature of the flow through the narrows was poorly documented. We will collaborate with other VTMX investigators (primarily Bob Banta at NOAA/ETL and J. Horel of U Utah) in order to better document and understand the gap flow. The nature of the flow through the narrows varied greatly with a tendency for non-steadiness and pulsing to occur during the night. The nocturnal situation is relatively complex, as several different air masses can exist in a vertical column over the site. For example, the lowest layer is characterized by air through the narrows, the layer above this flow is lake breeze air that has been lifted by the flow through the narrows, a third layer above these air masses is characterized by remnants of a deep boundary layer advected into the area from the desert to the south and finally these layers are all capped by air in the "free" troposphere. The characterization of these terrain-induced circulations is simply a starting point for understanding how these circulations pattern mixing and vertical transport. In order to answer these questions, we have been examining the aerosol layers measured by the SABL data to detect periods where terrain-induced circulations produced significant mixing events. One type of example of mixing and vertical transport is breaking Kelvin-Helmholtz waves at the interface between the gap flow and the lifted lake breeze air where large vertical shears in the horizontal wind are observed. Richardson number calculations are underway to study this interface using our radiosonde and TAOS data sets. Mixing and vertical transport events also occur as weak fronts from the north are channeled through the valley from the north. Although these events produce only subtle changes in the surface wind field, they produce significant dramatic local wind changes lasting only several minutes producing correspondingly sharp changes in the nature of the aerosol fields. Another example of vertical transport is short-period gravity waves that frequently develop, especially during the first half of the VTMX experiment, during the transition from lake breeze to gap flow at the surface. We are planning to work with Joe Fernando at ASU on the causes for these wave events.

• The second question we intend to focus on is how to best measure mixing and vertical transport in stable, urban basins? We feel that we are uniquely poised to answer this question since our field efforts included acoustic, radar, lidar, and in-situ sensing systems with the METEK sodar, MAPR, SABL and TAOS, respectively. On the positive side, we found that the sodar performance was better than expected with a diurnal cycle in the signal-to-noise-ratio (SNR) that peaked during the night when these stable conditions existed. The sensitivity of the sodar backscatter on temperature variations (changes in CT) explains this behavior. In contrast, we found that the dry, stable, and calm conditions are challenging for wind profilers as these systems have a stronger sensitive to turbulent variations in the humidity field. The SNR for these systems, in contrast to the SODAR, is actually a minimum at night when these stable events were measured during VTMX. We also found that the complete thermodynamic and wind field characterization simultaneously measured at several different levels with TAOS afforded a unique behavior into explaining mixing and transport events, especially when combined with the SABL measurements. Also, the lidar worked well during the fall events observed during VTMX as these cases were clear-air situations. We are also investigating using serial radiosonde events to depict the "footprints" of mixing and transport events through comparing observed temperature profiles against those predicted by radiational cooling.

• The third question we will investigate is how well do mesoscale models predict topographically induced circulations and their associated patterning of mixing and transport? We are just beginning this phase of our work and we will have results to show in next year's report. To date we have begun simulations with version 5 of the Penn. State-NCAR mesoscale model (MM5).