Comparison of mixed layer heights from airborne high spectral resolution lidar, ground-based measurements, and the WRF-Chem model during CalNex and CARES

摘要

The California Research at the Nexus of Air Quality and Climate Change(CalNex) and Carbonaceous Aerosol and Radiative Effects Study (CARES) fieldcampaigns during May and June 2010 provided a data set appropriate forstudying the structure of the atmospheric boundary layer (BL). The NASALangley Research Center (LaRC) airborne high spectral resolution lidar (HSRL)was deployed to California onboard the NASA LaRC B-200 aircraft to aid incharacterizing aerosol properties during these two field campaigns.Measurements of aerosol extinction (532 nm), backscatter (532 and 1064 nm),and depolarization (532 and 1064 nm) profiles during 31 flights, many incoordination with other research aircraft and ground sites, constitute adiverse data set for use in characterizing the spatial and temporaldistribution of aerosols, as well as the depth and variability of the daytimemixed layer (ML) height. The paper describes the modified Haar waveletcovariance transform method used to derive the ML heights from HSRLbackscatter profiles. HSRL ML heights are validated using ML heights derivedfrom two radiosonde profile sites during CARES. Comparisons between MLheights from HSRL and a Vaisala ceilometer operated during CalNex were usedto evaluate the representativeness of a fixed measurement over a largerregion. In the Los Angeles basin, comparisons of ML heights derived from HSRLmeasurements and ML heights derived from the ceilometer result in a very goodagreement (mean bias difference of 10 m and correlation coefficient of 0.89)up to 30 km away from the ceilometer site, but are essentially uncorrelatedfor larger distances, indicating that the spatial variability of the MLheight is significant over these distances and not necessarily well capturedby limited ground stations. The HSRL ML heights are also used to evaluate theperformance in simulating the temporal and spatial variability of ML heightsfrom the Weather Research and Forecasting Chemistry (WRF-Chem) communitymodel. When compared to aerosol ML heights from HSRL, thermodynamic MLheights from WRF-Chem were underpredicted in the CalNex and CARES regions,shown by a bias difference value of −157 m and −29 m, respectively.Better agreement over the Central Valley than in mountainous regions suggeststhat some variability in the ML height is not well captured at the 4 km gridresolution of the model. A small but significant number of cases have pooragreement when WRF-Chem consistently overestimates the ML height in the lateafternoon. Additional comparisons with WRF-Chem aerosol mixed layer heightsshow no significant improvement over thermodynamic ML heights, confirmingthat any differences between measurement and model are not due to themethodology of ML height determination.