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

Abstract

Abstract. The California Research at the Nexus of Air Quality and Climate Change (CalNex) and Carbonaceous Aerosol and Radiative Effects Study (CARES) field campaigns during May and June 2010 provided a data set appropriate for studying the structure of the atmospheric boundary layer (BL). The NASA Langley Research Center (LaRC) airborne high spectral resolution lidar (HSRL) was deployed to California onboard the NASA LaRC B-200 aircraft to aid in characterizing 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 in coordination with other research aircraft and ground sites, constitute a diverse data set for use in characterizing the spatial and temporal distribution of aerosols, as well as the depth and variability of the daytime mixed layer (ML) height. The paper describes the modified Haar wavelet covariance transform method used to derive the ML heights from HSRL backscatter profiles. HSRL ML heights are validated using ML heights derived from two radiosonde profile sites during CARES. Comparisons between ML heights from HSRL and a Vaisala ceilometer operated during CalNex were used to evaluate the representativeness of a fixed measurement over a larger region. In the Los Angeles basin, comparisons of ML heights derived from HSRL measurements and ML heights derived from the ceilometer result in a very good agreement (mean bias difference of 10 m and correlation coefficient of 0.89) up to 30 km away from the ceilometer site, but are essentially uncorrelated for larger distances, indicating that the spatial variability of the ML height is significant over these distances and not necessarily well captured by limited ground stations. The HSRL ML heights are also used to evaluate the performance in simulating the temporal and spatial variability of ML heights from the Weather Research and Forecasting Chemistry (WRF-Chem) community model. When compared to aerosol ML heights from HSRL, thermodynamic ML heights 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 suggests that some variability in the ML height is not well captured at the 4 km grid resolution of the model. A small but significant number of cases have poor agreement when WRF-Chem consistently overestimates the ML height in the late afternoon. Additional comparisons with WRFChem aerosol mixed layer heights show no significant improvement over thermodynamic ML heights, confirming that any differences between measurement and model are not due to the methodology of ML height determination.

Comparison of aerosol lidar retrieval methods for boundary layer height detection using ceilometer aerosol backscatter data

“…As with previous studies (Brooks, 2003;Baars et al, 2008;Compton et al, 2013;Scarino et al, 2014), the dilation factor a affects the number of covariance wavelet transform coefficient (CWTC) local minimums. Larger values create a few large local minimums (Fig.…”

“…Out of the three methods tested in this study, it is suggested to employ the wavelet method in future studies, in particular for longterm seasonal and diurnal boundary layer studies and spatial analysis of the BL using multiple lidar aerosol backscatter measurements. A combination of the wavelet method BLH retrievals during clear skies and the cluster analysis method's ability to calculate cloud depth is also recommended for more robust BL studies to retrieve more information about the boundary layer under both conditions, as both the wavelet and cluster analysis methods were seen to perform well using various lidar instruments in studies such as Compton et al (2013), Scarino et al (2014), and Toledo et al (2014). Although not tested in this study, recent work by de Bruine et al (2016) and Poltera et al (2017) show promising results using an automated method which reduces incorrect detection of the BLH using graph theory.…”

“…Apart from a few occasions (e.g., André and Mahrt, 1982;Berman et al, 1999;Day et al, 2010), NSL measurements are particularly uncommon since most radiosonde launches are performed during daytime ML hours. In recent years, remote-sensing techniques such as light detection and ranging (lidar), radio acoustic sounding systems (RASS), and sonic detection and ranging (sodar) systems have allowed for the continuous monitoring of the BL (Cohn and Angevine, 2000;Schäfer et al, 2011;Seibert et al, 2000;Emeis et al, 2004Emeis et al, , 2006Emeis et al, , 2012Eresmaa et al, 2006;Baars et al, 2008;McKendry et al, 2009;Muñoz and Undurraga, 2010;Haman et al, 2012;Milroy et al, 2012;Compton et al, 2013;Scarino et al, 2014;Wiegner and Gasteiger, 2015;Uzan et al, 2016). Ceilometers in particular offer a low-maintenance and low-cost solution to constantly monitoring the ML using aerosol backscatter while also facilitating the monitoring of the nocturnal stable layer, internal aerosol layers, and the nighttime residual layer (Haman et al, 2012Pandolfi et al, 2013;Peña et al, 2013).…”

WRF simulation of downslope wind events in coastal Santa Barbara County

“…Model configuration-2 (WRF-2) is a sensitivity test for which we replaced the boundary layer configuration used in WRF-1 with Mellor-Yamada-Nakanishi-Niino (MYNN; Nakanishi and Niino, 2009). Previous studies evaluating WRF performance in Southern California have identified PBL schemes as a primary source of error (Huang et al, 2013;Scarino et al, 2014). These studies established that available parameterization schemes produce significant differences in PBL height compared to large-eddy simulated (Huang et al, 2013) and light detection and ranging (LIDAR) estimates (Scarino et al, 2014), and that these issues are exacerbated near the coastal zone, which adversely affects the simulation of sundowner wind influence over SB (communication with NWS LA/Oxnard office).…”

“…Previous studies evaluating WRF performance in Southern California have identified PBL schemes as a primary source of error (Huang et al, 2013;Scarino et al, 2014). These studies established that available parameterization schemes produce significant differences in PBL height compared to large-eddy simulated (Huang et al, 2013) and light detection and ranging (LIDAR) estimates (Scarino et al, 2014), and that these issues are exacerbated near the coastal zone, which adversely affects the simulation of sundowner wind influence over SB (communication with NWS LA/Oxnard office). The MYNN PBL parameterization was adopted in WRF-2 because of the reported improvement of the turbulent length scale that realistically increases with decreasing stability, and the improvement of the expression for stability functions for momentum and heat.…”