Performance of a dust model to predict the vertical mass concentration of an extreme Saharan dust event in the Iberian Peninsula: Comparison with continuous, elastic, polarization-sensitive lidars

Abstract An intense dusty event unusually occurred in wintertime over the Iberian Peninsula was detected over two Spanish NASA/MPLNET sites: the temporary Torrejon Observational Tower for Environmental Monitoring (TOTEM, 40.5oN 3.5oW) and the Barcelona station (BCN, 41.4oN 2.1oE). The highest dust incidence was observed from 22 to 23 February 2017; this two-day dusty scenario is examined in order to evaluate the performance of the operational NMMB/BSC-Dust model on forecasted mass concentration profiling in comparison with polarized Micro-Pulse (P-MPL) lidar-derived mass estimates for dust particles. First, the optical properties of the dust (DD) were effectively separated from the non-dust (ND) component by using the combined P-MPL/POLIPHON method, retrieving the lidar-derived extinction profiles for the separated DD component. Maximums of the lidar-derived DD optical depth were reached before at TOTEM (1.6 ± 0.1) than at BCN (1.7 ± 0.1). Typical features for dust were obtained: linear particle depolarization ratios between 0.3 and 0.4, corresponding to a predominance of dust coarse particles (>1 μm diameter); and mean lidar ratios of 61 ± 6 and 58 ± 10 sr, respectively, for TOTEM and BCN sites. AERONET Angstrom exponents of 0.12 ± 0.04 and 0.5 ± 0.3, as reported for TOTEM and BCN, respectively, confirm the presence of larger dust particles over TOTEM than those observed at BCN. HYSPLIT back-trajectory analysis shows air masses coming from the Sahara region, mostly transporting dust particles. AERONET-derived Mass Extinction Efficiencies (MEE) under dusty conditions were used for the extinction-to-mass conversion procedure as applied to the P-MPL measurements. Reported MEE values were lower at TOTEM (0.57 ± 0.01 m 2  g −1 ) than those found at BCN (0.87 ± 0.10 m 2  g −1 ), indicating dust particles were predominantly larger at TOTEM than those observed at BCN, as also stated before. Those results reveal that the longer transport of dust particles from the Sahara sources to BCN could favour a higher gravitational settling of coarser particles before reaching BCN than TOTEM. Simultaneous comparisons of profiles of the lidar DD component of the mass concentration with those forecasted by the NMMB/BSC-Dust model (25 available dusty profiles) were performed. The degree of agreement between both datasets was determined by the percentage of dusty cases satisfying selected model performance criteria (favourable cases) of two proxies: the Mean Fractional Bias, M F B , and the correlation coefficient, C C . A good agreement is found (72% and 76%, respectively, of favourable cases); however, large discrepancies are found at low altitudes between the dust model and the lidar observations, mostly at early stages of the arrival of the dust intrusion. Moreover, the mean lidar-derived centre-of-mass (CoM) of the dust event at TOTEM was found around 1.8 ± 0.2 and 2.2 ± 0.4 km height, respectively, on 22 and 23 February; likewise, the CoM was around 2.3 ± 0.3 and 2.2 ± 0.1 km height at BCN, respectively, on those days. Higher model-derived CoM are found in 60% of the cases (with differences