Aerosol impacts on California winter clouds and precipitation during CalWater 2011: local pollution versus long-range transported dust

Abstract

Abstract. Mineral dust aerosols often observed over California in winter and spring, associated with long-range transport from Asia and the Sahara, have been linked to enhanced precipitation based on observations. Local anthropogenic pollution, on the other hand, was shown in previous observational and modeling studies to reduce precipitation. Here we incorporate recent developments in ice nucleation parameterizations to link aerosols with ice crystal formation in a spectral-bin cloud microphysical model coupled with the Weather Research and Forecasting (WRF) model in order to examine the relative and combined impacts of dust and local pollution particles on cloud properties and precipitation type and intensity. Simulations are carried out for two cloud cases (from the CalWater 2011 field campaign) with contrasting meteorology and cloud dynamics that occurred on 16 February (FEB16) and 2 March (MAR02). In both cases, observations show the presence of dust and biological particles in a relative pristine environment. The simulated cloud microphysical properties and precipitation show reasonable agreement with aircraft and surface measurements. Model sensitivity experiments indicate that in the pristine environment, the dust and biological aerosol layers increase the accumulated precipitation by 10–20% from the Central Valley to the Sierra Nevada for both FEB16 and MAR02 due to a ~40% increase in snow formation, validating the observational hypothesis. Model results show that local pollution increases precipitation over the windward slope of the mountains by a few percent due to increased snow formation when dust is present, but reduces precipitation by 5–8% if dust is removed on FEB16. The effects of local pollution on cloud microphysics and precipitation strongly depend on meteorology, including cloud dynamics and the strength of the Sierra Barrier Jet. This study further underscores the importance of the interactions between local pollution, dust, and environmental conditions for assessing aerosol effects on cold-season precipitation in California.

Comparing parameterized versus measured microphysical properties of tropical convective cloud bases during the ACRIDICON–CHUVA campaign

“…These spectra are separated by the threshold of r e for significant coalescence, which varies as a function of the drizzle water content (DWC) for 1 s cloud passes (Freud and Rosenfeld, 2012). In addition, droplets with diameters < 10 µm are captured less efficiently by the hot-wire probe, resulting in an underestimation of CWC h .…”

“…The DSDs from CCP-CDP and CAS-DPOL are used to calculate the CWC, defined here as the mass of the drops integrated over the diameter range of 3-50 µm. Similarly, DSDs from CCP-CIP are used to calculate the DWC, defined here as the mass of the drops integrated over the diameter range of 75-250 µm (Freud and Rosenfeld, 2012). Figure 3 shows the dependency of calculated r e as a function of altitude for cloud passes during flights over different aerosol concentration conditions (AC13 very polluted, AC18 polluted, and AC19 clean).…”

“…After nucleation, the cloud droplets in rising cloud parcels grow with height mainly by condensation. Raindrops start forming when r e reaches 13-14 µm and coalescence becomes efficient (Freud and Rosenfeld, 2012;Rosenfeld and Gutman, 1994). Accurate documentation of the vertical evolution of cloud and rain DSDs is essential for analyzing these types of microphysical processes within clouds.…”

“…The goal of cloud microphysical models is to reproduce atmospheric processes based on physical relationships developed from field experiments and remote sensing observations in different parts of the globe (Silva Dias et al, 2002;Machado et al, 2014;Fan et al, 2014;Rosenfeld et al, 2014b). Data from aircraft probes provide opportunities to validate and improve cloud models and satellite retrievals of cloud microphysical properties.…”

Improving our fundamental understanding of the role of aerosol−cloud interactions in the climate system

“…Aerosol composition can play a significant role in mixed-phase and ice clouds. For example, Ault et al (54) and Fan et al (55) showed that two mixed-phase storms classified as "atmospheric rivers" with essentially identical meteorology and dynamics-one with clouds seeded with pollution aerosols and the second seeded with dust-produced 40% more precipitation with dust as seeds. Finally, ice production from riming and ice−ice collisions in warm mixed-phase clouds can profoundly impact cloud lifetime and precipitation.…”