Abstract
A large fraction of annual precipitation over the western United States comes from wintertime orographic clouds associated with atmospheric rivers (ARs). The transported African and Asian dust and marine aerosols from the Pacific Ocean may act as ice-nucleating particles (INPs) to affect cloud and precipitation properties over the region. Here we explored the effects of INPs from marine aerosols on orographic mixed-phase clouds and precipitation at different AR stages for an AR event observed during the 2015 ACAPEX field campaign under low dust (< 0.02 cm-3) conditions. Simulations were conducted using the chemistry version of the Weather Research and Forecasting model coupled with the spectral-bin microphysics at 1-km grid spacing, with ice nucleation connected with dust and marine aerosols. By comparing against airborne and ground-based observations, accounting for marine INP effects improves the simulation of cloud phase state and precipitation. The marine INPs enhance the formation of ice and snow, leading to less shallow warm clouds but more mixed-phase and deep clouds, and increased ice water path (over 5 times) and snow precipitation (over 40 times). The responses of cloud and precipitation to marine INPs vary with the AR stages with more significant effects before AR landfall and post-AR than after AR landfall, mainly because the moisture and temperature conditions change with the AR evolution. This work suggests weather and climate models need to consider the impacts of marine INPs since their contribution is notable under low dust conditions despite the much lower relative ice nucleation efficiency of marine INPs.
Highlights
Atmospheric river (AR) events have great impacts on atmospheric and hydrological processes in the western United States during winter
The simulated aerosol number concentration over the size range of 0.067 - 3 μm is comparable to the observations over the same size range estimated by combining data from the Ultra-High-Sensitivity Aerosol Spectrometer (UHSAS) and the Passive Cavity Aerosol Spectrometer Probe (PCASP) at below 2-km altitude
Associated with atmospheric river (AR) events. This is done by carrying out simulations at a cloud-resolving scale (1 km) using WRF-Chem coupled with the spectral-bin microphysics (SBM) scheme for an AR event observed during the 2015 Atmospheric Radiation Measurement
Summary
Atmospheric river (AR) events have great impacts on atmospheric and hydrological processes in the western United States during winter. These measurements were made in conjunction with clouds and aerosols, meteorological, hydrological, and oceanic measurements collected by instruments on three other aircraft and Ron Brown and at a coastal surface station These data provide a unique opportunity to examine the complex interactions among aerosols, orographic clouds, and ARs. A major AR event spanning over 5 - 9 February 2015 occurred during the ACAPEX campaign and made landfall on the coast of Northern California, producing heavy rainfall with some regions receiving up to 400 mm of total precipitation during the event (Ralph et al, 2016; Cordeira et al, 2017). In our previous modeling studies (Fan et al, 2014, 2017b), we implemented an immersion freezing parameterization for dust particles (DeMott et al 2015) in a spectral-bin microphysics (SBM) scheme to examine the long-range dust effects on AR-associated orographic mixed-phase clouds and precipitation during CalWater 2011. The size distribution of each aerosol component is unknown in the model and any assumption on the size distribution might introduce uncertainty
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