Strong Precipitation Suppression by Aerosols in Marine Low Clouds

Abstract

The adjustment of cloud amount to aerosol effects occurs to a large extent in response to the aerosol effect on precipitation. Here the marine boundary layer clouds were studied by analyzing the dependence of rain intensity measured by Global Precipitation Measurement on cloud properties. We showed that detectable rain initiates when the drop effective radius at the cloud top (r e ) exceeds 14 μm, and precipitation is strongly suppressed with increasing cloud drop concentration (N d ), which contributes to the strong dependence of cloud amount on aerosols. The rain rate increases sharply with cloud thickness (CGT) and r e when r e > 14 μm. The dependence of rain rate on r e and CGT presents a simple framework for precipitation susceptibility to aerosols, which explains other previously observed relationships. We showed that sorting data by CGT and using alternative cloud condensation nuclei proxy rather than aerosol optical depth are critical for studying aerosol-cloud-precipitation interactions.Plain Language Summary Aerosol-cloud interaction remains the greatest uncertainty in future climate projection. Precipitation is a key process that mediates how the cloud amount responds to aerosol perturbations. Here we combined precipitation measured by the radar onboard the satellite of Global Precipitation Measurement (GPM) and cloud properties retrieved from Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Aqua satellite for studying the dependence of rain intensity on cloud properties for marine boundary layer water clouds over the Southern Hemisphere Ocean. Our results showed that rain is sharply intensified when droplets at the cloud top grow larger than 14 μm, and precipitation decreases with increasing cloud drop number concentration (N d ). A simple framework to explain the relationship between precipitation and aerosols is proposed here by showing the dependence of precipitation on N d and cloud geometric thickness. We also discussed why using aerosol optical depth (AOD) as CCN proxy in previous studies could lead to great uncertainties and why sorting cloud geometrical thickness is necessary. Key Points:• Precipitation is strongly suppressed with increasing cloud drop concentration • Sorting data by cloud thickness and using alternative CCN proxy rather than AOD are critical for studying aerosol-cloud interactions • Detectable rain initiates when the drop effective radius at the cloud top exceeds 14 μm Supporting Information:• Supporting Information S1

Evaluation of Cloud and Precipitation Response to Aerosols in WRF‐Chem With Satellite Observations

“…MOR suffers less from this issue than LIN, mainly because the autoconversion process in MOR is suppressed to a much larger extent by aerosol loading than LIN, although it is smaller than that in the satellite observations (Table 2). The strong dependence of cloud properties and precipitation characteristics on aerosols in this study agree well with the recent observations of the marine boundary layer clouds in Fan et al (2020) and Rosenfeld et al (2019), where the clouds are also sorted by CGT. Here the underestimated S con results in smaller S R and S POP in WRF‐Chem simulations, and it is closely related to the underestimated S CF , as the cloud lifetime is extended by the less suppressed scavenging by precipitation.…”

“…Precipitation in MOR and LIN intensifies quickly when the cloud effective radius (R e ) is larger than 9 μm. As a comparison, the initiation of rain usually occurs when R e reaches around 14 μm, in agreement with large eddy simulations (Rosenfeld et al, 2012) and measurements (Fan et al, 2020;Freud & Rosenfeld, 2012).…”

“…In this way, the aerosol effect on clouds is taken apart from the meteorology. The strong dependence of CF on N d is consistent with a large suppression of surface precipitation rate by N d in different CGT bins based on Global Precipitation Measurements (GPM) and MODIS observations (Fan et al, 2020). These findings motivate us to apply this newly developed approach to examine the ACI in climate models.…”

“…The rate of cloud droplet coalescence is determined mainly by droplet size, and intensified precipitation occurs when droplet effective radius exceeds 12-14 μm (Freud & Rosenfeld, 2012). A recent study have shown that detectable rain measured by GPM initiates when R e exceeds 14 μm, below which the precipitation is strongly suppressed by large cloud droplet number concentration (Fan et al, 2020). From the CloudSat observation, the 14 μm is also critical for the initiation of precipitation, and the averaged surface rain rate of clouds start to increases sharply with R e when it is larger than 14 μm, as shown in Figure 7d.…”

The Dependence of Ship‐Polluted Marine Cloud Properties and Radiative Forcing on Background Drop Concentrations

“…Fan et al. (2020) showed that precipitation starts when N d‐core is less than ∼60 cm −3 and increasing steeply with lower N d‐core (see supporting information Figure ). It indicates that the enhanced Cf and albedo are responses to partial rain suppression by the ship emissions.…”