Rain‐aerosol relationships influenced by wind speed

Abstract

Aerosol optical depth (AOD) has been shown to correlate with precipitation rate (R) in recent studies. The R‐AOD relationships over oceans are examined in this study using 150 year simulations with the Community Earth System Model. Through partial correlation analysis, with the influence of 10 m wind speed removed, R‐AOD relationships exert a change from positive to negative over the midlatitude oceans, indicating that wind speed makes a large contribution to the relationships by changing the sea‐salt emissions. A simulation with prescribed sea‐salt emissions shows that wind speed leads to increasing R by +0.99 mm d−1 averaged globally, offsetting 64% of the wet scavenging‐induced decrease between polluted and clean conditions, defined according to percentiles of AOD. These demonstrate that wind speed is one of the major drivers of R‐AOD relationships. Relative humidity at 915 hPa can also result in the positive relationships; however, its role is smaller than that of wind speed.

Investigation of the aerosol–cloud–rainfall association over the Indian summer monsoon region

“…aerosol-cloud-rainfall associations were found over continental conditions in global simulations (Grandey et al, 2013Gryspeerdt et al, 2015;Yang et al, 2016). Unlike continental conditions, a lack of high emission rates at the ocean surface might also contribute to the dominant effect of wet scavenging on the aerosol-cloud-rainfall association.…”

“…Thus, the wet-scavenging effect might not be accu- rately represented in the MODIS-retrieved AOD dataset used in our study. Modeling studies suggest that this artifact in the satellite-retrieved AOD values can significantly affect the magnitude as well as the sign of the aerosol-cloud-rainfall associations (Grandey et al, 2013Yang et al, 2016). At the same time, Gryspeerdt et al (2015) have recently illustrated that the aerosol in neighboring cloud-free regions may be more representative for aerosol-cloud interaction studies than the below-cloud aerosol using a high-resolution regional model; this justifies the methodology used in their study.…”

“…It is well documented that the aerosol-cloud correlation analysis using satellite data can be affected by one or more of the following factors: (1) positive correlation of variability in aerosol and cloud-rainfall fields with meteorological variations, which are the true modifiers of cloud and rainfall properties (Chakraborty et al, 2016;Kourtidis et al, 2015;Ten Hoeve et al, 2011), (2) cloud contamination of retrieved AOD values due to the aerosol humidification effect (Boucher and Quaas, 2013;Gryspeerdt et al, 2014), and (3) inaccurate representation of the wet-scavenging effect in the satellite-retrieved AOD dataset (Grandey et al, 2013Yang et al, 2016). Therefore, we have critically investigated the plausible role of these factors in our analyses as presented below.…”

“…This difference in sign of the association in modeling studies is mainly attributed to the inclusion of the wet-scavenging effect in models and the probable lack of the same in satellite samples (Grandey et al, 2013Yang et al, 2016). However, global modeling studies have their own inherent limitations and uncertainties in addressing aerosol-cloud-rainfall associations.…”

“…Besides, aerosol-cloud-rainfall associations are highly sensitive to variation in thermodynamical and environmental conditions, cloud properties, and aerosol types (Khain et al, 2008;Lee, 2011;Tao et al, 2012), further complicating these interactions. Moreover, clouds and precipitation can also interact with aerosols through wet-scavenging process (Grandey et al, 2013Yang et al, 2016). Global model simulations illustrated that wet scavenging can cause a strong negative cloud-fraction-AOD (aerosol optical depth) correlation over the tropics (Grandey et al, 2013).…”