Do aerosols induce the differences in low cloud frequency between eastern China and eastern United States?

Abstract

Low cloud exerts significant cooling effect on climate system. Nevertheless, the occurrence frequency of low clouds (OFLC) with height and its influential mechanism remain unclear, especially at the regional scale. In this study, high-vertical-resolution radiosonde measurements during the period 2018–2019 reveal that mean OFLC in eastern China (13.5 ± 6.4 %) is 21 % less than that in eastern United States (17.0 ± 4.5 %). Coincidentally, the regional distribution of aerosol optical depth (AOD) is opposite, with the mean AOD in eastern China being 150 % larger than that in eastern United States. In the meantime, the spatial pattern of surface relative humidity (RH) is similar to OFLC, with lower RH in eastern China than in eastern United States. The impacts of AOD and RH on the OFLC are explored. Results show that higher AOD usually increases the lower-tropospheric stability (LTS) and decreases the planetary boundary layer height (PBLH), while lower RH increases the lifting condensation level (LCL) for a given air temperature. Lower PBLH and higher LCL jointly decrease the difference between PBLH and LCL, which decrease the chances for the low cloud formation, resulting in lower OFLC in eastern China. The effect of AOD on OFLC depends on the RH conditions. When RH < 40 %, AOD has almost no effect on OFLC in both regions. When 40 % ≤ RH ≤ 70 %, OFLC tends to decrease as AOD increases in eastern United States, but no obvious OFLC trend exists in eastern China. While AOD exerts significantly negative roles on OFLC to different extent in the two regions when RH > 70 %. As AOD increases, the OFLC in eastern United States shows a significantly decreasing tends, with the correlation coefficient (R) being −0.90 between OFLC and AOD. Similarly, in eastern China, the OFLC decreases significantly when AOD > 0.4, with a R of −0.80. These results deepen our understanding of regional-scale impact mechanisms of the aerosol on low cloud's occurrence and help to reduce the uncertainties in simulating the height-resolved OFLC in predicting future climate change.