Impact of the Oyashio Extension SST Front on Synoptic Variability of Oceanic Low-Level Cloud in Summertime Based on WRF Numerical Simulation

The sea surface temperature (SST) front in the North Pacific (NP) has a potential to modulate the atmospheric boundary layer and cloud properties within it. We investigated the impact of the SST gradient along with the Oyashio Extension on low-level cloud properties during summertime based on a Weather Research and Forecasting (WRF) numerical simulation. To reveal the SST gradient impact, we conducted two experiments with different boundary conditions from July to August for 3 years from 2014 to 2016: the first with 0.25° daily SST data (CTL experiment) and the second with spatially smoothed SST without SST frontal characteristics (SMO experiment). The period mean cloud water mixing ratio of marine fog on the northern flank of the front in the CTL experiment was larger than that in the SMO experiment by about 20% of the mean value. The SST front affected not only the mean state but also the synoptic variability of the marine fog, and the magnitude of the effects depended on the meridional wind across the SST front. We found two competing physical processes modulating the marine fog on the northern flank. First, a local cold SST anomaly reduced the saturated water vapor pressure near the surface, which is favorable for fog formation (SST anomaly effect). Second, warm temperature advection from southern to northern flanks suppressed the fog formation, and the suppression was effective when the horizontal gradient of SST anomaly was large (SST frontal effect). Our results indicated the importance of the SST gradient in summertime Oyashio Extension for the marine fog formation.