Marine atmospheric boundary layer and low-level cloud responses to the Kuroshio Extension front in the early summer of 2012: three-vessel simultaneous observations and numerical simulations

Abstract

Intensive atmospheric observations were carried out with five research vessels in total across the sea surface temperature (SST) front along the Kuroshio Extension in the early summer of 2012, to identify the effects of the front on the thermal structure and cloud formation in the marine atmospheric boundary layer (MABL). Three of the vessels were aligned together along the 143°E meridian with latitudinal separation of as small as 30′ or 45′, going back and forth across the SST front for in situ observations during 2–6 July. The SST front was quite sharp and moved northward by about 50 km in 3 days, which was not well represented in objectively analyzed SST data sets. The observations captured rapid changes of the mesoscale MABL structure across the SST front, which were particularly evident in cloud base height and downward longwave radiation (DLR) at the surface. The higher base of low-level clouds observed over the warmer water resulted from stronger turbulent mixing in the MABL, which became prominent under the northerlies. The most frequently measured DLR value was greater by 20 W m−2 to the south of the SST front than to the north. High-resolution atmospheric model experiments conducted with and without the frontal SST gradient have confirmed its critical importance for the MABL structure and low-level clouds. These imprints of the SST front simulated in the models are sensitive to SST data assigned at the lower boundary of the model.