Low-Level Cloud Response to the Gulf Stream Front in Winter Using CALIPSO*

Abstract

A sharp sea surface temperature front develops between the warm water of the Gulf Stream and cold continental shelf water in boreal winter. This front has a substantial impact on the marine boundary layer. The present study analyzes and synthesizes satellite observations and reanalysis data to examine how the sea surface temperature front influences the three-dimensional structure of low-level clouds. The CloudAerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite captures a sharp lowlevel cloud transition across the Gulf Stream front, a structure frequently observed under the northerly condition. Low-level cloud top (,4 km) increases by about 500 m from the cold to the warm flank of the front. The sea surface temperature front induces a secondary low-level circulation through sea level pressure adjustment with ascending motion over the warm water and descending motion over cold water. The secondary circulation further contributes to the cross-frontal transition of low-level clouds. Composite analysis shows that surface meridional advection over the front plays an important role in the development of the marine atmospheric boundary layer and low-level clouds. Under cold northerly advection over the Gulf Stream front, strong near-surface instability leads to a well-mixed boundary layer over the Gulf Stream, causing southward deepening of low-level clouds across the sea surface temperature front. Moreover, the front affects the freezing level by transferring heat to the atmosphere and therefore influences the cross-frontal variation of the cloud phase.

Atmospheric Research Over the Western North Atlantic Ocean Region and North American East Coast: A Review of Past Work and Challenges Ahead

“…This feature can alter the cloud field through other processes in addition to CAOs. The SST gradients near the Gulf Stream, resulting in strong heat flux gradients, can give rise to circulations resembling sea‐breeze fronts (Sublette & Young, ), with higher cloud tops toward the warm side of the front (Liu et al, ). The warmer SSTs can reduce the local atmospheric stability, affecting the low cloud field and modulating the surface winds through the vertical momentum flux (Chelton et al, ).…”

Influence of the Subtropical High and Storm Track on Low-Cloud Fraction and Its Seasonality over the South Indian Ocean

“…This rectifying effect of storm-track activity on climatological SHF seems prominent around the Agulhas SST front. Note that submonthly atmospheric eddies may also be important for climatological-mean lowcloud top, since climatological cross-frontal contrast of low-cloud top around the SST front along the Gulf Stream resembles the corresponding contrast under cold advection events (Liu et al 2014).…”

A potential vorticity signature for the cold sector of winter extratropical cyclones

“…Liu et al . (), in an attempt to explain the enhancement of cloud cover and the increase of cloud‐top level over the Gulf Stream front, compared southerly and northerly regimes: under cold northerly advection, strong near‐surface instability led to a well‐mixed boundary layer over the Gulf Stream, causing a southward deepening of low‐level clouds across the SST front.…”

“…On the oceanic side, the Gulf Stream current, which flows along the North American continental margin, carries warmer waters northward, leading to strong sea-surface temperature (SST) gradients off the coast and further east. The SST front has a substantial impact on the marine atmospheric boundary layer above: QuikSCAT satellite data revealed increased wind speed over warmer waters and coherent structures of surface wind convergence along the SST gradient (Chelton et al, 2004), while CALIPSO satellite data captured a sharp low-level cloud transition across the front Liu et al, 2014). Minobe et al (2008) showed that the wind convergence in the boundary layer anchored a band of precipitation and was accompanied by vertical wind reaching the tropopause.…”