Low Cloud Type over the Ocean from Surface Observations. Part III: Relationship to Vertical Motion and the Regional Surface Synoptic Environment

Abstract

Composite large-scale dynamical fields contemporaneous with low cloud types observed at midlatitude Ocean Weather Station (OWS) C and eastern subtropical OWS N are used to establish representative relationships between low cloud type and the synoptic environment. The composites are constructed by averaging meteorological observations of surface wind and sea level pressure from volunteering observing ships (VOS) and analyses of sea level pressure, 1000-mb wind, and 700-mb pressure vertical velocity from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis project on those dates and times of day when a particular low cloud type was reported at the OWS. VOS and NCEP results for OWS C during summer show that bad-weather stratus occurs with strong convergence and ascent slightly ahead of a surface low center and trough. Cumulus-under-stratocumulus and moderate and large cumulus occur with divergence and subsidence in the cold sector of an extratropical cyclone. Both sky-obscuring fog and no-low-cloud typically occur with southwesterly flow from regions of warmer sea surface temperature and differ primarily according to slight surface convergence and stronger warm advection in the case of sky-obscuring fog or surface divergence and weaker warm advection in the case of no-low-cloud. Fair-weather stratus and ordinary stratocumulus are associated with a mixture of meteorological conditions, but differ with respect to vertical motion in the environment. Fair-weather stratus occurs most commonly in the presence of slight convergence and ascent, while stratocumulus often occurs in the presence of divergence and subsidence. Surface divergence and estimated subsidence at the top of the boundary layer are calculated from VOS observations. At both OWS C and OWS N during summer and winter these values are large for ordinary stratocumulus, less for cumulus-under-stratocumulus, and least (and sometimes slightly negative) for moderate and large cumulus. Subsidence interpolated from NCEP analyses to the top of the boundary layer does not exhibit such variation, but the discrepancy may be due to deficiencies in the analysis procedure or the boundary layer parameterization of the NCEP model. The VOS results suggest that decreasing divergence and subsidence in addition to increasing sea surface temperature may promote the transition from stratocumulus to trade cumulus observed over low-latitude oceans.

Relationship between Low Stratiform Cloud Amount and Estimated Inversion Strength in the Lower Troposphere over the Global Ocean in Terms of Cloud Types

“…In the subtropical eastern Pacific, an increase in the LSC amount, most of which are Sc, is associated with cold advection Klein 1997;Xu et al 2005;Mansbach and Norris 2007). In contrast, FOG typically forms with warm advection over the cold ocean in the midlatitudes (Norris and Klein 2000;Tokinaga and Xie 2009). Moreover, across the strong SST fronts in the North Pacific during the summer, changes in wind direction over the SST gradient cause a cloud regime transition between Sc or St and FOG (Norris and Iacobellis 2005;Tanimoto et al 2009).…”

“…In addition, it is found that a transitional area between warm and cold SST regimes can be seen in the SST range of approximately 16-20°C, where the variance of EIS is maximum; the seasonal variation in the Australian LSC region is similar to those in the midlatitude regions. Norris and Klein (2000) have shown that Sc is associated with divergence and subsidence, whereas St and FOG are associated with slight convergence and ascent, from observational composites at a midlatitude ocean weather station. Although not shown, we also note that we briefly analyzed the distribution of the 700-hPa pressure vertical velocity (ω 700 ) from ERA-40 in this SST-EIS field; results for the 500-and 850-hPa levels are quite similar.…”

“…5b) With these variations, the air-sea temperature difference increases from negative to positive values, suggesting the transition from cold to warm advection over the ocean. The warm advection often leads to a surface-based inversion, which is reflected in the increase in EIS sfc 925 , due to cooling of warm air by heat transfer to the cold ocean; if the air is sufficiently moist, the cooling causes it to reach saturation and FOG forms (Norris and Klein 2000;Tokinaga and Xie 2009). In addition, the meridional SST gradient is generally strong in the cold SST regime.…”

Characteristics of the Cloud Top Heights of Marine Boundary Layer Clouds and the Frequency of Marine Fog over Mid-Latitudes

“…Wood and Bretherton (2006) defined a stability index that has a high correlation with the cloud fraction of stratiform low clouds, including mid-latitude low clouds. Several studies investigated the morphology of mid-latitude low clouds and their relationship with meteorological fields using synoptic surface observation data (e.g., Norris 1998a, b;Norris and Klein 2000;Koshiro and Shiotani 2014). However, MBLCs over the mid-latitudes (ca.…”

Dynamical and thermodynamical coupling between the North Atlantic subtropical high and the marine boundary layer clouds in boreal summer

“…The northerly flow along the eastern flank of the North Pacific Subtropical High (NPSH) generates cold air advection from high latitudes and coastal upwelling along the west coast of the continent. The latter lowers local SSTs and favors enhanced MBL cloudiness by modifying the structure of the planetary boundary layer (Klein and Hartmann 1993;Norris and Klein 2000;Clement et al 2009). The strong longwave radiative cooling and enhanced subsidence induced by MBL clouds off the California coast in return tend to intensify the NPSH to its observed strength (Seager et al 2003;Li et al 2012b).…”