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Abstract Large, abrupt clearing events have been documented in the marine stratocumulus cloud deck over the subtropical Southeast Atlantic Ocean. In these events, clouds are rapidly eroded along a line hundreds–to–thousands of kilometers in length that generally moves westward away from the coast. Because marine stratocumulus clouds exert a strong cooling effect on the planet, any phenomenon that acts to erode large areas of low clouds may be climatically important. Previous satellite-based research suggests that the cloud-eroding boundaries may be caused by westward-propagating atmospheric gravity waves rather than simple advection of the cloud. The behavior of the coastal offshore flow, which is proposed as a fundamental physical mechanism associated with the clearing events, is explored using the Weather Research and Forecasting model. Results are presented from several week-long simulations in the month of May when cloud-eroding boundaries exhibit maximum frequency. Two simulations cover periods containing multiple cloud-eroding boundaries (active periods), and two other simulations cover periods without any cloud-eroding boundaries (null periods). Passive tracers and an analysis of mass flux are used to assess the character of the diurnal west-African coastal circulation. Results indicate that the active periods containing cloud-eroding boundaries regularly experience stronger and deeper nocturnal offshore flow from the continent above the marine boundary layer, compared to the null periods. Additionally, we find that the boundary layer height is higher in the null periods than in the active periods, suggesting that the active periods are associated with areas of thinner clouds that may be more susceptible to cloud erosion. 

Abstract. In the current global climate models (GCMs), the nonlinearity effect ofsubgrid cloud variations on the parameterization of warm-rain process, e.g.,the autoconversion rate, is often treated by multiplying the resolved-scalewarm-rain process rates by a so-called enhancement factor (EF). In thisstudy, we investigate the subgrid-scale horizontal variations andcovariation of cloud water content (qc) and cloud droplet numberconcentration (Nc) in marine boundary layer (MBL) clouds based on thein situ measurements from a recent field campaign and study the implicationsfor the autoconversion rate EF in GCMs. Based on a few carefully selectedcases from the field campaign, we found that in contrast to the enhancingeffect of qc and Nc variations that tends to make EF > 1, the strong positive correlation between qc and Nc results in asuppressing effect that tends to make EF < 1. This effect isespecially strong at cloud top, where the qc and Nc correlation canbe as high as 0.95. We also found that the physically complete EF thataccounts for the covariation of qc and Nc is significantly smallerthan its counterpart that accounts only for the subgrid variation ofqc, especially at cloud top. Although this study is based on limitedcases, it suggests that the subgrid variations of Nc and itscorrelation with qc both need to be considered for an accuratesimulation of the autoconversion process in GCMs. 

By combining measurements from MODIS and the CloudSat radar, we develop a parameterization scheme to quantify the combined microphysical controls by liquid water path (LWP) and cloud droplet number concentration (CDNC) of the probability of precipitation (PoP) in marine low cloud over tropical oceans. We demonstrate that the spatial‐temporal variation of grid‐mean in‐cloud can be largely explained by the variation of the joint probability density function of LWP and CDNC in the phase space specified by the bivariate PoP (LWP and CDNC) function. Through a series of sensitivity tests guided by this understanding, we find that in the Southeastern Pacific and Atlantic the stratocumulus to cumulus transition of the is mainly due to the variation of CDNC while the annual cycle is mainly due to the variation of LWP. The results of this study provide a viable way to diagnose the root cause of warm rain problems in global climate models.

 

We document rapid and abrupt clearings of large portions of the subtropical marine low cloud deck that have implications for the global radiation balance and climate sensitivity. Over the southeast Atlantic, large areas of stratocumulus are quickly eroded, yielding partial or complete clearing along sharp transitions hundreds to thousands of kilometers in length that move westward at 8 to 12 meters per second and travel as far as 1000+ kilometers from the African coast. The westward-moving cloudiness reductions have an annual peak in occurrence in the period from April through June. The cloud erosion boundaries reduce cloud at ≈10-kilometer scale in less than 15 minutes, move approximately perpendicular to the mean flow, and are often accompanied by small-scale wave features. Observations suggest that the cloud erosion is caused by atmospheric gravity waves. 

Entrainment of warm, dry air from above the boundary layer into the cloud layer has a significant impact on stratocumulus clouds in the marine boundary layer. During the MAGIC field campaign, the Atmospheric Radiation Measurement (ARM) mobile facility was deployed aboard a container ship that made regular transects between Los Angeles, California and Honolulu, Hawaii. Observations made during MAGIC transects were collocated with observations from the Geostationary Operational Environmental Satellite (GOES‐15) and European Centre for Medium‐range Weather Forecasting (ECMWF) reanalysis model. From these data, hourly estimates of entrainment velocities in closed cellular stratocumulus cloud conditions were calculated from the mixed‐layer mass budget equation, modified to accommodate observations sampled from a moving platform. The technique is demonstrated using observations collected during Leg 15A (46 h) and then extended to 178 h of data. The average entrainment velocity was 7.83 ± 5.23 mm/s, and the average large‐scale vertical air motion at cloud top (obtained from reanalysis) was −2.56 ± 3.31 mm/s. The vertical air motion at cloud top was positive (upward) during 36 h (∼20%) with a mean of 2.68 mm/s. Entrainment velocity is highly variable and on average the MAGIC observations show no dependence of entrainment velocity on longitude or any pronounced diurnal cycle. When binned by inversion strength, the mean entrainment velocity and mean large‐scale vertical air motion mirrored each other, with both exhibiting substantial variability. Collectively, our results suggest a mean entrainment‐velocity behaviour associated with the background state, with large changes in entrainment velocity forced by strong variability in internal boundary‐layer properties like turbulence, radiation, and inversion strength. This cautions against using climatological mean estimates of entrainment velocities or neglecting instances with upward large‐scale vertical air motion.