- Award ID(s):
- 1639868
- NSF-PAR ID:
- 10039762
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 44
- Issue:
- 15
- ISSN:
- 0094-8276
- Page Range / eLocation ID:
- 8002 to 8010
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
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.
-
more » « less
Abstract Drizzle is ubiquitous in marine boundary layer stratocumulus clouds with much of it evaporating before reaching the surface. Ten days of observations made at the Atmospheric Radiation Measurement's Eastern North Atlantic site during closed cellular stratocumulus cloud conditions are used to characterize drizzle below the cloud base and its impact on the boundary layer turbulence. Cloud and drizzle microphysical and macrophysical properties were retrieved by combining the data from vertically pointing Doppler cloud radar, ceilometer, and microwave radiometer. On average, the drizzle shafts were 28.14 km wide, with cloud base rain rate and modal diameter of 0.98 mm/day and 138.62 μm, respectively. The rain rate at the surface was negligible yielding an average diabatic cooling of −28.68 W/m2in the subcloud layer. The liquid water path and turbulence within the boundary layer increased with an increase in the cloud top radiative cooling; however, none of these variables exhibited any relationship with cloud base rain rate. For a similar amount of radiative cooling at the cloud top, the average variance of vertical velocity in the subcloud layer was about 16% lower during strongly precipitating conditions as compared to lightly precipitating conditions. The reduction in the variance of vertical velocity due to drizzle evaporation was primarily confined to the upper half of the subcloud layer and was due to reduction in the strengths of the downdrafts. Collectively, our results show substantial impact of drizzle evaporation on turbulence below stratocumulus clouds, necessitating its accurate representation in the Earth system models.
