Environmental Science Division (EVS)a Division of Argonne National Laboratory
Predictive environmental understanding

EVS research characterizes entrainment within marine stratocumulus clouds

April 23, 2019

Clouds cover vast areas (~70%) of the Earth's oceans. Marine stratocumulus clouds occur at the Eastern edges of subtropical oceans like the California coast, blanket about a quarter of the ocean's surface, and persist almost year-round. These decks of marine stratocumulus clouds are transported by winds from the California coast towards the Hawaiian region that has higher ocean surface temperatures, higher humidity, as well as lower atmospheric stability. These changes in the environment cause the stratocumulus clouds to transition to marine cumulus clouds. Because of the differences in the radiative properties of stratocumulus and cumulus clouds, this cloud transition carries important consequences for the Earth's radiative balance. It is therefore important to properly understand the mechanisms driving the transition from stratocumulus to the cumulus cloud regime, and to adequately represent that transition in Earth System Models used for predicting future climate.

Entrainment of dry air into the cloud from above the cloud layer plays a critical role in this transition between cloud regimes. However, limited observational estimates of entrainment rates exist, primarily due to a lack of detailed observations needed to derive them. The Marine ARM (Atmospheric Radiation Measurement) GPCI Investigation of Clouds (MAGIC) field campaign provided such detailed observations of cloud, radiation, dynamic, and thermodynamic variables needed for deriving entrainment rates. During the MAGIC field campaign, the ARM Mobile Facility (AMF) was deployed onboard a container ship that made regular transects from Los Angeles, California, to Honolulu, Hawaii, from the fall of 2012 to the summer of 2013. Multiple instruments, including vertically pointing Doppler cloud radars, lidars, and radiometers, were part of the AMF. EVS climate scientists combined the data from these instruments with data from a Geostationary satellite and from global reanalysis models to retrieve estimates of entrainment rates and their relative variability at hourly timescales.

The average entrainment rate was 7.86 mm/s, with a large-scale downward vertical air motion of 2.56 mm/s at the cloud top. One of the findings of this study was that, despite the study region falling under a broad descending branch of the large-scale Hadley circulation, upward vertical motion at the cloud top occurred for about 20% of the time. This is the first study to report entrainment rates during occurrences of both downward and upward large-scale vertical air motions, providing in-depth knowledge of their spatial and temporal variability. Another important and surprising finding was that the mean entrainment velocity did not exhibit a systematic diurnal cycle and was minimally impacted by large scale processes. Rather, this study finds that changes in entrainment are modulated by boundary layer internal properties like turbulence. These findings have important implications for climate models as they collectively suggest that entrainment cannot be successfully represented using resolved large-scale variables, such as inversion strength and subsidence, further complicating parameterizing clouds in the Earth System Models.

This study was conducted in collaboration with David Mechem (University of Kansas), Edwin Eloranta (University of Wisconsin), Michael Jensen (Brookhaven National Laboratory), and William Smith (NASA Langley Research Center).

Terms of the boundary layer mass-budget equation that were used to retrieve entrainment rates for the period shown in Figure 1. The vertical bars denote the variability of the terms within 2°.
Terms of the boundary layer mass-budget equation that were used to retrieve entrainment rates for the period shown in Figure 1. The vertical bars denote the variability of the terms within 2°. [Source: Ghate et al., 2019]
(Top panel) Time-height profile of radar reflectivity (shades) and cloud base height (black). (Bottom panels). Coincident satellite images along with the ship track (yellow) and location of the ship when the image was taken (red dots). The boundary layer deepened towards west with transition from close to open cellular stratocumulus cloud organization.
(Top panel) Time-height profile of radar reflectivity (shades) and cloud base height (black). (Bottom panels). Coincident satellite images along with the ship track (yellow) and location of the ship when the image was taken (red dots). The boundary layer deepened towards west with transition from close to open cellular stratocumulus cloud organization. [Source: Ghate et al., 2019]
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