The Dependence of Shallow Cumulus Macrophysical Properties on Large‐Scale Meteorology as Observed in ASTER Imagery

Abstract

This study identifies meteorological variables that control the macrophysical properties of shallow cumulus cloud fields over the tropical ocean. We use 1,158 high‐resolution Advanced Spaceborn Thermal Emission and Reflection Radiometer (ASTER) images to derive properties of shallow cumuli, such as their size distribution, cloud top heights, fractal dimensions, and spatial organization, as well as cloud amount. The large‐scale meteorology is characterized by the lower‐tropospheric stability, subsidence rate, sea surface temperature, total column water vapor, wind speed, wind shear, and Bowen ratio. The surface wind speed emerges as the most powerful control factor. With increasing wind speed the cloud amount and cloud top heights show a robust increase accompanied by a marked shift in the cloud size distribution toward larger clouds with smoother shapes. These results lend observational support to the deepening response of a wind‐driven marine boundary layer as simulated by large‐eddy models. The other control factors cause smaller changes in the cloud field properties. We find a robust increase in cloud amount with increasing stability and decreasing sea surface temperature, respectively, which confirms a well‐known behavior of marine stratocumulus also for shallow cumulus clouds. Due to the high resolution of cloud images, we are able to study the lower end of the cloud size distribution and find a robust double power law behavior with a scale break at 590 m. We find a variation in the shape of the cloud size distribution with Bowen ratio, qualitatively consistent with modeling results and suggesting the Bowen ratio as a new potential control factor on shallow cumulus clouds.

Comparing ground‐based observations and a large‐eddy simulation of shallow cumuli by isolating the main controlling factors of the mass flux distribution

“…In the study of Nuijens and Stevens (), wind speed was found to influence the deepening of the boundary layer and cause deeper cloud layers. The effect of the wind on the boundary layer clouds is also confirmed by observations (Brueck et al ., ; Mieslinger et al ., ). Nuijens and Stevens () further emphasize that shallow cumuli get deeper with stronger winds, but not more numerous or more energetic, while the wind speed does not change the buoyancy flux in the mixed layer in equilibrium.…”

“…Although there is a lack of observational studies on the large-scale control of the mass flux distribution, the cloud size distribution has been studied extensively. In a recent study by Mieslinger et al (2019), ASTER satellite imagery and the ERA-Interim reanalysis are used to test the controls of the large-scale meteorological conditions on the shallow cloud properties over the tropical oceans. Wind speed is recognized as a major factor controlling the cloud size distribution, among other cloud-based statistics.…”

“…In a recent study by Mieslinger et al . (), ASTER satellite imagery and the ERA‐Interim reanalysis are used to test the controls of the large‐scale meteorological conditions on the shallow cloud properties over the tropical oceans. Wind speed is recognized as a major factor controlling the cloud size distribution, among other cloud‐based statistics.…”

EUREC⁴A

“…The second, vertically pointing ATR cloud radar, allows a characterization of the aspect ratio of clouds, which may help infer the mesoscale circulations within the cloud field. These measurements, associated with new methods developed to estimate the cloud-base mass flux (Vogel et al, 2020), and to characterize the mesoscale cloud patterns from GOES-16, MODIS or ASTER satellite observations Mieslinger et al, 2019;Bony et al, 2020;Denby, 2020;Rasp et al, 2020), will make it possible to test cloud feedback mechanisms and advance understanding as to whether mesoscale cloud patterns influence the hypothesized feedback mechanisms.…”

A-Train estimates of the sensitivity of warm rain likelihood and efficiency to cloud size, environmental moisture, and aerosols

“…While perhaps not as important as organization (Minor et al, 2011) or cloud size (Jiang and Feingold, 2006), it is widely understood that aerosol concentrations act to suppress warm-rain production (Twomey, 1974;Albrecht, 1989) by increasing the cloud droplet concentration and reducing cloud droplet sizes (Squires, 1958). Albrecht (1989) found that increasing precipitation efficiency within a model is equivalent to decreasing the amount of cloud concentration nuclei (CCN), which reduces the mass concentration of cloud water within a cloudy layer.…”