Twomey effect observed from collocated microphysical and remote sensing measurements over shallow cumulus

Abstract

Clear experimental evidence of the Twomey effect for shallow trade wind cumuli near Barbados is presented. Effective droplet radius (reff) and cloud optical thickness (τ), retrieved from helicopter‐borne spectral cloud‐reflected radiance measurements, and spectral cloud reflectivity (γλ) are correlated with collocated in situ observations of the number concentration of aerosol particles from the subcloud layer (N). N denotes the concentration of particles larger than 80 nm in diameter and represents particles in the activation mode. In situ cloud microphysical and aerosol parameters were sampled by the Airborne Cloud Turbulence Observation System (ACTOS). Spectral cloud‐reflected radiance data were collected by the Spectral Modular Airborne Radiation measurement sysTem (SMART‐HELIOS). With increasing N a shift in the probability density functions of τ and γλ toward larger values is observed, while the mean values and observed ranges of retrieved reff decrease. The relative susceptibilities (RS) of reff, τ, and γλ to N are derived for bins of constant liquid water path. The resulting values of RS are in the range of 0.35 for reff and τ, and 0.27 for γλ. These results are close to the maximum susceptibility possible from theory. Overall, the shallow cumuli sampled near Barbados show characteristics of homogeneous, plane‐parallel clouds. Comparisons of RS derived from in situ measured reff and from a microphysical parcel model are in close agreement.

Strong aerosol–cloud interaction in altocumulus during updraft periods: lidar observations over central Europe

“…(2001) investigated aerosol-cloud relationships over the Indian Ocean and found only a slight increase in the mean ACI N values from 0.63 to 0.67 and 0.7 for data sets, considering only data for which the vertical winds were < 0.5, > 0.5-2, and > 2 m s −1 in tropical cloud layers, respectively. Werner et al (2014) found that updraft velocity variations from 0.5 to 4 m s −1 caused variations in the derived ACI r values by 0.02 and in ACI N by 0.06. They concluded that updraft velocity strength is of minor importance in aerosol-cloud interaction studies of short-lived tropical trade-wind cumuli over the tropical Atlantic.…”

“…The studied short-lived cumuli fields or aged stratocumulus layers mostly develop within a well-mixed, undisturbed marine boundary layer at almost adiabatic-like stratification of the water content resulting in an height-independent CDNC from cloud base to top (Painemal and Zuidema, 2013). Effects of vertical motions (updrafts, turbulent mixing, and entrainment of drier air into the clouds) may then be comparably weak (Twohy et al, 2005;Terai et al, 2012;Werner et al, 2014). In contrast, much more complex aerosol and meteorological conditions occur over land, as mentioned above.…”

A tandem approach for collocated measurements of microphysical and radiative cirrus properties

“…Such constellations are well suited to investigate the interaction of cloud microphysical and radiative properties as demonstrated by Werner et al (2014) for shallow cumulus. However, the approach by Werner et al (2014) for analysing the collocated number concentration and cloud remote sens- ing works only if the radiation measurements are taken well above the cloud.…”

“…However, the approach by Werner et al (2014) for analysing the collocated number concentration and cloud remote sens- ing works only if the radiation measurements are taken well above the cloud. In the case of the AIRTOSS-Learjet tandem this would limit the analysis to the uppermost cirrus layer.…”

“…Werner et al (2014) showed that such observations can be used to link cloud microphysical and radiative properties and estimate the Twomey effect in shallow cumulus. However, such helicopter measurements are limited to altitudes below 3000 m and therefore are not suited for investigating cirrus.…”