Analyzing Inhomogeneity of Trade Wind Cumuli from Collocated Radiation and Microphysical Observations

The inhomogeneity of shallow trade wind cumuli near Barbados is analyzed and quantified by means of spectral upward radiances (I_λ), sampled by the Spectral Modular Airborne Radiation measurements sysTem (SMART-HELIOS) during the CARRIBA project. Statistical analysis, radiance-ratios, and photos from a downward-facing camera installed on SMART-HELIOS are used to identify cloud data that are influenced by 3D radiative effects. It is shown that cloud holes and cloud shadows can be reliably identified from I_λ measurements, while illuminated cloud parts and sections with high optical thickness are difficult to discriminate. A measure of cloud inhomogeneity (δ) is derived from PDFs of I_λ, ranging between 0 (very homogeneous) – 1 (very inhomogeneous). The CARRIBA data show δ = 0.5-75 for different flights, indicating that trade wind cumuli are characterized by medium to high inhomogeneity regarding I_λ. These values are compared to in situ observations of the droplet diameter (d) and cloud liquid water content (LWC) sampled simultaneously by instrumentation installed on the helicopter payload ACTOS (Airborne Cloud Turbulence Observation System). While δ from the LWC data shows a similar degree of inhomogeneity, d shows a significantly more homogeneous characteristic.