Publications

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2014
Flatau, PJ, Draine BT.  2014.  Light scattering by hexagonal columns in the discrete dipole approximation. Optics Express. 22:21834-21846.   10.1364/oe.22.021834   AbstractWebsite

Scattering by infinite hexagonal ice prisms is calculated using Maxwell's equations in the discrete dipole approximation for size parameters x = pi D/lambda up to x = 400 (D = prism diameter). Birefringence is included in the calculations. Applicability of the geometric optics approximation is investigated. Excellent agreement between wave optics and geometric optics is observed for large size parameter in the outer part of the 22 degree halo feature. For smaller ice crystals halo broadening is predicted, and there is appreciable "spillover" of the halo into shadow scattering angles < 22 degrees. Ways to retrieve ice crystal sizes are suggested based on the full width at half-maximum of the halo, the power at < 22deg, and the halo polarization. (C) 2014 Optical Society of America

Schmidt, JM, Flatau PJ, Yates RD.  2014.  Convective cells in altocumulus observed with a high-resolution radar. Journal of the Atmospheric Sciences. 71:2130-2154.   10.1175/jas-d-13-0172.1   AbstractWebsite

Very-high-resolution Doppler radar observations are used together with aircraft measurements to document the dynamic and thermodynamic structure of a dissipating altocumulus cloud system associated with a deep virga layer. The cloud layer circulation is shown to consist of shallow vertical velocity couplets near cloud top and a series of subkilometer-scale Rayleigh-Benard-like cells that extend vertically through the depth of the cloud layer. The subcloud layer was observed to contain a number of narrow virga fall streaks that developed below the more dominant Rayleigh-Benard updraft circulations in the cloud layer. These features were discovered to be associated with kilometer-scale horizontally orientated rotor circulations that formed along the lateral flanks of the streaks collocated downdraft circulation. The Doppler analysis further reveals that a layer mean descent was present throughout both the cloud and subcloud layers. This characteristic of the circulation is analyzed with regard to the diabatic and radiative forcing on horizontal length scales ranging from the Rayleigh-Benard circulations to the overall cloud layer width. In particular, linear analytical results indicate that a deep and broad mesoscale region of subsidence is quickly established in middle-level cloud layers of finite width when a layer-wide horizontal gradient in the cloud-top radiative cooling rate is present. A conceptual model summarizing the primary observed and inferred circulation features of the altocumulus layer is presented.