Turbulence and Mixed Phase Microphysics in an Altocumulus Cloud at High Vertical Resolution, Derived from in Situ Aircraft Measurements.

Mixed phase altocumulus layer clouds are ubiquitous in the atmosphere but in situ measurement from an aircraft is notoriously difficult, in part due to the shallow vertical extent which they occupy, and their often transient nature which makes them a challenge to forecast. Understanding the phase partition in altocumulus clouds is important for determining the radiative balance of the clouds. The turbulent nature of the clouds, driven by long-wave radiative cooling from the top of the liquid cloud layer is crucial in driving the evolution of the clouds. Ice virga often precipitate from these clouds.

Here we present turbulence observations from an isolated single layer mixed phase altocumulus cloud with a cloud top temperature of -30 °C. A liquid layer cloud of 200 m vertical extent was found above precipitating ice virga,which fell through a thermodynamically well mixed layer some 600 m deep. The altocumulus layer cloud was found to reside under an isentropic surface that was sloped in the vertical, which made flight legs at a constant altitude from cloud top impossible.

Geometrically horizontal flight legs and vertical slant profiles were flown and three dimensional wind components measured at high resolution (32 Hz) with a 5-port turbulence probe located on the nose of the aircraft. Turbulent fluctuations of the vertical wind component were generated from high-pass Butterworth-filtered time series data, for a range of filter lengths from 1.5 km to 16 km.

The Butterworth-filtered data was combined with a derived estimate of the cloud top height, which varied temporally and spatially, constrained by the in-situ observations. The data, filtered by the range of filter lengths then permitted a detailed analysis of the turbulence structure in the vertical and as a function of mixing-length scale.

Profiles of turbulence kinetic energy and the skewness of the distribution of turbulent fluctuations of vertical velocity were used to test the model of mixing in isolated altocumulus layer clouds proposed by Schmidt (JAS 2014). Similar structures are observed in our in situ observations as those shown and inferred from their ground based remotely sensed observations and the derived model. Observations of the bulk and microphysical properties of the liquid cloud layer are shown to be consistent with the implied spectra of turbulent circulations. This work then justifies the further use of ground based observations allowing for a longer timeseries to be collated, and potentially for climatologies to be constructed.

Reference
Schmidt et al., Convective cells in altocumulus observed with a high-resolution radar. JAS 2014.