The Structure of Turbulence and mixed-phase Cloud Microphysics in a Highly Supercooled Altocumulus Cloud

Abstract. Observations of vertically resolved turbulence and cloud microphysics in a mixed-phase altocumulus cloud are presented using in situ measurements from an instrumented aircraft. The turbulence spectrum is observed to have an increasingly negative skewness with distance below cloud top, confirming that longwave radiative cooling from the liquid layer cloud is the source of turbulence kinetic energy. Turbulence data are presented from both the liquid cloud layer and ice virga below. Vertical profiles of both bulk and microphysical liquid and ice cloud properties indicate that ice is produced within the liquid cloud layer at a temperature of -30° C. These high resolution in situ measurements support previous remotely-sensed observations from both ground based and space borne instruments, and could be used to evaluate numerical model simulations of altocumulus clouds at all scales from eddy resolving to climate.

Introduction

Mixed-phase layer clouds are common in the Earth`s atmosphere (Zhang el ah. 2010: Warren el al.. 1988). from the tropics where detrainment from convection forms long-lived altocumulus layers (Stein et al., 2011), to the mid-latitudes where humidity is brought to the mid-troposphere by cyclonic activity (Rauber and Tokay. 1991). Upwards air-motion associated with gravity-waves may also generate altocumulus cells.

Carey et al. (2008) observed that mid-latitude altocumulus layer clouds were of mixed-phase composition on more than two-thirds of occasions and that mixed-phase conditions began within a few lens of metres of observable cloud top. Peak LWC was found at cloud top, and ice water content (IWC> reached a maximum in the lower half of the cloud system and the similarity to Arctic boundary layer mixed-phase stratocumulus was noted. More than half of the observed clouds are thinner than 500 m. with mean liquid water content (LWC) of 0.14 g m_;t (Korolev and Field, 2008). with thinner clouds being correlated with lower temperatures. Fleishauer et al. (2002) found, for altocumulus in the mid-latitudes, that cloud systems can consist of single and multiple layers. The maintenance of altocumulus clouds is the result of a complex network of processes relating supercooled water to ice through long-wave radiative cooling (LWRC). turbulence, underlying aerosol properties and entrainment. similar to that found in Arctic boundary layer clouds (Morrison et a).. 2012).

The glaciation of a liquid cloud has significant consequences for fractional cloud coverage and albedo. A liquid or mixed-phase altocumulus cloud may have large area! coverage and significant optical depth, although the amount of condensed water may be relatively low. with 50 % of clouds having liquid water path (LWP) < 100 g m~2 (Korolev et al.. 2007). as observed by in sim instrumented aircraft. Radiative transfer calculations performed by Hogan et al. (2003b) suggest that the radiative impact of the liquid layer is extremely significant. Once glaciated the coverage can be much reduced and the optical depth of the ice cloud much lower and so understanding the processes involved in the production of ice particles is crucial for being able to quantify the radiative balance of the global climate system (Sun and Shine. 1995). Marsham et al. (2006) showed that maintenance of the supercooled liquid layer in a large eddy simulation of mixed-phase altocumulus was dependant on good representation of the distribution of vertical velocity fluctuations as derived from ground based radar and lidar. Previous in situ observations show the range of turbulent fluctuations in altocumulus in the UK to be typically ± 1 m s and with a root-mean-square value of 0.5 m s in the middle of the (liquid) cloud (Watson. 1967). Similar results were found by Fleishauer et al.