Precipitation Properties of Arctic Single-Layer Mixed-Phase Clouds

Abstract:

In the Arctic, single-layer mixed-phase clouds rooted in the boundary layer with cloud top heights between several hundred meters and 2 km are frequently observed. Ice particles grow fast in these mixed-phase clouds and thus produce solid precipitation. Understanding the water cycle in the Arctic region requires an extensive study of the precipitation properties of the Arctic single-layer mixed-phase clouds.

The US Department of Energy Atmospheric Radiation Measurement (ARM) facility at Barrow, North Slope of Alaska conducts long-term measurements of mixed-phase clouds using a combination of active and passive sensors. Using the data of 35-GHz (Ka-band) zenith pointing radar (KAZR), ceilometer, and microwave radiometer, 1-hour averaged estimates of mixed-phase clouds are produced. A total of 553 hours of single-layer mixed-phase clouds were visually identified between October 2011 and December 2014.

Using the KAZR radar reflectivity measured just one radar range gate (30 m) below the liquid base of the mixed-phase, ice water content (IWC) estimates are derived. The correlation between the hourly-mean liquid water path (LWP) and IWC below the base is not high. On the other hand, the KAZR mean Doppler velocity (V_d) at the same level exhibits high correlation with the LWP. In the LWP regime above 220 g m^-2, V_d is larger than 1.0 m s^-1. V_d approached to 1.8 m s^-1 at even higher LWP values. We assumed that the 1-hour averaged V_d values are representative of the reflectivity-weighted mean terminal velocity of the ice particles. The large terminal velocities cannot be explained if the ice particles grow only via the deposition process. It is suggested that the riming process contributed to the growth of the particles when large terminal velocities are observed. The correlation between LWP and the KAZR V_d near the surface is similarly high. From high correlations between LWP and V_d near the cloud base and near the surface, it is clear that the LWP exhibits some control over the precipitating ice mass flux. The observed relationships will provide constrains to numerical simulations of mixed-phase clouds.