Statistical properties of cloud liquid water path at Barrow, Alaska, and at the Greenland Summit station

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Description

Accurate knowledge of arctic clouds, their phase, and their microphysical properties are important to correctly model cloud forcing and the Earth's radiation budget. It is important therefore to be able to detect the presence of liquid water in clouds and to quantify the liquid water path (LWP) as accurately as possible. The retrieval of liquid water path in the Arctic is challenging due to the low amounts of water vapor and the fact that a majority of clouds have very small amounts of liquid water. In 2006 the Atmospheric Radiation Measurement Climate Research Facility deployed a high-frequency microwave radiometer in Barrow, Alaska, to help improve the retrieval of water vapor and liquid water path. The G-band vapor radiometer (GVR)has been collecting data for seven years now, and retrieval of water vapor and liquid water path from this instrument has been developed. The vapor retrievals have an uncertainty of ~0.3 mm, and the liquid water path retrievals have an estimated uncertainty of 8 g/m2 (compared to the 25 g/m2 of the two-channel microwave radiometer). The retrievals provide improved information on the presence of small amount of liquid water, especially in supercooled clouds.

In this poster we use data from the microwave radiometers, radiosondes, and Vaisala ceilometer and analyze seven years (2006–2012) of improved liquid water path in Barrow, Alaska from GVR measurements. We compare them with three years (2010–2012) of liquid water path derived from the microwave radiometers at the Summit station in Greenland. The estimated uncertainty of this retrieval is approximately 5 g/m2.

At both sites we examine how the presence of liquid water and the LWP depend on the cloud temperature. We also examine some seasonal characteristics of clouds where liquid water was not detected by the radiometers. The analysis shows some interesting similarities between the two sites, but also some noticeable differences. For example, in both locations water in liquid phase was detected in about 20% of the cases when the cloud temperature was between -25°C and -30°C. Differences in the seasonal LWP average are noticeable at the two sites, the average LWP at Summit being much less than at Barrow in all seasons, and especially in spring and fall. Overall, the two data sets provide valuable information on some important cloud properties at the two sites and underscore the importance of long-term observations to improve our knowledge of cloud properties.