Abstract
Supercooled liquid clouds are very frequent in high-latitude regions. In addition to their substantial effect on visible and infrared radiation, they affect the signal of millimeter radars by producing nonnegligible attenuation. Such attenuation must be properly corrected if the information of millimeter radars is used in quantitative retrievals for inferring ice microphysical properties. This study proposes a multisensor scheme for refining the vertical distribution of supercooled liquid water content (SLWC) compared to state-of-the-art methods that equipartition the liquid water path measured by microwave radiometer to all pixels identified as cloudy by the radars and warmer than −40 °C. Our methodology is applicable in high-latitude, mixed-phase environments based on the synergy between radar and lidar binary cloud phase masking, microwave radiometer, and radio sounding observations. The technique is demonstrated via data collected by the U.S. Department of Energy (DoE) Atmospheric Radiation Measurement (ARM) Program climate research facility at the North Slope of Alaska (NSA) and compared with the state-of-the-art methods. Path integrated attenuation (PIA) at W- and G-band frequencies (>95 GHz) is then assessed. Results indicate that the different in-cloud distributions of the liquid condensate lead to round-trip PIA discrepancies of cloudy volumes that range in [2, 5] dB at W- and G-band frequencies. These differences far exceed those encountered when changing some of the algorithm’s arbitrary assumptions and weighting functions.
Highlights
In the past 30 years millimeter-wavelength radars have played a paramount role in atmospheric cloud research by providing a better understanding of clouds and precipitation and their feedbacks in the Earth’s climate system ( [1])
Thanks to their improved sensitivity to smaller particles these radars have been proposed for a variety of cloud microphysics applications when operated in synergy with lower frequency cloud radars ( [6]; [7]; [5]) and for profiling water vapor when designed with multiple tones within a water vapor absorption line ( [8]; [9]; [10])
Millimeter-radar signal attenuation cannot be neglected, yet it remains challenging given the significant uncertainty of the supercooled liquid water (SLW) location, more so when depolarization lidar -based information is not available due to complete lidar extinction from intervening liquid condensate or layers of optically thicker ice
Summary
In the past 30 years millimeter-wavelength radars have played a paramount role in atmospheric cloud research by providing a better understanding of clouds and precipitation and their feedbacks in the Earth’s climate system ( [1]). The focus of this study is on high latitude clouds generated in cold and dry environment below freezing temperatures, characterized by persistent mixed-phase clouds ( [12]) In such conditions we can exclude the presence of liquid precipitation, which is generally an important source of attenuation even at the lower frequencies ( [13]); while wavebands up to 40 GHz can be practically considered not attenuated, radiation at frequencies close to 100 GHz and above may still suffer non negligible attenuation caused by atmospheric gases, supercooled liquid droplets and ice (rimed) crystals ( [14]). The methodology is based on an optimal combination of all these instruments in the characterization of the vertical profile of gases and supecooled liquid layers This enables a more realistic liquid partitioning of the total LWP retrieved by the passive microwave radiometer in the column sensed by the radars. The radio soundings are co-registered via a temporally-weighted average scheme that applies at least the two recent-most deployments, which are never more than 12 hrs apart
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