Liquid water in snowing clouds: Implications for satellite remote sensing of snowfall

To study the impact of cloud liquid water on passive microwave snowfall remote sensing, we analyzed 4 years of liquid water path data retrieved from microwave radiometer on Aqua satellite that are collocated with CloudSat snowfall observations. Results showed that cloud liquid water commonly occurs in snowing clouds (2-m air temperature lower than 2 °C); about 72% of these clouds have a retrieved liquid water path greater than 0. The mean liquid water path for all snowing clouds is about 74 g m − 2, higher for horizontally extended clouds (70–100 g m− 2) and lower for isolated (~ 50 g m− 2). There is a clear tendency that snowing clouds are less likely to contain liquid water as 2-m air temperature decreases. However, the variation of the mode values of liquid water path with 2-m air temperature seems to be cloud type dependent, particularly for colder environment with 2-m air temperature lower than 263 K. On average, larger values of liquid water path occur when near-surface radar reflectivity ranges from − 10 to 0 dBZ, corresponding to relatively weak snowfall of 0.02 to 0.15 mm h − 1, rather than to the heaviest snowfall observed. The impact of cloud liquid water on passive microwave satellite remote sensing of snowfall has been investigated using radiative transfer simulations. It is concluded that for frequencies higher than 80 GHz the brightness temperature warming caused by cloud liquid water emission has a similar magnitude to the brightness temperature cooling caused by snowflakes' scattering. Therefore, while ice scattering is the primary signature for retrieving snowfall, it is equally important to take into account the impact by cloud liquid water when developing snowfall retrieval algorithms using high-frequency satellite observations.