Revisiting Liquid Water Content Retrievals in Warm Stratified Clouds: The Modified Frisch

Accurate observations of liquid water content (LWC) in warm stratiform clouds are important for quantifying their radiative and hydrological effects and for studying aerosol-cloud interactions. Retrieving LWC from radar reflectivity under drizzling or nondrizzling conditions has been investigated for several decades by the cloud remote sensing community. However, no physically plausible framework exists to address the biases introduced by drizzle on existing retrieval techniques. We present the modified Frisch retrieval (ModFrisch), which combines radar and microwave radiometer measurements to retrieve LWC in both nondrizzling and drizzling conditions. It is shown, using a 1-D steady state microphysical model and a radar simulator, that the uncertainty of ModFrisch is up to four times smaller than the uncertainty of similar retrievals under drizzling conditions, enabling LWC profiling with an accuracy of 20%. The performance of the ModFrisch technique is evaluated using 1year of observations. Plain Language Summary It is important to measure the vertical structure of clouds accurately to create reliable climatological records and to investigate how well clouds can be predicted by weather and climate models. A commonly used technique to determine the liquid water content of stratified clouds is based on two standard instruments: a radar, providing profile information, and a microwave radiometer providing the total amount of liquid in the cloud. However, if the cloud droplets are too big (larger than about 50-70m), the vertical profile of the radar measurement cannot be related to the profile of liquid water content. Unfortunately, most stratus clouds contain liquid drops (drizzle) that exceed that critical size leading to a big lack of accurate data. In our study, we separate the cloud into two regions: the upper part of the cloud where drizzle has no significant influence and the lower part where drizzle is present. In the upper part, we apply a commonly used retrieval while assuming a linearly increasing liquid water content in the lower part, which has been shown to be valid in previous studies. Thus, we can potentially provide a more reliable basis of observational liquid water contents to constrain model simulations.