Radar–Radiometer-Based Liquid Water Content Retrievals of Warm Low-Level Clouds: How the Measurement Setup Affects Retrieval Uncertainties

Abstract

Here, we propose a new methodology that increases the understanding of uncertainty sources of liquid water content (LWC) retrievals, which are caused by the combination of instruments having different beam widths and are horizontally displaced. Furthermore, we give first quantitative uncertainty estimates. This paper is based on a case study of a single-layer, warm, stratiform cloud observed at the Julich Observatory for Cloud Evolution. The LWC profiles of this cloud have been forward-simulated with the passive and active radiative transfer model providing radar and microwave radiometer (MWR) observables for all cloud columns. These observables have been converted back into LWC profiles, whereas, in this case, the MWR and radar observables from different columns were combined, representing horizontal displacement. We investigate the influence of horizontal distance between a radar and an MWR on a commonly used retrieval for LWC, which scales radar reflectivity profiles with the liquid water path given by the MWR. We found that a displacement of only 10 m already introduces an additional relative uncertainty of 10%. At 100 m displacement, the relative error grows up to 30%. Additionally, different beam widths decrease the retrieval accuracy by a few percent; however, at large displacements, radiometers with larger beam widths slightly decrease the error due to the displacement. Finally, we show that cloud edge studies require optimally matched beams between the radar and the radiometer, and already a displacement of 10 m leads to unreasonable results.