Abstract
Abstract. This study presents a new algorithm that combines W-band reflectivity measurements from the Airborne Precipitation Radar – third generation (APR-3) passive radiometric cloud optical depth and effective radius retrievals from the Research Scanning Polarimeter (RSP) to estimate total liquid water path in warm clouds and identify the contributions from cloud water path (CWP) and rainwater path (RWP). The resulting CWP estimates are primarily determined by the optical depth input, although reflectivity measurements contribute ∼10 %–50 % of the uncertainty due to attenuation through the profile. Uncertainties in CWP estimates across all conditions are 25 % to 35 %, while RWP uncertainty estimates frequently exceed 100 %. Two-thirds of all radar-detected clouds observed during the ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) campaign that took place from 2016–2018 over the southeast Atlantic Ocean have CWP between 41 and 168 g m−2 and almost all CWPs (99 %) between 6 to 445 g m−2. RWP, by contrast, typically makes up a much smaller fraction of total liquid water path (LWP), with more than 70 % of raining clouds having less than 10 g m−2 of rainwater. In heavier warm rain (i.e., rain rate exceeding 40 mm h−1 or 1000 mm d−1), however, RWP is observed to exceed 2500 g m−2. CWP (RWP) is found to be approximately 30 g m−2 (7 g m−2) larger in unstable environments compared to stable environments. Surface precipitation is also more than twice as likely in unstable environments. Comparisons against in situ cloud microphysical probe data spanning the range of thermodynamic stability and meteorological conditions encountered across the southeast Atlantic basin demonstrate that the combined APR-3 and RSP dataset enable a robust joint cloud–precipitation retrieval algorithm to support future ORACLES precipitation susceptibility and cloud–aerosol–precipitation interaction studies.
Highlights
Stratocumulus clouds (Sc hereafter) are the dominant cloud type over Earth’s subtropical oceans
We develop and test a joint radar–radiometer cloud water path (CWP) and rainwater path (RWP) retrieval using W-band reflectivity profiles from the Airborne Precipitation Radar – third generation (APR-3) and cloud properties from the Research Scanning Polarimeter (RSP), both of which flew aboard the NASA P-3 aircraft during all of ORACLES
This is especially difficult for precipitation, where comparing precipitation estimates is challenging due to collocation issues (e.g., Meneghini et al, 2001), small-scale heterogeneity along short distances (e.g., Tridon et al, 2019), and general complications arising from diagnostics used for comparing precipitation variables between datasets (Kay et al, 2018)
Summary
Stratocumulus clouds (Sc hereafter) are the dominant cloud type over Earth’s subtropical oceans. Significant advances in our understanding of subtropical Sc have been documented in many previous studies including their typical cloud fractions (Stephens et al, 2012; King et al, 2013; L’Ecuyer et al, 2019), radiative impacts (Hang et al, 2019), and associated precipitation processes using data from past field experiments (Stevens et al, 2003), long-term ground-based data collection (Mann et al, 2014; Yang et al, 2018), and spaceborne remote sensors (Fox and Illingworth, 1997; L’Ecuyer and Stephens, 2002; Lebsock et al, 2011; Chen et al, 2011; Lebsock et al, 2011; Douglas and L’Ecuyer, 2019; Werner and Deneke, 2020) Such measurements of the southeast Atlantic Sc deck are critically needed, given the general lack of process-level understanding of cloud–aerosol–precipitation processes (Hou et al, 2018; Mülmenstädt et al, 2020) and the consequent uncertainties they propagate in modern-day global climate models (e.g., Stephens et al, 2010; Sockol and Small Griswold, 2017; Cesana et al, 2019)
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