Abstract
Abstract. Marine stratocumulus (Sc) clouds play an essential role in the earth radiation budget. Here, we compare liquid water path (LWP), cloud optical thickness (τ), and cloud droplet effective radius (re) retrievals from 2 years of collocated Spinning Enhanced Visible and Infrared Imager (SEVIRI), Moderate Resolution Imaging Spectroradiometer (MODIS), and Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) observations; estimate the effect of biomass burning smoke on passive imager retrievals; and evaluate the diurnal cycle of South Atlantic marine Sc clouds.The effect of absorbing aerosols from biomass burning on the retrievals was investigated using the aerosol index (AI) obtained from the Ozone Monitoring Instrument (OMI). SEVIRI and MODIS LWPs were found to decrease with increasing AI relative to TMI LWP, consistent with well-known negative visible/near-infrared (VIS/NIR) retrieval biases in τ and re. In the aerosol-affected months of July–August–September, SEVIRI LWP – based on the 1.6 µm re – was biased low by 14 g m−2 ( ∼ 16 %) compared to TMI in overcast scenes, while MODIS LWP showed a smaller low bias of 4 g m−2 ( ∼ 5 %) for the 1.6 µm channel and a high bias of 8 g m−2 ( ∼ 10 %) for the 3.7 µm channel compared to TMI. Neglecting aerosol-affected pixels reduced the mean SEVIRI–TMI LWP bias considerably. For 2 years of data, SEVIRI LWP had a correlation with TMI and MODIS LWP of about 0.86 and 0.94, respectively, and biases of only 4–8 g m−2 (5 %–10 %) for overcast cases.The SEVIRI LWP diurnal cycle was in good overall agreement with TMI except in the aerosol-affected months. Both TMI and SEVIRI LWP decreased from morning to late afternoon, after which a slow increase was observed. Terra and Aqua MODIS mean LWPs also suggested a similar diurnal variation. The relative amplitude of the 2-year-mean and seasonal-mean LWP diurnal cycle varied between 35 % and 40 % from morning to late afternoon for overcast cases. The diurnal variation in SEVIRI LWP was mainly due to changes in τ, while re showed only little diurnal variability.
Highlights
Changes in marine boundary layer (MBL) clouds over eastern subtropical oceans and associated differences in cloud radiative forcing are thought to be the main source of uncertainty in climate feedback simulations (Bony and Dufresne, 2005; Meehl et al, 2007; Zelinka et al, 2017)
Previous research (Hobbs, 2002; McGill et al, 2003; Wilcox, 2010) has shown that the smoke is typically located in layers that are vertically separated from the Sc clouds below and, direct microphysical interaction between the aerosols and the Sc is often inhibited by the strong temperature inversion above the cloud layer
The presence of absorbing aerosols can be diagnosed using the Ozone Monitoring Instrument (OMI) aerosol index (AI), because large positive AIs correspond to absorbing aerosols, such as dust and smoke, and small positive or negative AIs correspond to non-absorbing aerosols and clouds
Summary
Changes in marine boundary layer (MBL) clouds over eastern subtropical oceans and associated differences in cloud radiative forcing are thought to be the main source of uncertainty in climate feedback simulations (Bony and Dufresne, 2005; Meehl et al, 2007; Zelinka et al, 2017). Understanding MBL cloud variability and its driving mechanisms remains crucial. Several studies investigated the synoptic to inter-annual variability and the driving mechanisms of these clouds from both an observational and a modeling perspective (e.g., Klein and Hartmann, 1993; Klein et al, 1995; Bretherton and Wyant, 1997; Wood and Bretherton, 2006; Eastman et al, 2011; Wood, 2012; Painemal et al, 2012, 2013a, 2015; Adebiyi et al, 2015; Adebiyi and Zuidema, 2016; Horowitz et al, 2017; Kar et al, 2018; Lu et al, 2018)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
