Abstract
Abstract. Microphysical measurements collected during eleven profiles, by the UK BAe-146 aircraft, through marine stratocumulus as part of the Variability of the American Monsoon Systems (VAMOS) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) are compared to collocated overpasses of the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua and Terra satellite platforms. The full depth of the cloud is sampled in each case using a Cloud Droplet Probe (CDP) and a Two-Dimensional Stereo Probe (2DS) together sizing cloud and precipitation droplets in the diameter range 2–1260 μm. This allows the total optical depth (τc) of the cloud and effective radius (re) of the droplet size distribution to be compared to MODIS cloud retrievals of the same quantities along with the secondarily derived total liquid water path. When compared to the effective radius at cloud top, the MODIS retrieved re using the 2.1 μm wavelength channel overestimates the in situ measurements on average by 13% with the largest overestimations coinciding with the detection by the 2DS of drizzle sized droplets. We show through consideration of the full vertical profile and penetration depths of the wavelengths used in the retrieval that the expected retrieved values are less than those at cloud top thus increasing the apparent bias in re retrievals particularly when using the 1.6 and 2.1 μm channels, with the 3.7 μm channel retrievals displaying the best agreement with in situ values. Retrievals of τc also tend to overestimate in situ values which, coupled with a high bias in re retrievals, lead to an overestimation of liquid water path. There is little apparent correlation between the variation of the three near-infrared re retrievals and the vertical structure of the cloud observed in situ. Retrievals are performed using measured profiles of water vapour and temperature along with an accurate knowledge of the width of the droplet size distribution which improve agreement between in situ and retrieved values but cannot completely explain the observed biases. Additionally we show that cloud heterogeneity and three-dimensional radiative effects may high skew the mean when averaging over comparison domains but cannot explain all of the apparent high bias. An intercomparison between in situ measurements from the BAe-146 and C-130 platforms is also presented, highlighting the uncertainties associated with in situ observations.
Highlights
Low-level marine stratocumulus clouds cover large regions of the global oceans on a quasi-permanent basis and are critical modulators of the global radiation budget (Klein and Hartmann, 1993)
This study focuses on measurements of cloud microphysics using a Droplet Measurement Technologies, Cloud Droplet Probe (CDP) aand a Stratton Park Engineering Company Two-Dimensional Stereo Probe (a 1-D array version used in VOCALS, which will be referred to as the 2DS in this paper)
During VOCALS-REx allowed us to asses the quality of collocated Moderate Resolution Imaging Spectroradiometer (MODIS) cloud retrievals
Summary
Low-level marine stratocumulus clouds cover large regions of the global oceans on a quasi-permanent basis and are critical modulators of the global radiation budget (Klein and Hartmann, 1993). Their complex interactions with aerosols (Lohmann and Feichter, 2005) and their spatial and temporal variability mean that their accurate representation in global models remains one of the largest uncertainties in modelling future climate (Forster et al, 2007). Together they specify the total liquid water path of the cloud and are used as indicators of aerosol-cloud-precipitation interactions (Twomey, 1991; Albrecht, 1989; Lohmann and Feichter, 2005).
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