Abstract
Differences in cloud droplet effective radius and cloud droplet number concentration (CDNC) estimates inferred from the Aqua MODIS Collections 5.1 and 6 cloud products (MYD06) are examined for warm clouds over global oceans for the year 2008. Individual pixel level retrievals for both collections are aggregated to 1° × 1° and compared globally and regionally for the three main spectral channel pairs used for MODIS cloud optical property retrievals. Comparisons between both collections are performed for cases in which all three effective radii retrievals are classified by the MODIS Cloud Product as valid. The contribution to the observed differences of several key MYD06 Collection 6 algorithm updates are also explored, with a focus on changes to the surface reflectance model, assumed solar irradiance, above cloud emission, cloud top pressure, and pixel registration. Global results show a neutral to positive (> 50 cm-3) change for C6-derived CDNC relative to C5.1 for the 1.6 µm and 2.1 µm channel retrievals, corresponding to a neutral to -2 µm difference in droplet effective radius. For 3.7 µm retrievals, CDNC results show a negative change in the tropics, with differences transitioning toward positive values with increasing latitude spanning -25 to +50 cm-3 related to a +2.5 to -1 µm transition in effective radius. Cloud optical thickness differences were small relative to effective radius, and found to not significantly impact CDNC estimates. Regionally, the magnitude and behavior of the annual CDNC cycle are compared for each effective radius retrieval. Results from this study indicate significant intercollection differences in aggregated values of effective radius due to changes to the pre-computed retrieval lookup tables for ocean scenes, changes to retrieved cloud top pressure, solar irradiance, or above cloud thermal emission, depending upon spectral channel. The observed differences between collections may have implications for existing MODIS derived climatologies and validation studies of effective radius and CDNC.
Highlights
Marine stratocumuli play a large role in the modulation of the Earth’s radiative balance and hydrological cycle, owing to their persistence and large areal extent
We investigate and highlight the differences in retrieved cloud microphysical properties manifested both in effective radius and cloud droplet number concentration (CDNC) between both collections using 1 year of Aqua– Moderate Resolution Imaging Spectroradiometer (MODIS) observations of warm clouds over global oceans in an attempt to understand the differences in CDNC from the respective products
A common pixel selection allows for an objective comparison of effective radius and CDNC estimates between both collections, free of the influence of pixel population differences due to, for example, reclassification of cloud phase due to changes to the cloud thermodynamic phase algorithm (Marchant et al, 2016), or where one or more effective radius retrievals have failed between collections (Cho et al, 2015)
Summary
Marine stratocumuli play a large role in the modulation of the Earth’s radiative balance and hydrological cycle, owing to their persistence and large areal extent. Through the first indirect aerosol effect, anthropogenic influences potentially alter the degree of reflection of shortwave radiation by distributing a given quantity of cloud water over a larger number of droplets compared to an unperturbed cloud by supplying additional cloud condensation nuclei (CCN; Twomey, 1974). This increase in cloud albedo provides a local cooling effect as the cloud-top temperature (CTT) is near the sea surface temperature, producing a negligible longwave radiative ef-. We investigate and highlight the differences in retrieved cloud microphysical properties manifested both in effective radius and CDNC between both collections using 1 year of Aqua– MODIS observations of warm clouds over global oceans in an attempt to understand the differences in CDNC from the respective products
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