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We present and evaluate a climatology of cloud droplet number concentration (CDNC) based on 13 years of Aqua-MODIS observations. The climatology provides monthly mean 1×1° CDNC values plus associated uncertainties over the global ice-free oceans. All values are incloud values, i.e. the reported CDNC value will be valid for the cloudy part of the grid box. Here, we provide an overview of how the climatology was generated and assess and quantify potential systematic error sources including effects of broken clouds, and remaining artefacts caused by the retrieval process or related to observation geometry. Retrievals and evaluations were performed at the scale of initial MODIS observations (in contrast to some earlier climatologies, which were created based on already gridded data). This allowed us to implement additional screening criteria, so that observations inconsistent with key assumptions made in the CDNC retrieval could be rejected. Application of these additional screening criteria led to significant changes in the annual cycle of CDNC in terms of both its phase and magnitude. After an optimal screening was established a final CDNC climatology was generated. Resulting CDNC uncertainties are reported as monthly-mean standard deviations of CDNC over each 1Γ1° grid box. These uncertainties are of the order of 30% in the stratocumulus regions and 60 to 80% elsewhere. [ABSTRACT FROM AUTHOR]

Differences in cloud droplet effective radius and cloud droplet number concentration (CDNC) estimates inferred from the Aqua-MODIS (Moderate Resolution Imaging Spectroradiometer) collections 5.1 (C5.1) and 6 (C6) 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 (CTP), 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 and 2.1 μm channel retrievals, corresponding to a neutral to--2 μm difference in droplet effective radius (re/. 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 (τ) 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 inter-collection differences in aggregated values of effective radius due to changes to the precomputed retrieval lookup tables (LUTs) 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. [ABSTRACT FROM AUTHOR]

Differences in cloud 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 channel pairs are successfully retrieved for each pixel and for cases where one or more channel pairs are successful. The contribution to the observed differences of several key MYD06 Collection 6 algorithm updates are also explored. Global results show a neutral to positive (>50cm-3) change for C6-derived CDNC relative to C5.1 for the 1.6 and 2.1μm channel retrievals corresponding to a neutral to -2μm difference in droplet effective radius. For 3.7μm retrievals, CDNC results show a negative change in the tropics, with differences transitioning towards positive values with increasing latitude spanning (-25 to +50cm-3) related to a (+2.5 to -1μm) transition in effective radius. Regionally, the magnitude and behavior of the annual CDNC cycle are compared for each retrieval channel pair. Results from this study indicate significant intercollection differences in aggregated values of microphysical parameters which may have implications for existing MODIS derived climatologies and validation studies. [ABSTRACT FROM AUTHOR]