Differences in liquid cloud droplet effective radius and number concentration estimates between MODIS collections 5.1 and 6 over global oceans
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...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-06-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | http://www.atmos-meas-tech.net/10/2105/2017/amt-10-2105-2017.pdf |
| Summary: | 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<sup>−3</sup>) 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 (<i>r</i><sub>e</sub>). 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<sup>−3</sup> related to a +2.5 to −1 µm transition
in effective radius. Cloud optical thickness (<i>τ</i>) 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. |
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| ISSN: | 1867-1381 1867-8548 |