Cloud top heights and aerosol columnar properties from combined EarthCARE lidar and imager observations: the AM-CTH and AM-ACD products
Abstract. The Earth Clouds, Aerosols and Radiation Explorer (EarthCARE) is the combination of multiple active and passive instruments on a single platform. The Atmospheric Lidar (ATLID) provides vertical information of clouds and aerosol particles along the satellite track. In addition, the Multi-Spectral Imager (MSI) collects the multispectral information from the visible till the infrared wavelengths over a swath width of 150 km across the track. The ATLID–MSI Column Products processor (AM-COL) described in this paper combines the high vertical resolution of the lidar along track and the horizontal resolution of the imager across track to better characterize the 3-dimensional scene. ATLID Level 2a (L2a) data from the ATLID Layer Products processor (A-LAY) and MSI L2a data from the MSI Cloud Products processor (M-CLD) and the MSI Aerosol Optical Thickness processor (M-AOT) as well as MSI Level 1c (L1c) data are used as input to produce the synergistic columnar products: the ATLID–MSI Cloud Top Height (AM-CTH) and the ATLID–MSI Aerosol Column Descriptor (AM-ACD). The coupling of ATLID (measuring at 355 nm) and MSI (at ≥ 670 nm) provides multispectral observations of the aerosol properties. Especially, the Ångström exponent from the spectral aerosol optical thickness (AOT 355 nm/670 nm) adds valuable information for aerosol typing. The AOT across track, the Ångström exponent and the dominant aerosol type are stored in the AM-ACD product. The accurate detection of the Cloud Top Height (CTH) with lidar is limited to the ATLID track. The difference of the CTH detected by ATLID and MSI is calculated along track. The similarity of MSI pixels across track with those along track is used to transfer the calculated CTH difference to the entire MSI swath. In this way, the accuracy of the CTH is increased to achieve the EarthCARE mission goal aiming to derive the radiative flux at the top of the atmosphere with an accuracy of 10 Wm−2 for a 100 km2 snapshot view of the atmosphere. The synergistic CTH difference is stored in the AM-CTH product. The quality status depending on day/night conditions or the presence of multiple cloud or aerosol layers is provided with the products. The algorithm was successfully tested using the common EarthCARE test scenes. Two definitions of the CTH from the model-truth cloud extinction fields are compared: An extinction-based threshold of 20 Mm−1 provides the geometric CTH and a cloud-optical-thickness threshold of 0.25 describes the radiative CTH. The first one is detected with ATLID, the second one with MSI.
Moritz Haarig et al.
Status: open (until 23 Jun 2023)
- RC1: 'Comment on egusphere-2023-327', Anonymous Referee #1, 02 Jun 2023 reply
Moritz Haarig et al.
Moritz Haarig et al.
Viewed (geographical distribution)
This paper introduces the interesting method to transfer the observed cloud and aerosol parameters at the collocated observation of lidar and imager to the imager-only observation pixels developed by using the model simulation results for the EarthCARE satellite mission. I recommend the publication of this manuscript after the revision of the following two points.
Figure 2: It is not clear to me to understand which product is input to the processing of “scene classification” and “check for multi-layer cloud”. The solid arrows in figure are overlapped.
Line 196: Does “same value” mean same cloud phase and surface type?
Line 244: Is ice included in the aerosol classification? However, ice means cirrus in the sentence. Please add an appropriate explanation.
Lines 304-310: It is difficult to understand the aerosol and cloud distributions. Please add the figures of simulation true results of Halifax scenes or refer to the figure number in Donovan et al. 2023a.
Line 317: Does this sentence only focus on the cloud at 2-3 km altitude at 55oN? Half of CTH differences are lower than 1 km, while the other half are about 2 km between 55oN and 60oN. A more detailed discussion is needed.
Figure 5: This quality status means the summarized information of five criteria. The influence of the five criteria is not clear. In addition, the differences of results between day and night are not discussed. See also general comment 1.
Line 387: Is the cloud class in Figure 9 determined by using an extinction threshold of 20Mm-1, pressure, and temperature?
Figure 11: Is the true AOT shown regardless of clouds? How about true AOT at 670 and 875 nm? Is the wavelength dependency of marine AOT small?
Line 422: The overestimation of AOT is about 0.05. It is possible to detect thin cirrus. Please show the figure of attenuated backscatter and discuss this issue.
Figure 12: Why are the patterns of AOT and quality status different? Is AOT at 355 nm estimated in the region which the values of quality status are -1 and 4?
Line 449: Do you plan to examine the other cases. See general comment 2.
Line 450: Which AOT product is validated?
Line 38: clouds and aerosol layers --> cloud and aerosol layers
Line 158: after the complete ATLID L2a … --> after the ATLID L2a …
Line 193-194: The phrases of criteria 4 and 5 are different from those of criteria 1, 2, and 3. Please rephrase them. For example, the phrase of criterion 4 is written as “Satisfaction of the criterion of the brightness temperature (10.8 mm) difference threshold (Equation 1).”
Line 435: Ångström exponent usually has positive value. Does Ångström exponent in the sentence include minus sign of a negative power?