Validation of the Cloud_CCI cloud products in the Arctic
Abstract. The role of clouds in the Arctic radiation budget is not well understood. Ground-based and airborne measurements provide valuable data to test and improve our understanding. However, the ground-based measurements are intrinsically sparse, and the airborne observations are snapshots in time and space. Passive remote sensing measurements from satellite sensors offer high spatial coverage and an evolving time series, having lengths potentially of decades. However, detecting clouds by passive satellite remote sensing sensors is challenging over the Arctic because of the brightness of snow and ice in the ultraviolet and visible spectral regions, and because of the small brightness temperature contrast to the surface. Consequently, the quality of the resulting cloud data products needs to be assessed quantitatively. In this study, we validate the cloud data products retrieved from the Advanced Very High Resolution Radiometer (AVHRR) post meridiem (PM) data from the polar-orbiting NOAA-19 satellite and compare them with those derived from the ground-based instruments during the sunlit months. The AVHRR cloud data products by the European Space Agency’s (ESA) Cloud Climate Change Initiative (Cloud_CCI) project, which uses the observations in the visible and IR bands to determine cloud properties. The ground-based measurements from four high-latitude sites have been selected for this investigation: Hyytiälä (61.84° N, 24.29° E), North Slope of Alaska (NSA, 71.32° N, 156.61° W), Ny-Ålesund (Ny-Å, 78.92° N, 11.93° E), and Summit (72.59° N, 38.42° W). The Liquid Water Path (LWP) ground-based data are retrieved from microwave radiometers, while the Cloud Top Height (CTH) has been determined from the integrated lidar-radar measurements. The quality of the satellite products, Cloud Mask and Cloud Optical Depth (COD), have been assessed using data from NSA, whereas LWP and CTH have been investigated over Hyytiälä, NSA, Ny-Å, and Summit.
The Cloud_CCI COD results for liquid water clouds are in better agreement with the NSA radiometer data than those for ice clouds. For liquid water clouds, the Cloud_CCI COD is underestimated roughly by 2.8 Optical Depth (OD) units. When ice clouds are included, the underestimation increases to about 4.6 OD units. The Cloud_CCI LWP is overestimated over Hyytiälä by ≈ 7 gm−2, over NSA by ≈ 16 gm−2, and over Ny-Å by ≈ 24 gm−2. Over Summit, CCI LWP is overestimated for values ≤ 20 gm−2 and underestimated for values > 20 gm−2. Overall the results of the CCI LWP retrievals are within the ground-based instrument uncertainties. For CTH retrievals, the Cloud_CCI product overestimates single-layer clouds. To understand the effects of multi layer clouds on the CTH retrievals, the statistics are compared between the single layer clouds and all types (single + multi layer). When the multi layer clouds are included (i.e., all types), the observed CTH overestimation become underestimations of about 360–420 m. The CTH results over Summit station showed the highest biases compared to the other three sites. To understand the scale-dependent differences between the satellite and ground-based data, the Bland-Altman method is applied. This method does not identify any scale-dependent differences for all the selected cloud parameters except for the retrievals over the Summit station. In summary, the Cloud_CCI cloud data products investigated agree reasonably well with those retrieved from ground-based measurements, made at the four high-latitude sites.
Kameswara S. Vinjamuri et al.
Status: final response (author comments only)
- RC1: 'Comment on amt-2022-312', Anonymous Referee #2, 20 Feb 2023
- RC2: 'Comment on amt-2022-312', Abhay Devasthale, 20 Feb 2023
Kameswara S. Vinjamuri et al.
Kameswara S. Vinjamuri et al.
Viewed (geographical distribution)
Review of the manuscript 'Validation of the Cloud_CCI cloud products in the Arctic' by Vinjamuri et al.
The manuscript describes a validation of the ESA Cloud CCI cloud products for four sites in the Arctic. Being the first of its kind the manuscript includes novel results worthy publication.
The manuscript is well-organised, well-written and include detailed descriptions and discussions of the results. It is recommended for publication after consideration of the minor comments below.
Line 60: The term ``reasonable agreement'' does not contain much useful information. Please quantify this ``reasonable agreement''.
Line 62: Please give a number for the high viewing angles for which cloud detection problems happen.
Lines 118-130: Here the COD product, among others, is described. Although mentioned later, please state that the COD refers to the SW (visible) COD.
Line 157: Change ``from an MWR'' to ``from MWR''.
Lines 171-188: Somewhere in this paragraph mention whether the CTH refers to height above sea level or height above ground level.
Lines 272-274: It is claimed that Fig. 3 ``... shows better agreement ...''. Most of the points in the scatter plot is lumped together for COD smaller than about 40. This collection of points hardly shows better agreement as claimed. To better show the behaviour of the points for COD<40 maybe try using log scale or add a zoom-in-plot for this region.
Line 276: Please state that these underestimates are biases.
Lines 288-289: For the COD all decimal values from Table 1 were included when reporting biases (line 276). For the LWP the decimal values are omitted compared to Table 2. Is there a reason for this? If not please be consistent. In any case, 15.6 for NSA rounds up to 16 and not down to 15.
Lines 315-317: Please clarify this sentence.
Lines 351-383: Based on your findings: do you find the Cloud_CCI cloud products to be of such a quality to be useful in studies of the Arctic Amplification? If not, why not? And how can it be improved to be useful?
General comment: This study assumes that there are no trends in the cloud products due to instrument drifts or other changes. May you please comment on this? Also, have the temporal behaviour in the differences between the ground-based and Cloud_CCI products been looked at? This may be done by for example plotting the differences between ground-based and Cloud_CCI cloud products as a function of time.