Reconciling Ground-Based and Space-Based Estimates of the Frequency of Occurrence and Radiative Effect of Clouds around Darwin, Australia

Abstract

International audienceThe objective of this paper is to investigate whether estimates of the cloud frequency of occurrence and associated cloud radiative forcing as derived from ground-based and satellite active remote sensing and radiative transfer calculations can be reconciled over a well-instrumented active remote sensing site located in Darwin, Australia, despite the very different viewing geometry and instrument characteristics. It is found that the ground-based radar-lidar combination at Darwin does not detect most of the cirrus clouds above 10 km (due to limited lidar detection capability and signal obscuration by low-level clouds) and that the CloudSat radar - Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) combination underreports the hydrometeor frequency of occurrence below 2 km height, due to instrument limitations at these heights. The radiative impact associated with these differences in cloud frequency of occurrence is large on the surface downwelling shortwave fluxes (ground and satellite) and the top-of-atmosphere upwelling shortwave and longwave fluxes (ground). Good agreement is found for other radiative fluxes. Large differences in radiative heating rate as derived from ground and satellite radar-lidar instruments and RT calculations are also found above 10 km (up to 0.35 Kday-1 for the shortwave and 0.8 Kday-1 for the longwave). Given that the ground-based and satellite estimates of cloud frequency of occurrence and radiative impact cannot be fully reconciled over Darwin, caution should be exercised when evaluating the representation of clouds and cloud-radiation interactions in large-scale models and limitations of each set of instrumentation should be considered when interpreting model-observations differences

Characterizing Observed Midtopped Cloud Regimes Associated with Southern Ocean Shortwave Radiation Biases

“…M4 is distinguished from the other midtopped cloud regimes by the high frequency of midlevel cloud above 3 km. The apparent steep reduction of cloud fraction below around 1.50 km for all cloud regimes is consistent with the underestimation of cloud near the surface identified in Protat et al (2014).…”

“…The strong interaction of lidar with liquid water droplets leads to significant attenuation through these clouds; where lidar is extinguished but radar suggests cloud, DARDAR returns the ''unknown'' classification. For this reason we expect the DARDAR classification mask to underestimate the frequency of occurrence of the known cloud phase categories in the lowest part of the profile, specifically below 1.5 km and especially below thick cloud, below liquid water, or in layered scenes (Protat et al 2014). Hence, when identifying cloud frequency of occurrence from the DARDAR cloud classification mask we make a cautious upper estimate of cloud amount by including the unknown category, which is likely to represent cloud where the lidar is extinguished.…”

“…An evaluation of DARDAR against ground-based observations at the Atmospheric Radiation Measurement Program (ARM) station in Darwin have shown that the satellite observations underestimate cloud below 1.50 km (Protat et al 2014). This is a considerable limitation over the Southern Ocean, where low cloud is nearly ubiquitous (e.g., Huang et al 2012b;Haynes et al 2011).…”

Ice crystal number concentration estimates from lidar-radar satellite remote sensing. Part 1: Method and evaluation

“…Battaglia and Delanoë, 2013;Protat et al, 2014;Feofilov et al, 2015;Massie et al, 2016). Its operational retrievals have also been evaluated against products from a similar lidar-radar method (CloudSat 2C-ICE; Deng et al, 2010) and in situ observations.…”

Macrophysical properties of specific cloud types from radiosonde and surface active remote sensing measurements over the ARM Southern Great Plains site

“…However, when one or more of the lower-level cloud types (low, ML, and mid clouds) is/ are present, the occurrence frequencies with high clouds detected are 10% larger for the radiosonde (19%) relative to the ARSCL (9%) results. Therefore, aside from the spatial discrepancy of the two datasets mentioned above, the attenuation effect of the lower-level clouds on the radar-lidar signals may also lead to a detection deficiency in terms of high clouds in the ARSCL data (Protat et al 2014). The occurrence frequencies of overlap between low and HM clouds are 4% for the radiosonde and 2% for the ARSCL data, respectively.…”