A simulator for the CLARA-A2 cloud climate data record and its application to assess EC-Earth polar cloudiness
Authors: Eliasson Salomon , Karlsson Karl-Göran , Willén Ulrika 
Published: January 2020

Abstract. This paper describes a new satellite simulator for the CLARA-A2
climate data record (CDR). This simulator takes into account the variable skill in cloud
detection in the CLARA-A2 CDR by using a different approach to other
similar satellite simulators to emulate the ability to detect
clouds. In particular, the paper describes three methods to filter out
clouds from climate models undetectable by observations. The first
method is comparable to the current simulators in the Cloud Feedback
Model Intercomparison Project (CFMIP) Observation Simulator Package
(COSP), since it relies on a single visible cloud optical depth at
550 nm (τc) threshold applied globally to delineate
cloudy and cloud-free conditions. Methods two and three apply
long/lat-gridded values separated by daytime and nighttime
conditions. Method two uses gridded varying τc as opposed to
method one, which uses just a τc threshold, and method three
uses a cloud probability of detection (POD) depending on the model
τc. The gridded POD values are from the CLARA-A2 validation
study by Karlsson and Håkansson (2018). Methods two and three replicate the relative ease or difficulty for
cloud retrievals depending on the region and illumination. They
increase the cloud sensitivity where the cloud retrievals are
relatively straightforward, such as over midlatitude oceans, and
they decrease the sensitivity where cloud retrievals are notoriously
tricky, such as where thick clouds may be inseparable from cold
snow-covered surfaces, as well as in areas with an abundance of
broken and small-scale cumulus clouds such as the atmospheric
subsidence regions over the ocean. The simulator, together with the International Satellite Cloud
Climatology Project (ISCCP) simulator of the COSP, is used to assess
Arctic clouds in the EC-Earth climate model compared to the CLARA-A2
and ISCCP H-Series (ISCCP-H) CDRs. Compared to CLARA-A2, EC-Earth
generally underestimates cloudiness in the Arctic. However, compared
to ISCCP and its simulator, the opposite conclusion is
reached. Based on EC-Earth, this paper shows that the simulated
cloud mask of CLARA-A2, using method three, is more representative of
the CDR than method one used for the ISCCP simulator. The simulator substantially improves the simulation of the CLARA-A2-detected clouds, especially in the polar regions, by accounting for
the variable cloud detection skill over the year. The approach to
cloud simulation based on the POD of clouds depending on their
τc, location, and illumination is the preferred one as it
reduces cloudiness over a range of cloud optical depths. Climate
model comparisons with satellite-derived information can be
significantly improved by this approach, mainly by reducing the risk
of misinterpreting problems with satellite retrievals as cloudiness
features. Since previous studies found that the CLARA-A2 CDR
performs well in the Arctic during the summer months, and that method
three is more representative than method one, the conclusion is that
EC-Earth likely underestimates clouds in the Arctic summer.