Content area
Full text
1. Introduction
Accurate estimation of cloud cover over the ocean is critically important for many applications. Clouds largely determine shortwave and longwave radiation fluxes over the ocean, and the cloud fraction is a key parameter of many parameterizations of radiative fluxes (Zillman 1972; Reed 1977; Dobson and Smith 1988; Malevsky et al. 1992; Gulev 1995; Josey et al. 2003; Fairall et al. 2008; Dupont and Haeffelin 2008; Kalisch and Macke 2008, 2012; Hanschmann et al. 2012; Bumke et al. 2014). Clouds play an important role in regulating key climate feedbacks, representing one of the most effectively used parameters for diagnosing the climate sensitivity (Curry et al. 1996; Bony et al. 2004, 2006, 2015; Fasullo and Trenberth 2012; Taylor et al. 2013; Bellomo et al. 2014). Clouds determine the earth’s albedo (Stephens et al. 2015), as well as the occurrence of dimming and brightening periods (Wild et al. 2007; Dallafior et al. 2015). Variability in cloud cover over the oceans is important to enable a better understanding of the mechanisms underlying leading climate modes, such as El Niño, monsoons, ITCZ shifts, midlatitudinal oscillations (i.e., the NAO, PNA pattern, and PDO), and Arctic climate changes, all of which have cloud cover signatures (Lietzke et al. 2001; Bedacht et al. 2007; Previdi and Veron 2007; Kay and Gettelman 2009; Chiacchio and Wild 2010; Chiacchio et al. 2010; Mendoza et al. 2014; Parding et al. 2014; Radley et al. 2014; Schwartz et al. 2014; Chernokulsky et al. 2017). Analysis of cloud cover over the oceans may also help us to better quantify and understand the mechanisms behind regional air-sea interactions in the western boundary currents (Minobe et al. 2008; Hand et al. 2014) and the upwelling regions (e.g., Hernández et al. 2012).
All mentioned processes are associated not only with the changes in the mean cloud amount (the measure that is widely used), but also with changes in the probability distributions of cloud cover. In the midlatitudes, the same monthly mean can originate from very different cloud regimes: it can be a continuous period of broken clouds (40%-60% cloud fraction) or short, alternating periods of clear skies and overcast. In this case, monthly mean (which can be similar in both cases) will tell a little about the changes and...