Constraining the Twomey effect from satellite observations: issues and perspectives by Quaas, J, Antti, A, Cairns, B, Christensen, M, Deneke, H, Ekman, AML, Feingold, G, Fridlind, A, Gryspeerdt, E, Hasekamp, O, Li, Z, Lipponen, A, Ma, P-L, Muelmenstaedt, J, Nenes, A, Penner, J, Rosenfeld, R, Schroedner, R, Sinclair, K, Sourdeval, O, Stier, P, Tesche, M, Dieedenhoven, B, Wendisch, M