Roll and cell convection in wintertime arctic cold-air outbreaks

Cold-air outbreaks from the polar ice caps or winterly continents over the open ocean lead to organized convection that typically starts as longitudin al roll patterns and changes to cellular patterns in downstream direction. During the field experiments ARKTIS 1991 and ARKTIS 1993, aircraft missions were conducted in 13 cold-air outbreak events over the Greenland and Baren ts Seas to determine the characteristic parameters of both the mean (primar y) flow and the superimposed organized convection (secondary flow). The mea surements are classified into four categories with respect to the convectiv e pattern form: longitudinal rolls with small and wider horizontal waveleng ths, transitional forms between rolls and cells, and cells. Rolls were observed for boundary layer depths h < 1 km with horizontal wave lengths: lambda < 5 km and aspect ratios lambda/h between 2.6 and 6.5. Dist inct cellular structures occurred for h > 1.4 km with lambda > 8 km and lam bda/h between 4 and 12. The amplitudes of the secondary flow-scale variatio ns of the temperature theta(R), moisture m(R), and the longitudinal, u(R); transversal, v(R), and vertical, w(R), wind components were on the order of 0.1-0.4 K, 0.03-0.30 g kg(-1), 0.6-2.5 m s(-1), 0.8-2.5 m s(-1), and 0.4-1 .8 m s(-1), respectively, generally increasing from the roll to the cell re gion. The same is true for the ratio u(R)/v(R) (from about 0.6 to nearly 1) and for the ratio Lm(R)/c(P)theta(R) (from 0.7 to more than 2), hinting at increasing importance of moisture processes in the cell compared to the ro ll region. The importance of the secondary-flow transports of heat and momentum in rel ation to the total vertical transports increases with height and from rolls to cells. Particularly clear is the vertical profile of the vertical moist ure transport m(R)w(R), which exhibits a maximum around cloud base and is o n the average related to the surface moisture flux as (m(R)w(R))(max) = 0.3 5(m'w')(o). The thermodynamic conditions of the basic flow are characterized by the Ray leigh number Ra, the stability of the capping inversion, and the net conden sation rate in the cloud layer. Here Ra is clearly overcritical in the whol e cold-air outbreak region; it is around 10(5) in the roll region and aroun d 2 x 10(6) in the cell region. The Monin-Obukhov stability parameter does not appear to be suitable measure to distinguish between roll and cell conv ection. The stability above the boundary layer is about two to three times larger for rolls than for cells. The net condensation in clouds is three ti mes larger in cell than in roll regions and the resulting heating of the bo undary layer exceeds that of the surface heat flux in the cell region. The kinematic conditions of the basic flow are characterized by a larger shear of the longitudinal wind component ii in the roil than in the cell region. The curvature of the u profile is mostly overcritical in rolls and always s ubcritical in cells. The secondary flow-scale kinetic energy E-kin,E-R is related to Ra. The bes t least squares fit is given by E-kin,E-R = 3.7Ra(0.4).