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Trade Cumulus: Observations and Modeling. Chapter 4 (pp 99-126) in The Physics and Parameterization of Moist Atmospheric Convection

This talk reviews some work on convective boundary layer (CBL) equilibrium and radiative-convective equilibrium in the Tropics. It is based primarily on Betts and Albrecht (1987), Betts and Ridgway (1988, 1989: BR88 and BR89), and on the use of conserved thermodynamic variables, as discussed in several papers (eg. Betts, 1975, 1982a,b, 1984, 1985a,b, 1992).
A convective boundary layer (CBL) of shallow non-precipitating cumulus clouds covers most of the tropical oceans away from the atmospheric convergence zones. It is remarkably uniform with the cloud-base near 950 mb, an inversion above the top of the cloud layer near 800 mb. (eg. Firestone and Albrecht, 1986); and a subcloud layer equivalent potential temperature theta sub e, typically in the range 345-350K. This boundary layer in turn feeds the deep convection in the ascending branches of the Hadley and Walker circulations, so that the height of the tropopause and tropical tropospheric temperature are directly coupled to the thetae of the CBL over the oceans (Sarachik, 1978). This rather stable equilibrium is a result of a subtle balance between the radiation field, the subsidence, the convective fluxes, the cloud field, the surface wind and the sea surface temperature. Many timescales are involved. For this talk I shall address this equilibrium balance on two timescales over the oceans, using a simple energy balance model. We shall explore the dependence of the equilibrium low level thetae, height of cloud-base and CBL top, and the surface fluxes of heat and moisture, on sea surface temperature, surface wind speed, and the moisture above the CBL. The early studies of the Tradewinds emphasized the downstream moistening and rise of the top of the cloud layer on equatorward trajectories towards Hawaii (Riehl et al, 1951; Malkus, 1956). We shall show this is largely an equilibrium response to the rise of sea surface temperature along the trajectory. Over most of the tropical Pacific where the sea surface temperature has weaker gradients, the CBL top is near 800 mb. These early papers used kinematic methods to estimate the mean subsidence in the Trades and found values ≈ 6 mb day-1; much lower than that required for mass balance in the subsiding branch of the Hadley circulation. Neiburger (1960), however, used trajectories and a radiative budget to estimate the subsidence at CBL top to be ≈ 40 mb day-1. Subsequent mass budget studies of the Tradewinds in the Atlantic (Holland and Rasmusson, 1973; Nitta and Esbensen, 1974; Augstein et al, 1973) confirmed that the subsidence in the Trade inversion was of order 40-60 mb day-1. Sarachik, (1978) and Betts and Ridgway (1988) showed that the radiatively driven subsidence in the subsiding branches of the mean tropical circulation was of this same magnitude ≈ 40 mb day-1. BR88 used a coupled radiative-CBL model and observed CBL data over the equatorial Pacific (from Betts and Albrecht, 1987) to show that on climatic timescales, the radiation field was the primary control on the surface fluxes.

This is the published version of a paper presented at the Kloster Seeon workshop Aug 4-16, 1996 in Bavaria, Germany.

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Betts, A.K., 1997: Trade Cumulus: Observations and Modeling. Chapter 4 (pp 99-126) in The Physics and Parameterization of Moist Atmospheric Convection, Ed. R. K. Smith, NATO ASI Series C: Vol. 505, Kluwer Academic Publishers, Dordrecht, 498pp.