Does the boundary layer drive Hadley cell dynamics?

Abstract:

The Hadley cell organizes moist convection at large scales, latent heating in turn affects the lapse rate and ultimately governs vertical velocities in downdrafts. It is undisputed that Hadley cell dynamics is fundamental to moist convection and cloud formation in the Tropics. Traditional Hadley cell theory derives from consideration of balanced dynamics in the upper troposphere. We investigate the possible role of ageostrophic boundary layer mass transport – Ekman pumping – and thermodynamic throttling of vertical velocities – the omega governor – on Hadley cell dynamics. Significantly, the resulting scaling is independent of the Rossby number. We test this scaling against idealized GCM simulations and explore the relevance to three problems. First, a longstanding problem is how Hadley cell widths and strengths scale with planetary parameters. A closely related problem is that Hadley cell widths and strengths have a much weaker scaling with planetary rotation than expected from angular-momentum-conserving theory. A third problem is that simulated Hadley cells closely follow single-power-law scalings despite large changes in the Rossby number.