Representing land-ocean heterogeneity via convective adjustment timescale

Representing land-ocean heterogeneity via convective
adjustment timescale
Bidyut Goswami¹ , Andrea Polesello¹ , Caroline Muller¹ .
¹Department of Earth Science, Institute of Science and Technology Austria, Klosterneuburg, Austria
January 2024

Abstract

The time needed by deep convection to bring the atmosphere back to equilibrium
is called convective adjustment timescale or simply adjustment timescale, typically
denoted by τ . In the Community Atmospheric Model version 6 (CAM6), convection
is parameterized through the Zhang-McFarlan scheme [1], where CAPE undergoes
an exponential consumption, of which τ is the time constant. τ is a tunable pa-
rameter in CAM6 and it has a default value of 1 hour, worldwide, on both ocean
and land. Albeit, there is no justified reason why one adjustment timescale value
should work over land and ocean both. Continental and oceanic convection is dif-
ferent in terms of the vigor of updraft and hence can have different durations.[2, 3]
So it is logical to investigate the prescription of two different convective adjustment
timescales for land (τ_L ) and ocean (τ_L ). To understand the impact of representing
land-ocean heterogeneity via τ , we investigated CAM climate simulations for two
different convective adjustment timescales for land and ocean in contrast to having
one value globally.
Following a comparative analysis of 5-year-long climate simulations, we find
τ_O =4hr and τ_L =1hr to yield the best results. In particular, we obtain a better
description of the Madden-Julian Oscillation (MJO). Although these τ values were
chosen empirically and require further tuning, the conclusion of our finding remains
the same, which is, to use two different τ values for land and ocean.
References
[1] G. Zhang and N. A. McFarlane, “Sensitivity of climate simulations to the parameterization of
cumulus convection in the canadian climate centre general circulation model,” Atmosphere-
Ocean, vol. 33, no. 3, pp. 407–446, 1995.
[2] C. Lucas, E. J. Zipser, and M. A. Lemone, “Vertical Velocity in Oceanic Convection off
Tropical Australia,” Journal of the Atmospheric Sciences, vol. 51, pp. 3183–3193, 11 1994.
[3] R. Roca, T. Fiolleau, and D. Bouniol, “A Simple Model of the Life Cycle of Mesoscale
Convective Systems Cloud Shield in the Tropics,” Journal of Climate, vol. 30, pp. 4283–
4298, 6 2017.