Investigations of tropical rainfall biases and climate dynamics in the era of convection-permitting global climate models
Tropical precipitation in modern global climate models (GCMs) still shows large errors in both spatial and temporal patterns, despite notable improvement of GCMs over past decades. As tropical precipitation biases are linked to large scale circulation errors and in turn have global impacts in simulated climate, tropical precipitation bias is one of the primary issues to be tackled to improve the accuracy of GCM output. This dissertation addresses key issues in the underlying model dynamics and physics responsible for tropical precipitation biases in modern, state-of-art GCMs.
The first of the three research projects presented in this dissertation systematically tests the time step sensitivity of a superparameterized (SP) GCM to better understand the physical essence of superparameterized convection and its impact on global climatology. The time step in an SPGCM dictates the frequency of information exchange between two scales (be- tween the GCM and the cloud resolving model (CRM) embedded in each grid cell), and, accordingly, a different time step artificially imposes a constraint on model behavior. In an SP version of the Community Atmosphere Model 3.0, several important sensitivities in the tropical climatology are identified, e.g. a faster scale coupling causes weaker cloud forcing,temperature gradients. These sensitivities are distinct from the time step sensitivities of conventionally parameterized GCMs and have implications for understanding emergent be- haviors of multiscale deep convective organization in superparameterized GCMs.
The second research project examines the impact of superparameterization on the initial growth of the double ITCZ (Intertropical Convergence Zone) bias in a dynamic ocean coupled GCM, with the initial intention to test if superparameterization can be a useful tool to mitigate this long-standing model problem by avoiding convective parameterizations that have been known as one of major double ITCZ bias sources. In an SP version of Community Earth System Model (CESM) 1.1, a fast bias growth, e.g. days for precipitation biases and weeks for sea surface temperature biases (also known as the cold tongue SST bias), is observed and validates our novel approach using short, ensemble hindcast simulations. The SP simulations show weaker double ITCZ biases than non-SP simulations, but this improvement is due to non-physical causes. A key discovery is that the historical neglect of convective momentum transport in SP-CESM causes unrealistically strong zonal wind shear near the surface that effectively suppresses the increase of zonal surface wind stress even when stronger overlying easterlies are present. The results highlight a less-appreciated role of convective momentum transport as a potential double ITCZ bias source, providing a new perspective to ITCZ dynamics and also suggesting future model development strategies.
The third research project focuses on a marquee issue of modern tropical climate dynam- ics by exploring how oceanic circulation response buffers forced ITCZ shift when a top-of- atmosphere solar flux perturbation is present at the different latitudinal bands. Particular attention is paid to heat transport associated with Atlantic meridional overturning circula- tion (AMOC) that, unlike subtropical cell, does not have mechanical constraints coupled to Hadley circulation allowing potentially more efficient ITCZ shift buffering. A set of 200-year simulations using CESM 1.2 shows that the heat transport partitioning becomes more ocean centric as the solar forcing is located at higher latitudes and that such sensitivity is indeed driven by the AMOC responses. On the other hand, a previously identified STC (subtrop- ical cell) damping mechanism does not respond sensitively to the solar forcing locations, as a result of compensating circulation strength and depth responses. The findings demand the inclusion of the previously underappreciated AMOC heat transport to the current ITCZ migration framework to fully capture the atmosphere-ocean coupling. Besides, the results have some practical implication in GCM development strategy suggesting fixing tropical bias would be more effective to alleviate tropical precipitation biases in GCMs than fixing extratropical biases.