Abstract
In this paper, we have investigated the impact of radiation–cloud–convection–circulation interaction (RC3I) on structural changes and variability of the Inter-tropical Convergence Zone (ITCZ) using the Goddard Multi-scale Modeling Framework, where cloud processes are super-parameterized, i.e., explicitly resolved with 2-D cloud resolving models embedded in each coarse grid of the host Goddard Earth Observing System-Version 5 global climate model. Experiments have been conducted under prescribed sea surface temperature conditions for 10 years (2007–2016), with and without cloud radiation feedback in the atmosphere, respectively. Diagnostic analyses separately for January and July show that RC3I leads to an enhanced and expanded Hadley Circulation characterized by (1) a quasi-uniform warming and moistening of the tropical atmosphere and a sharpening of the ITCZ with enhanced deep convection, more intense precipitation and higher clouds, (2) extended drying of the tropical marginal convective zones, and extratropical mid- to lower troposphere, and (3) a cooling of the polar regions, with increased baroclinicity and midlatitude storm track activities. Computations based on the zonal mean thermodynamic energy balance equation show that the radiative warming and cooling are strongly balanced by local adiabatic processes associated with changes in large-scale vertical motions, as well as horizontal atmospheric heat transport. In the tropics, enhanced short-wave absorption and longwave water vapor greenhouse effects by high clouds play key roles in providing strong positive feedback to the tropospheric warming. In the extratropics, increased atmospheric heat transport associated with changes in the Hadley circulation is balanced by strong longwave cooling above, and warming below due to increased high clouds. We also find a strong positive correlation between daily and pentad heavy rain in the ITCZ core, and expansion of the drier zones coupled to a contraction of the highly convective zones in the ITCZ, indicating a strong tendency RC3I-induced convective aggregation in tropical clouds i.e., wet-regions-get-wetter and contracted, and dry-areas-get-drier and expanded.
Highlights
Differential solar and longwave radiative forcing, manifested as surplus in radiant energy in the tropics and deficit in polar regions, is well known to be the fundamental driver of the general circulation and the hydrologic cycle of the earth’s climate system (Lorenz 1967; Wallace and Hobbs 1977)
Using the Goddard Multi-scale Modeling Framework, we have investigated the multi-scale interactions involving radiation, clouds, convection, and circulation (RC3I) in affecting the structure and variability of the Intertropical Convergence Zone (ITCZ)
A near-uniform warmer and moister tropics, with a sharpened ITCZ characterized by increased deep clouds, intensified precipitation, in association with increased ascent and a narrowing of the rising branch of the Hadley Circulation (HC)
Summary
Differential solar and longwave radiative forcing, manifested as surplus in radiant energy in the tropics and deficit in polar regions, is well known to be the fundamental driver of the general circulation and the hydrologic cycle of the earth’s climate system (Lorenz 1967; Wallace and Hobbs 1977). Rising motions in the moist atmosphere generate clouds, which interact with the large-scale circulation via feedback processes involving radiative transfer, phase changes, and convective processes (Stephens and Webster 1979; Stephens 2004). This interaction, hereafter referred to as radiation-clouds-convection-circulation-interaction (RC3I), further alters the heat and water balance, inducing changes. First and foremost, on the global scale, dynamical adjustments stemming from equator-to-pole differential radiative forcing give rise to the development and variability of large-scale atmospheric structures such as the Intertropical Convergence Zone (ITCZ), the Hadley and Walker circulations, mid-latitude storm tracks, and associated regional extreme precipitation. Dynamical adjustments involving the atmosphere, land, ocean, cryosphere, and biosphere can operate on diverse spatial and temporal scales, from diurnal, subseasonal-seasonal, interannual, inter-decadal to climate change (IPCC 2013)
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