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Regional Climate Change
Regional Climate Change
The Earth’s climate, with its daunting complexity, is undergoing profound anthropogenic changes. The impacts of climate change are known to be regionally dependent, particularly on the hydrological cycle and extremes. My research has looked into regional changes in seasonal hydroclimate, temporal variability and extremes that arise from fundamental (predicted by theory) alterations in atmosphere dynamics over a broad spectrum of spatial and temporal scales.
Deep-tropical contraction and subtropical monsoon shift
Deep-tropical contraction and subtropical monsoon shift
Under anthropogenic warming, deep-tropical ascent of ITCZ is projected to contract equatorward while subtropical descent associated with the Hadley cell edge is predicted to expand poleward. These changes have important implications for regional climate, but their mechanisms are not well understood. We revealed a key role of enhanced equatorial surface warming (EEW) in driving the deep-tropical contraction and modulating the Hadley expansion. By shifting the seasonally warm SST equatorward, EEW reduces the seasonal migration of the ITCZ and causes an annual-mean deep-tropical contraction. This process further contracts the subtropical circulation, as seen during El Niño, and counteracts the Hadley expansion caused by the global-scale warming. The EEW-induced contraction even dominates in the northern hemisphere early summer, leading to a seasonal Hadley contraction that shifts the East Asian subtropical monsoon equatorward.
Zhou, W.*, S-P Xie and D. Yang (2019), Enhanced equatorial warming causes deep-tropical contraction and subtropical monsoon shift. Nature Climate Change, 1-6. [Link]
EEW.pptxTropical temperature profile and extreme convection
Tropical temperature profile and extreme convection
The tropical tropospheric temperature is close to but typically cooler than that of the moist adiabat. This negative temperature deviation, manifested as a C-shape profile, is projected to increase and stretch upward under warming in both comprehensive climate models and idealized radiative-convective equilibrium (RCE, without considering circulation changes). This leads to a larger convective available potential energy (CAPE). Interestingly, the extreme convective updraft velocity intensifies at a much smaller fractional rate. The increasing CAPE has been previously explained by a zero-buoyancy plume model, showing that the enhanced background saturation deficit leads to larger temperature deviation. The model however does not reproduce the C-shape profile and its upward stretch under warming. We have formulated a new conceptual model by replacing the bulk zero-buoyancy plume with a spectrum of entraining plumes that are positively buoyant until their neutral-buoyancy level. This allows the spectral plume model to reproduce all the projected features. It further illustrates that the smaller fractional increase in the extreme convection is due to the upward-stretched deviation, which reduces the ratio between the integrated plume buoyancy and CAPE.
Zhou, W. and S-P Xie (2019), A spectral plume model for understanding tropical temperature profile and convective updraft velocities. Journal of the Atmospheric Sciences, 76, 2801–2814.
Spectral_Plume.pptxTropical Dynamics
Tropical Dynamics
In the tropics, moist convection is organized into various forms, ranging from convective cluster, to synoptic tropical cyclones (TCs), to regional monsoons, and to planetary-scale Intertropical Convergence Zone (ITCZ). I have a general interest in these convectively coupled phenomena, and have investigated some basic yet still challenging questions, with exampled including: Is the spontaneous convective aggregation robust to different models? how the large-scale state influences TC intensity and genesis? what sets the extent and onset of the monsoon? and why there are double-ITCZ bias in the global climate models?
Model sensitivity of convective self-aggregation
Model sensitivity of convective self-aggregation
Convective self-aggregation is studied by performing three-dimensional simulations of non-rotating radiative-convective equilibrium in two cloud-resolving models: the System for Atmospheric Modeling (SAM) and the Weather and Research Forecasting Model (WRF). In contrast to previous SAM studies with a coarse horizontal resolution (3 km), the spontaneous aggregation is not found in the WRF run with the same horizontal resolution (3 km) and the SAM run with a finer horizontal resolution (1 km). In both runs, convective cells are much smaller and no sustainable drier patch is triggered. Such model sensitivity is robust to the variation of physics parameterizations. Self-aggregation does appear in the WRF model by using a much higher SST along with a larger domain size or applying a finite-amplitude perturbation which triggers a initial drier patch.
Zhou, W., Model sensitivity of convective self-aggregation. In Preparation.
SA.pptxDistill TC dynamics in idealized models
Distill TC dynamics in idealized models
TCs are a major threat to coastal populations. One difficulty for studying TCs is that they are often embedded in complex and evolving large-scale state, which prevents a clear view of their interactions with environmental factors. By coupling radiative-convective physics to f-plane dynamics, I developed an idealized model (Rotating Radiative-Convective Equilibrium, RRCE) that simulates a world filled with TCs but without large-scale flows. This provides a basic framework for focusing on TC dynamics and their sensitivity to specific environmental factors. Coupling this basic framework with ocean, I showed that the coupling-induced surface cooling is less effective than conventionally thought in weakening the TC. A stratified atmospheric boundary layer develops and limits the surface influence on the free-tropospheric vortex. By introducing vertical wind shears, I found in another work that the sensitivity of TC genesis to large-scale thermodynamic state is distinct between conditions with and without wind shears, as wind shears profoundly change the dynamic procedure of TC genesis.
Zhou, W., I. M. Held, and S. T. Garner (2014), Parameter study of tropical cyclones in rotating radiative–convective equilibrium with column physics and resolution of a 25-km GCM. Journal of the Atmospheric Sciences, 71, 1058–1069.
Zhou, W., I. M. Held, and S. T. Garner (2017), Tropical cyclones in rotating radiative-convective equilibrium with coupled SST. Journal of the Atmospheric Science, 74 (3), 879-892.
Zhou, W.* (2015), The impact of vertical shear on the sensitivity of tropical cyclogenesis to environmental rotation and thermodynamic state. Journal of Advances in Modeling Earth Systems, 7 (4), 1872-1884. [link]
TC_dynamics.pptxConceptual understanding of monsoon development
Conceptual understanding of monsoon development
A large proportion of the world's population depends on the seasonal rainfall delivered by monsoons. What controls the extent and onset of the monsoon however remains an open question. I studied how the observed monsoon extent emerges from individual contributing factors such as seasonal insolation, land-sea geometry and soil moisture, by adding the ingredients one by one into an idealized model. For an idealized monsoon with zonally-uniform land-sea geometry, its inland extent is analytically predictable, lagging the seasonal insolation peak by an intrinsic timescale that arises from the energetic capacity of the land and atmosphere. The monsoon extent is further reduced, by a zonally-confined continent which intrigues low-energy air advections from the subtropical ocean, and by a dry land which requires time to be saturated for supporting evaporation. Besides the large-scale dynamics, I am also interested in small-scale thermodynamic processes involved in the monsoon onset. In Particular, I found that the abrupt monsoon onset in the northwest Pacific is related to a rapid regime transition in the cloud buoyancy before and after the breakdown of capping inversion barrier
Zhou, W.* and S-P Xie (2018), A hierarchy of idealized monsoons in an intermediate GCM. Journal of Climate, 31 (22), 9021-9036. [link]Zhou, W.*, S-P Xie, and Z-Q Zhou (2016), Slow preconditioning for the abrupt convective jump over the northwest Pacific during summer. Journal of Climate, 29 (22), 8103-8113. [link]
Monsoons.pptxRemote effect from extratropical land on double-ITCZ bias
Remote effect from extratropical land on double-ITCZ bias
Global climate models (GCMs) have long suffered from biases of excessive tropical precipitation in the South Hemisphere (SH). The severity of the double Intertropical Convergence Zone (ITCZ) bias, defined here as the interhemispheric difference in zonal-mean tropical precipitation, varies strongly among models in the Coupled Model Intercomparison Project Phase 5. Models with a more severe double-ITCZ bias feature warmer tropical SST in the SH, coupled with weaker southeast trades. While previous studies focus on coupled ocean-atmosphere interactions, here we show that the intermodel spread in the tropical double-ITCZ bias is remotely related to land surface temperature biases, which can be further traced back to those in the Atmosphere Model Intercomparison Project (AMIP) simulations. By perturbing land temperature in models, we demonstrate that cooler land can lead to a more severe double-ITCZ bias by inducing the above coupled SST-trade wind pattern in the tropics. The responses can be theoretically explained from both the dynamic and energetic perspectives.
Zhou, W. and S-P Xie (2017), Intermodel spread of the double‐ITCZ bias in coupled GCMs tied to land surface temperature in AMIP GCMs. Geophysical Research Letters, 44 (15), 7975-7984.
double_itcz_land.pptxGoogle Sites
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