Understanding stratospheric water vapor variability and model-simulated rainfall biases from the perspective of convection

Convection and its interactions with transport of heat, water and momentum are essential in understanding many aspects of the climate system, for example, tracer transport in both the troposphere and lower stratosphere, the modelling of rainfall and prediction of climate change. The goal of this dissertation is to examine the role of convection variability in the related atmospheric composition transport and rainfall processes using both observations and climate models.

The relationships between deep convection and the global diabatic heat budget are studied to understand the coupling between convection, temperature and general circulation. By examining the influence of convection on diabatic heating in the troposphere and stratosphere, we can gain a comprehensive understanding of the tropical convection and its influence on large-scale atmospheric circulation and stratosphere composition, especially on the cross-tropospuase transport of water vapor. We show that transient variability in deep convection is highly correlated with diabatic heating throughout the troposphere and stratosphere. Enhanced deep convection is linked to amplified heating in the tropical troposphere and in the mid-latitude storm tracks, tied to latent heat release. Enhanced convection is also linked to radiative cooling in the lower stratosphere, due to weaker upwelling longwave (LW) from lower altitudes. Transient deep convection modulates LW and shortwave (SW) radiation in the troposphere, with compensating effects that are linked to variations in cloud fraction and liquid and ice water content.

Then we explore the the variability of lower stratospheric (LS) water vapor in the Northern Hemisphere (NH) monsoon regions based on satellite observations and trajectory model simulations driven by diabatic heating in reanalyses. The links between stratospheric water vapor, fluctuations in deep convection and large-scale circulation and temperature are quantified. Results suggest that temperature plays a dominant role on water vapor variations with stronger convection leads to cold dehydration temperatures and a relatively dry stratosphere. Besides, the seasonal increase of stratospheric water vapor can be attributed to the geographic variations of convection and resultant variations of the dehydration center, of which the influence is comparable to the influence of the local dehydration temperature increase. Specifically, the seasonal geographic shift of the dehydration center from the east to the west Asian monsoon region with warmer tropopause temperatures could increase water vapor significantly.

Dry biases over Southern Amazonia are observed in Community Atmosphere Model version 5 (CAM5). We use hindcast simulations to track the root causes for the biases. Results suggest that the dry bias is present by day 2 (24 to 48 hours) of model integrations and is very robust for all the seasons with largest bias magnitude during the southern summer (Dec-Feb, wet season). The near-surface-warm biases and low biases of humidity in the lower troposphere that exist since day 2 may be significant factors influencing the dry biases. The low biases of humidity are contributed by both physical components (shallow convective scheme and Zhang-McFarlane convective scheme, and maybe weak turbulence term), and dynamics (weak moisture convergence). We further evaluate the CAM5 with a higher-order turbulence closure scheme, named Cloud Layers Unified By Binomials (CLUBB), and a Multiscale Modeling Framework, referred as the “super-parameterization” (SP) with two different microphysics configurations to investigate their influences on rainfall simulations over Southern Amazonia. The two different microphysics configurations in SP are the one-moment cloud microphysics without aerosol treatment (SP1) and two-moment cloud microphysics coupled with aerosol treatment (SP2). Results show that both SP2 and CLUBB effectively reduce the low biases of rainfall, mainly during the wet season, and reduce low biases of humidity in the lower troposphere with further reduced shallow clouds and increased surface solar flux. These changes increase moist static energy, contribute to stronger convection and more rainfall. SP2 appears to realistically capture the observed increase of relative humidity prior to deep convection and it significantly increases rainfall in the afternoon; CLUBB significantly delays the afternoon peak time and produces more precipitation in the early morning, due to more gradual transition between shallow and deep convection. In CAM5 and CAM5 with CLUBB, occurrence of deep convection appears to be a result of stronger heating rather than higher relative humidity.