UTLS - Science - Modeling

Multi Scale Modeling of the
UTLS Region at NCAR

Several efforts are currently underway on modeling the UTLS region at various scales. These efforts focus on a variety of topics currently being conducted in various divisions of NCAR

UTLS chemistry

Understanding changing composition in the UTLS (Upper Troposphere/Lower Stratosphere) region at mid-latitudes is important for understanding changing climate. Perturbations to the distributions of trace gases such as O3, H2O, and aerosols in this region can lead to direct forcing on climate. Indirect effects through, for example, changing cirrus following new particle production or contrail formation from aircraft emissions can also impact the radiative balance in this region. In turn, climate change, through changing temperatures and transport patterns, has the potential to effect the chemical composition of the mid-latitude UTLS and thus the composition of the troposphere and stratosphere. Transport of ozone from the stratosphere to the troposphere may change in response to ozone recovery and greenhouse gas impacts in the stratosphere.

Researchers in the UTLS initiative are investigating various aspects of UTLS composition and chemistry using various scales of models. Chemical transport models using off-line meteorology are being used to compare to observations, and estimate global fluxes of chemical species between the stratosphere and troposphere.

Detailed cloud resolving models are being used to investigate transport of chemical species into the UTLS and their evolution.

Ice cloud microphysics

Ice cloud microphysics and nucleation is the least understood component of representing microphysics in global or regional models, and this makes work in the UTLS critical for understanding the global cloud properties, and potential changes to them (see below). Researchers in MMM and CGD are working together on detailed microphysics models and paramterizations of ice cloud microphysics for global models. Observations and theory show for example that ice clouds do not homogeneously form at 100% relative humidity over ice, and that supersaturation with respect to ice is frequently observed in the UTLS region.

Aerosol Indirect Effects

Clouds in the upper troposphere are critical for (1) earth's radiation balance and (2) understanding transport into the stratosphere. The formation, evolution and lifetime of these clouds may be strongly dependent on the aerosol disitrubiton and particulatrly the aerosols which are cloud condensation nuclei (CCN) and the subset of these CCN particles which effectively nucleate ice (Ice Nuclei or IN). These nuclei may be natural or anthropogenic, and anthropogenic nuclei are changing dramatically in many parts of the world, potentially affecting clouds, the radiative balance and even precipitation in regions with high aerosol concentrations (China, India, Eastern US, Europe) or downstream of these regions (Pacific & Atlantic Oceans).

Members of the UTLS initiative are conducting several investigations of the observed affects of aerosols on clouds using in-situ and remote sensing observations. Other groups within the initiative are developing better theory of cloud particle nucleation, particularly for ice. These observations and analysis are being combined with multi-scale models from the microscale to climate scale to simulate these effects.

The NCAR global climate model currently does not have aerosols interact with cloud particles. A new version of the microphysics for the global model that explicitly treats aerosol nucleation of cloud particles is being developed in a unique collaboration between CGD, MMM, ACD and RAL researchers.

Upper Troposphere Water Vapor Feedbacks

Upper tropospheric humidity is a critical part of the water vapor feedback, which may double the effect of the direct radiative forcing from anthropogenic greenhouse gases. The response of clouds and humidity in the UTLS to changes in radiative forcing is critical for understanding the response of the climate system.

Using remote sensing data and the NCAR Community Atmosphere Model, researchers in the climate and global dynamics division are exploring observations of upper tropospheric humidity and its variation, and comparing many scales of observed variability (daily-> seasonal in time and from 100km -> global in space) to climate simulations with CAM. The goal is to better constrain the climate sensitivity of the model, focusing on those processes in the UTLS which impact climate sensitivity.

South Asian Monsoon impacts on the UTLS

The South Asian Summer Monsoon dominates boreal summer (June-September) circulation and transport in the UTLS. Representing the transport in this region in summer in models is critical for properly understanding stratosphere-troposphere exchange, particularly for short lived tropospheric species that might be rapidly transported into the UTLS during deep monsoonal convection. This circulation also dominates simulated fluxes of water vapor into the stratosphere in global chemical transport simulations.

ACD researchers are using observations and global models to better understand the dynamics and transport in this critical region.