Objectives

GRIPS aims to understand how well we can represent the coupled troposphere-middle atmosphere system in middle atmosphere climate models. These models are general circulation models that extend from the surface to the stratosphere or beyond, which attempt to represent all processes that impact the atmospheric circulation and structure. GRIPS has three concurrent 'phases' each of which poses specific questions: Phase 1 asks how well the different models perform; Phase 2 examines some of the model components in detail; Phase 3 compares the response of the models to perturbations in the physical forcing mechanisms (such as volcanic aerosol loading, solar variability, and ozone changes). A very important question, motivating the Phase-3 studies, is how well can models represent climate variability over the past two decades?

Key findings

Many of the activities in Phase 1 of GRIPS are complete or are nearing completion. It was shown that most models have a cold bias at all levels (see Figure) and this has prompted an investigation of the performance of radiation schemes. The models display considerable differences in their ability to simulate the quasi-stationary wave structure in the troposphere and stratosphere. To understand the interannual variability, at least 20 years of integration are required, which is just becoming possible for many of the participating models. Travelling waves in the tropics differ vastly between the models, a result partly due to different representation of convective processes and partly due to model formulation. The models show very different spectra of resolved gravity waves, which have an important impact on what we attempt to parameterise.

Ongoing activities

Phase 2 research is examining the radiation schemes and the representation of gravity wave drag in the models. Off-line comparisons using identical temperature and trace gas fields reveal large differences between the radiation schemes presently used. Tests of gravity wave drag are beginning. Phase 3 studies the impacts of different forcing anomalies. Volcanic aerosols in the tropical lower stratosphere lead to perturbations to the radiative balance and induce a response in the extratropical stratosphere and troposphere in winter, an effect which is captured in some models. Changes in solar incoming radiation on 11-year timescales, coupled with their induced ozone changes, lead to a response in all models tested, but this can differ between the models.

Achievements

Two papers have appeared in the refered literature (Koshyk et al., and Pawson et al.) and several others are planned. Papers have been presented at international conferences. One of the most important achievements of GRIPS has been the success of the annual workshops, which provide a much-needed forum for informal discussion of the latest results and uncertainties of present day models.

Future Plans

The main plan is to complete what we have started: Phase 1 should be wrapped up by 2002 and the scientifically relevant impact studies of Phase 3 should be studied in much more detail. To date, GRIPS has given little attention to aspects of chemistry, which will be an avenue to explore as our understanding of dynamics and physics becomes more thorough.