Diagnostic subprojects - Overview

Diagnostic sub-projects aim to address some of more focused scientific questions in addition to the broader ones described in the overall CFMIP-1 e.g.:

What is the current spread in cloud feedbacks as determined by simple idealised (SST perturbation) climate change experiments and what are the main components responsible?
What is the degree of the spread in cloud feedback strength in current slab ocean climate models and what are the main contributing factors to that spread?
To what extent are the changes in clouds seen in 2*CO2 slab ocean models consistent with the change in clouds in different dynamical regimes?

As with CMIP, a proposal for a new diagnostic sub-project can be submitted at any time to either of the CFMIP convenors. The proposal need only be a few paragraphs describing what is intended to be done. Feedback on the proposal will be given quickly and will be concerned mostly with potential for overlap with other proposals. After approval the proposal will be posted on the CFMIP web site.

Subprojects using the +/-2K experiments
• Sub-Project 1: Comparison of clouds from the ISCCP simulator with July climatology.
• Sub-Project 2: Changes in clear sky radiation

Subprojects using the slab ocean experiments
• Sub-Project 3: Changes in "ISCCP clouds"
• Sub-Project 4a: Separation of feedbacks
• Sub-Project 4b: Analysis of large scale feedback mechanisms
• Sub-Project 5: Instantaneous CO2 radiative Forcing.
• Sub-Project 6: Changes in Surface Energy Balance
• Sub-Project 7: Relating model metrics to climate sensitivity

Sub-Project 1: Comparison of clouds from the ISCCP simulator with July climatology.

Objectives

This project will require participating models to use the “ISCCP simulator” to produce the “ISCCP clouds” corresponding to a fixed season. The general climatology of the simulated cloud distribution is to be compared with the ISCCP climatology. Also some exploration of the simulated cloud regimes in the fixed season case as compared to the full AMIP2 case would be of interest.

The project will augment the basic analysis of cloud sensitivities with an examination of changes to the ISCCP cloud types, including analysis in terms of dynamical regimes. In particular the project will perform a detailed evaluation of the model's simulation of clouds and their representation in different dynamical regimes for July (the '0K' control experiment) using ISCCP data and reanalyses.

The basic methodology for analysing the ±2K perturbation experiments in terms of climate sensitivity parameters is described in Cess et al (1990). This will require diagnostic package A. The analysis will be extended to consider cloud amounts in the categories defined by ISCCP according to cloud top pressure (CTP) and visible optical depth (tau). CTP-tau histograms of cloud amounts will be examined in different dynamical regimes, defined primarily in terms of mid-tropospheric vertical motion using pressure vertical velocity at 500 hPa (diagnostic package B/C).

Sub-Project Leader:

Mark Ringer (Hadley Centre)

Sub-Project 2: Changes in clear sky radiation.

Objectives

This project should explore the changes in clear sky radiation and its relation with the simulated changes in the temperature and water vapour profiles (diagnostic package D). Of particular interest may be an exploration of the possible different behaviour of the models in very dry and very moist areas.

Additional Data Requirements

Time averaged vertical profiles of air temperature, relative humidity and specific humidity.

Sub-Project Leader

Brian Soden (GFDL) and Laura Fowler (CSU)

Sub-Project 3: Changes in "ISCCP clouds"

Objectives

If different dynamical regimes in the current climate are be taken for proxies for future climate then it is important to relate the cloud changes seen in a climate change experiment to the shifted dynamical regimes. Thus a careful investigation of the changes in “ISCCP clouds” is needed. The changes in “ISCCP clouds” will need to be related to the changes in the cloud radiative forcing in order to explore more fully which aspects of the cloud distribution contribute most to the cloud feedback. Any consistency found between models would provide valuable information to assist in model evaluation studies in simulations of the current climate (diagnostic package E).

Compositing methods will be used to explore the cloud response to a doubling of CO2 in the slab model experiments as diagnosed from the ISCCP simulator. Methods similar to Williams et al (2003) (diagnostic package E) and other compositing studies (e.g. Norris and Weaver, 2001; Bony et al, 1997; Tselioudis et al., 2000; Jakob and Tselioudis, 2003) (diagnostic package F/G) will be used to investigate whether any primary relationships associated with the cloud response can be consistently identified in several models. If any consistency is found, comparison with composited observations would provide valuable information to assist in model evaluation. The changes in "ISCCP clouds" will need to be related to changes in the cloud radiative response in order to explore more fully which aspects of the cloud distribution contribute most to the cloud feedback.

Sub-Project Leader:

Keith Williams (Hadley Centre)

Sub-Project 4a: Separation of feedbacks

Objectives

It is understood that there are limitations in using changes in cloud radiative forcing to estimate cloud feedback (e.g. Zhang et al, 1994; Soden et al, 2004). This subproject uses the partial radiative perturbation (PRP) technique of Manabe and Wetherald (1988) and extended by Colman et al (1997) to calculate the feedback components. This analysis technique can separate out water vapour, lapse rate and albedo feedbacks from cloud feedbacks and allows a more consistent intercomparison of models in a spatial context. It is impractical for the partial derivatives to be calculated centrally, hence participants are encouraged to save the diagnostics listed in diagnostic package I and calculate the partial derivatives 'in house'. A draft document detailing the methodology to do this is available here (Word document).

Sub-Project Leader:

Robert Colman (BMRC)

Sub-Project 4b: Analysis of large sclae feedback mechanisms

Objectives

This is to be achieved by extending the regional Boer & Yu (2003) analysis to include use of the ISCCP simulator diagnostics.

A secondary aim of the sub-project is to assess various simple models which can be used to interpret changes in cloud radiative forcing in terms of the radiative perturbation diagnostics applied in Project4a.

The applicability of another (simpler) method following Gregory et al (2003) will also be tested across the range of models with data obtained from the models during their approach to equilibrium (diagnostic package J).

Sub-Project Leader:

Mark Webb (Hadley Centre)

Sub-Project 5: Instantaneous CO2 radiative Forcing.

Objectives

This sub-project requests modelling groups to diagnose radiative forcing, using the instantaneous change double radiation call approach in the 2xCO2 slab experiment. The forcing should be diagnosed at TOA, surface and 200mb, and split into SW and LW, up and down, clear-sky and all sky net fluxes (diagnostic package K). At a bare minimum at-least-5-year means of the geographical distribution should be provided. This should be linked to the WCRP (GEWEX Radiation Panel) Intercomparison of Radiation codes in Climate Models (ICRCCM).

Participants are also encouraged also to calculate the surface, TOA and 200mb "Hansen"/"relaxed" forcing (double CO2 but keep SST constant; Hansen et al, 1997).

Sub-Project Leader:

Bill Collins (NCAR)

Sub-Project 6: Changes in Surface Energy Balance.

Objectives

The intercomparison paper of Randall et al., 1992 (using additional model output from the Cess et al 1990 experiments) concluded that the major differences in the responses of models in the component of the surface energy budget to a 4K warming were due to:
      Differences in the simulated hydrologic cycles
     Parametrisation of long wave radiation (especially the water vapour continuum)
      Cumulus convection.
In contrast cloud-radiation effects were found to have only a secondary importance.
In both Cess et al 1990 and Randall et al 1992 the conclusions reached are consistent with problems and uncertainties with moist processes.

It was further argued that the wide range of model sensitivities that were found would not be narrowed through simply increasing model resolution but that improvements in the model physics would be needed.

While there have been many improvements to models since 1992, it is not self-evident that the differences in model response at the surface to a 4K SST perturbation have reduced especially since there remain many deficiencies in the representation of the hydrologic cycle in climate models.

Additional Data Required

To be agreed by participants but no doubt water vapour and temperature profiles will be required. Daily data may also be required in order (possibly) to determine the main sources of model differences (diagnostic package L).

Sub-Project Leader:

Volunteer Needed

Sub-Project 7: Relating model metrics to climate sensitivity

Objectives

One of the major shortcomings of all climate prediction assessments has been the inability to objectively and unambiguously relate the errors in model simulations (as judged by comparisons to observations) to uncertainty in climate sensitivity. One of the problems immediately obvious (and why this has not been accomplished to date) is that climate model error metrics vary with a) model variable b) spatial scale c) time scale d) single variable errors versus multiple correlated variable errors. The fundamental nonlinear nature of the climate system guarantees that this function is unlikely to be simple. To date attempts have been ad-hoc and have varied widely depending on physical feedback mechanism, variable, time and space scale.

This sub-project forms part of a more general approach to relate model metrics to climate sensitivity. Initially, the approach will utilise a parameter perturbation ensemble based on one GCM, however a critical extension will be to use the multi-model ensemble of CFMIP. Required diagnostics will be identified as the approach develops, however diagnostic packages E, F and Z are likely to be required.

Sub-Project Leaders:

David Sexton (Hadley Centre)