Solar Radiation and Climate Gordon Research Conference, Colby Sawyer College, New London, New Hampshire, USA. 13-18th July 2003.
V. Ramanathan talked on "Aerosols, Radiation and Climate: Regional and Global Perspectives", focusing on the INDOEX experiment and the previous debate on the apparent anomalous solar absorption in cloudy atmospheres. the influence of the surface radiation budget on the hydrological cycle was emphasised and the apparent underestimation of atmospheric solar absorption by climate models (should the surface absorb about 43% of the incoming solar?). The effects of the shift in black carbon from the extratropics to the tropics may have influence the hydrological cycle.
This talk formed the Cruickshank memorial lecture and this honour made up for a category of disasters that preceded the talk including a lost bag at New Delhi, damaged overheads in a hotel fire and a dead laptop.
Susan Soloman talked on the future IPCC 2007 report which will be a leaner and meaner edition. The big message that still needs to be conveyed to the public and policy makers is that even with stabilisation of emissions, there will still be a huge warming before stabilisation of the climate system and the resulting sea level rises are even more lagged. Alan Robock asked whether the radiative forcings diagram could be redrawn to denote what part of the pre-industrial to today forcings has yet to be realised.
Radiative Interactions in the Climate System
Q. Fu compared current radiative transfer codes and noted the need for lab. measurements of continuum absorption at about 1200 cm-1 as well as in the solar spectrum as well as more general experiments for the entire range of atmospheric conditions.
Robert Pincus talked on radiative transfer in large scale models. He argued that 1D radiative transfer in 3D clouds was "good enough" although you need to compute a PDF of sub-grid cloud structure. One novel approach was to produce Monte Carlo simulations where random sub-grid columns and wavelengths are chosen so as to reduce computation ("unbiased noise is good").
Steve Klein talked on the evaluating the treatment of cloud physical processes in climate models including discussion of the new GFDL model which is looking very good (NB however, Brian Soden noted that with very small changes in parametrizations and small changes in mean climate, the new model produced huge ranges of climate sensitivity). Steve Klein prefers better parametrizations to better tuning. The details of mass flux schemes, turbulence above strato-cumulus and its relation to radiative LW cooling, and the return of statistical cloud schemes to account for sub-grid processes.
CLOUD PROCESSES IN THE CLIMATE SYSTEM
Chris Bretherton gave an excellent review talk on Boundary Layer Clouds and their relation to static stability. He favoured the approach of Klein and Hartmann in looking at the environmental conditions that give rise to types of cloud and seeing how these conditions are constrained by the climate system. The diurnal cycle of strato cumulus (due to solar absorption) was discussed and the effects of early morning drizzle, gravity wave driven cloud top thickening and the effect of aerosol in reducing drizzle due to the increase in CCN and reduced effective radius (thereby a big potential sensitivity to aerosol). Leo Donner used longwave cloud forcing as a proxy for high cloud and discussed Convection, High Cloud and Climate. The microphysics of the cell appear crucial.
RADIATIVE FORCING OF CLIMATE CHANGE
Olivier Boucher talked on the radiative forcing of aerosol species and argued that account of scene dependent surface albedo was required to account for spectral changes due to absorption and scattering. The position of the aerosol above or below cloud is also crucial and cited work by Ben Johnson of Reading University (who obviously found a new career following 100m). Aerosol at the edges of cloud may also be important (e.g. clear but very high RH). (NB Ramaswamy asked whether the absorption by the myriads of trace organic compunds could be important - he obviously does)
Ulrike Lohmann talked as if to a junior school class on
"Modelling of the Indirect Effects of Aerosols". The vertical velocity is important in modelling cloud drops and therefore we need sub-gridscale omega. When the indirect effect of aerosols were included in ECHAM4, a southward shift in the ITCZ resulted due to the asymmetric hemispheric forcing of aerosols.
Bill Collins discussed the synthesis of models and observations to look at aerosols in the present day climate. Estimates of black and organic carbon should be refined. The presence of cloud and interaction with the hydrological cycle were discussed although Alan Betts decided that all conclusions with regard to the hydrological cycle were invalid as a fixed SST model had been used.
Jo Haigh talked about the difficulties of measuring total solar irradiance (accuracy a few Wm-2) although the variations over a solar cycle are known to be approximately 0.3 Wm-2. Longer term trends are not well known. Changes in UV are proportionally much larger than the total solar irradiance and therefore exert a significant impact on Ozone and therefore climate. Changes in solar irradiance appear to produce a distinct effect on the large-scale dynamics through weakening the jets.
Alan Robock gave an extremely entertaining talk on "Volcano's and climate" although it could equally have been called "The effect of Volcanic Eruptions on Frankenstein and my first Tenure Track Position...". The summer cooling and (limited) winter warming following explosive eruptions were discussed. Again the dynamical response to volcanic forcing is important. A very interesting point is that atmospheric CO2 reduces following eruptions because trees prefer the resulting higher diffuse:direct solar radiation ratio. Jim Haywood wondered if interannual changes in aerosol may have an impact on the carbon cycle in this way or whether this was a "hair brained scheme" (this soon became the conference catch phrase).
PALEOCLIMATE CHANGES
Otto-Bliesner talked on "Milankovitch Orbital Variations of Insolation and Interglacial Climates". Large feedbacks are required to amplify the small solar forcing over the Milankovitch cycles over which the simple climate sensitivity-radiative forcing relationship surely breaks down. Models don't capture all the feedbacks implicitly shown in the observing system.
Amy Clement discussed "The Link Between Solar Forcings and Climate on Millennial to Orbital Timescales". ENSO is linked to the seasonal cycle which is linked to orbital cycles over thousands of years. therefore ENSO may be modulated by orbital changes.T. Crowley talked on the "Role of Solar Variability in Decadal/ Millennial Scale Climate Change".
RECENT CLIMATE VARIATIONS AND CHANGE
Dian Seidel (nee Gaffen) talked on "Upper-Air Temperature Changes in Satellite and Radiosonde Datasets". The alternative MSU record of upper tropospheric temperature from Meares et al shows smaller changes in lapse rate more consistent with climate models than the Christy dataset. This is mainly due to differences in the inter-calibration of NOAA-9. The influence of the 1977 "regime-shift" on the radiosonde trends is profound with little change in lapse rate before of after this time.
Brian Soden talked on work with the GFDL model to try and understand the Wielicki et al. trends in the tropical radiation budget. There is independent evidence for changes in tropical cloud from ISCCP and surface cloud obs which point to a reduction in high cloud cover. Also, using surface rain gauges there appears to be an increase in heaviest rainfall events which ties in with an increase in the hydrological cycle. Brian asked, "how much do you have to strengthen the tropical circulation to reproduce the observed changes in OLR?". To do this he used the Clemment and Seager method of reducing ocean heat transport. However, you have to vastly increase the circulation to reproduce the observed changes in OLR. Another possibility is that model microphysics is inaccurate. For example, in the GFDL model, the precipitation efficiency of 0.979 is so sensitive that three sig. fig.s are required for tuning purposes. Observed values tend to be more like 0.6 although this is a bit of an apples and Oranges comparison. Brian also argued that a model with different precip. efficiency could produce the IRIS effect ("Dennis Baiting"). He also simulated TOVS radiances within the GFDL model to emulate the changes in equator crossing time in the satellite data.
CLIMATE FEEDBACKS AND SENSITIVITY
Alex Hall gave a talk on the surface albedo feedback. A northern hemisphere decrease in surface albedo of about 1-2% per K warming was mainly due to changes in snow cover. Can we see this in the satellite data? There is a decrease in snow area from 79-98 but clouds cloud the picture.
Sandrine Bony talked about her work teasing out information Tropical Cloud Feedback by separating changes in cloud or cloud forcing into a dynamic, thermodynamic and co-variation components. Using a +2K GCM experiments, the changes in LW cloud effects were dominated by the thermodynamic component with all 3 models producing a high sensitivity in subsidence regimes. The environmental conditions that control cloud type should be well understood before you can isolate the intrinsic sensitivity of clouds to Ts.
Dennis Hartmann talked on "Tropical Cloud, Water Vapour and Circulation Feedbacks". the near zero net cloud forcing in the tropics represents an ensemble of cloud types (NB also the net forcing at the surface and in the atmosphere are large and the changes over the diurnal cycle large also). He showed some interesting results from a simple conceptual model. Hartmann then went about killing off the IRIS hypotheses and described his FAT hypothesis (Fixed Anvil Temperature) which is based on the balance between maximum fall off in radiative longwave cooling and the latent heat release. Most of the anvil occurs at a lower altitude to the the minimum in clear-sky cooling.
Ben Santer and Peter Stott described all their Nature/Science papers on Detection Attribution although this was directly after the final Lobster meal with wine so was quite hard going.
Overall this was an excellent meeting although a feeling of helplessness in the face of overwhelming observational uncertainty tended to surface on a number of occasions. The format of the meeting was excellent: Almost all participants stayed the entire duration and discussion was encouraged not only by the length of time allocated after each talk, but also by the informal nature of the conference compounded by the provision of plenty of recreation time. Follow this link for details of the previous GRC meeting on Solar Radiation and Climate. By the way the dullest part of the meeting involved a 45 minute discussion on whether to change the name of the meeting to from Solar radiation and climate to Radiation and Climate. This was eventually agreed upon but only after the chair realised we were eating into our Lobster meal time.