Also, Why RF is likely 4.1 W/m2 and not 3.7 W/m2
In the previous post, Mimicking Myhre, I shared how I used the Column Radiation Model (CRM) to mimic ( if not replicate ) the Myhre et. al. calculations for Radiative Forcing (RF) which has been the standard reference for the IPCC. The concept of RF, as defined by the IPCC, is
“Radiative forcing is the change in the net, downward minus upward, radiative flux ( expressed in W/m^2 ) at the tropopause or top of the atmosphere due to a change in an external driver of climate change, such as, for example, a change in the concentration of carbon dioxide… The traditional radiative forcing is computed with all tropospheric properties held fixed at their unperturbed values, and after allowing for stratospheric temperatures, if perturbed, to readjust to radiative-dynamical equilibrium. …”
Discussions of atmospheric profiles to use in related papers discussed whether a single global average sounding was sufficient or whether the three regional average soundings of the TREX experiment were necessary. The conclusion was that the three soundings were necessary but the global set of soundings ( from 2.5 to 10 degrees resolution ) were not. Since the time these papers were authored, archived atmospheric data of 1 degree resolution ( which is 6.25 to 100 times as fine as those mentioned from the early 1990s ) is now publicly available. Because I was curious about how RF varied spatially and temporally, I decided to apply the CRM to the global CFS one degree analysis fields for given times.
CFS Analysis Data
The Climate Forecast System (CFS) analysis fields ( the zero hour forecast ) provide objective analysis for most of the input fields necessary for executing the CRM. Importantly, that includes cloud amount and cloud liquid water content. By contrast, the clouds in the TREX soundings were somewhat arbitrarily prescribed. By examining the difference between the net radiance for this atmosphere at preindustrial CO2 ( 279 ppm ) and twice that, we can assess the RF across the globe and over time. For all these results, the stratosphere is cooled by recursively applying the imposed cooling rate until the cooling rate is near zero ( this is the so called ‘adjusted’ forcing ).
Seasonal Variation of RF
Seasonal Variation of Global Average RF
Diurnal Variation of RF
Diurnal Variation of Global Average RF
Meridional Variation of RF
Vertical Variation of RF
- Figure 2 does display a remarkably consistency of global average RF.
- The analysis for the CFS data does indicate a value of RF around 10% higher (4.1 W/m^2) than the nominal value (3.7 W/m^2).
- The difference is not large ( around 10% ), but it appears that the average of RF around the globe is NOT the same as the RF of the average profile, or even three average profiles.
- Both the spatial and temporal variations appear to reflect the temperature of the atmosphere. Warmer profiles, with larger radiance, incur larger reductions in radiance by CO2 doubling. Cooler profiles, with smaller radiance, incur smaller reductions in radiance by CO2 doubling.
The exact evaluation of RF is probably not that significant because RF represents a ‘What If’ hypothetical. That is ‘What if the CO2 doubled while the atmosphere was held fixed?’. The atmosphere does not stand still, of course, so RF will forever remain likely but hypothetical. In the next post, I’ll show some results of further ‘What If’ variations of RF and and important vertical aspect.