Archive | 2021
Sir John Houghton (30 December 1931 — 15 April 2020) and radiation transfer
Abstract
<p>IPCC announced that the WGI contribution to AR6 will be dedicated to the memory of leading climate scientist Sir John Houghton. Sir John died of complications from COVID-19 one year ago. He helped creating the IPCC in 1988, and served as Chair and Co-Chair of WGI from 1988 to 2002. In this presentation we focus on two aspects of his work: radiation transfer and cloud radiative forcing. — His book “The Physics of Atmospheres” (third edition, 2002) says: “The equation of radiative transfer through the slab, which includes both absorption and emission, is sometimes known as Schwarzschild’s equation” (Eq. 2.3, p.11). Introducing a constant Ф net flux (Eq. 2.5) being equal to the outgoing radiation, the black-body function B of the atmosphere is given as a function of Ф and the optical depth as B = Ф(χ* + 1)/2π (Eq. 2.12). He says, “it is easy to show that there must be a temperature discontinuity at the lower boundary”: B<sub>g</sub> – B<sub>0</sub> = Ф/2π (Eq. 2.13). Fig. 2.4 displays the net flux at the boundary as half of the outgoing radiation, independently of the optical depth. He notes: “Such a steep lapse rate will soon be destroyed by the process of convection”, and continues: “Combining (2.12) and (2.13) we find Bg = Ф(χ* + 2)/2π ” (Eq. 2.15, section 2.5 The greenhouse effect). We controlled Eq. (2.13) on 20 years of clear-sky CERES EBAF Ed4.1 global mean data and found it satisfied with a difference of -2.28 Wm<sup>-2</sup>. The validity of this equation casts constraint on the surface net radiation and on the corresponding non-radiative fluxes in the hydrological cycle by connecting them unequivocally to half of the outgoing longwave radiation. We constructed the all-sky version of the equation by separating atmospheric radiation transfer from longwave cloud effect, and found it valid within 2.84 Wm<sup>-2</sup>. We computed Eq. (2.15) with a special optical depth of χ* = 2 for clear-sky; it is justified with a difference of -2.88 Wm<sup>-2</sup>. We also created its all-sky version; the difference is 2.46 Wm<sup>-2</sup>. Altogether, the four equations are satisfied on 20-yr of CERES data with a mean bias of 0.035 Wm<sup>-2</sup>. We show that the four equations together determine a clear-sky and an all-sky greenhouse factor as 1/3 and 0.4. Data from Wild et al. (2018) and IPCC AR5 (2013) show g(clear) = (398 – 267)/398 = 0.33 and g(all) = (398 – 239)/398 = 0.3995. The IPCC reports predict an enhanced greenhouse effect from human emissions. According to the above arithmetic solutions, Earth’s observed greenhouse factors are equal to the theoretical ones without any deviation or enhancement. — The first IPCC report states that cloud radiative forcing is governed by cloud properties as cloud amount, reflectivity, vertical distribution and optical depth. Here we show that the TOA net CRF (= SWCRF + LWCRF) in equilibrium is equivalent to TOA net clear-sky imbalance, hence to determine its magnitude only clear-sky fluxes are needed.</p>