Interactions among deep convection, sea surface temperature and radiation in the Asian monsoon region

The climatic interactions among deep convection, sea surface temperature and radiation in the Asian monsoon region have been examined using various satellite-derived data sets of the period 1983-90. Annual average Frequency of Deep Convection (FDC) is maximum over the equatorial east Indian ocean and adjoining west Pacific and Indonesian region. Maximum FDC zone shifts to Bay of Bengal during the monsoon (June-September) season. There is weak relationship between the variations in FDC and SST in the Indian ocean. Deep convective activity was suppressed over most of the tropical Indian ocean during El Nino of 1987 in spite of warmer SSTs. The pattern of inter-annual variation between FDC and SST behaves differently in the Indian ocean basin as compared to the Pacific ocean basin. Deep convective clouds interact with radiation very effectively in the Asian monsoon region to cause large net negative cloud radiative forcing. Variation in FDC explains more than 70% of the variation in surface short-wave cloud radiative forcing (SWCRF) and long wave cloud radiative forcing (LWCRF) in the atmosphere. On inter-annual scale, warmer SSTs may not necessarily increase deep convection in the Indian ocean. However, the inter-annual variation of deep convective clouds influences significantly the radiative budget of the surface-atmosphere system in the Asian monsoon region. The satellite observations suggest that warmer SSTs in the Indian ocean might have resulted from an increase in the absorbed solar radiation at the surface due to a reduction in deep convective cloud cover.

The Arctic Temperature Response to Global and Regional Anthropogenic Sulfate Aerosols

The mechanisms behind Arctic warming and associated climate changes are difficult to discern. Also, the complex local processes and feedbacks like aerosol-cloud-climate interactions are yet to be quantified. Here, using the Community Earth System Model (CAM5) experiments, with emission enhancement of anthropogenic sulfate 1) five-fold globally, 2) ten-times over Asia, and 3) ten-times over Europe we show that regional emissions of sulfate aerosols alter seasonal warming over the Arctic, i.e., colder summer and warmer winter. European emissions play a dominant role in cooling during the summer season (0.7 K), while Asian emissions dominate the warming during the winter season (maximum ∼0.6 K) in the Arctic surface. The cooling/warming is associated with a negative/positive cloud radiative forcing. During the summer season increase in low–mid level clouds, induced by sulfate emissions, favours the solar dimming effect that reduces the downwelling radiation to the surface and thus leads to surface cooling. Warmer winters are associated with enhanced high-level clouds that induce a positive radiative forcing at the top of the atmosphere. This study points to the importance of international strategies being implemented to control sulfate emissions to combat air pollution. Such strategies will also affect the Arctic cooling/warming associated with a cloud radiative forcing caused by sulfate emission change.

Atmospheric ionization and cloud radiative forcing

Atmospheric ionization produced by cosmic rays has been suspected to influence aerosols and clouds, but its actual importance has been questioned. If changes in atmospheric ionization have a substantial impact on clouds, one would expect to observe significant responses in Earth’s energy budget. Here it is shown that the average of the five strongest week-long decreases in atmospheric ionization coincides with changes in the average net radiative balance of 1.7 W/m$$^2$$ 2 (median value: 1.2 W/m$$^2$$ 2 ) using CERES satellite observations. Simultaneous satellite observations of clouds show that these variations are mainly caused by changes in the short-wave radiation of low liquid clouds along with small changes in the long-wave radiation, and are almost exclusively located over the pristine areas of the oceans. These observed radiation and cloud changes are consistent with a link in which atmospheric ionization modulates aerosol's formation and growth, which survive to cloud condensation nuclei and ultimately affect cloud formation and thereby temporarily the radiative balance of Earth.

Radiative energy budget and cloud radiative forcing in the daytime marginal sea ice zone during Arctic spring and summer

Airborne measurements of the surface radiative energy budget (REB) collected in the area of the marginal sea ice zone (MIZ) close to Svalbard (Norway) during two campaigns conducted in early spring and and early summer are presented. From the data, the cloud radiative forcing was derived. The analysis is focussed on the impact of changing atmospheric thermodynamic conditions on the REB and on the linkage of sea ice properties and cloud radiative forcing (CRF). The observed two-mode longwave net irradiance frequency distributions above sea ice are compared with measurements from previous studies. The transition of both states (cloudy and cloud-free) from winter towards summer and the associated broadening of the modes is discussed as a function of the seasonal thermodynamic profiles and the surface type. The influence of cold air outbreaks (CAO) and warm air intrusions on the REB is illustrated for several case studies, whereby the source and sink terms of REB in the evolving CAO boundary layer are quantified. Furthermore, the role of thermodynamic profiles and the vertical location of clouds during on-ice flow is illustrated. The sea ice concentration was identified as the main driver of the shortwave cooling by the clouds. The longwave warming of clouds, estimated to about 75 W m−2, seems to be representative for this region, as compared to other studies. Simplified radiative transfer simulations of the frequently observed low-level boundary layer clouds and average thermodynamic profiles represent the observed radiative quantities fairly well. The simulations illustrate the delicate interplay of surface and cloud properties that modify the REB and CRF, and the challenges in quantifying trends in the Arctic REB induced by potential changes of the cloud optical thickness.

Airborne observations of surface cloud radiative forcing over different surface types of the Arctic Ocean during late summer/early autumn

<p>Clouds can cause a significant change to the radiative energy budget of the Earth's surface compared to clear-sky conditions, which is referred to as surface cloud radiative forcing (CRF). The CRF in the Arctic strongly depends on the surface properties (absorbing open ocean vs. strongly reflecting sea ice) and is affected by the low or even absent sun and the special thermodynamic conditions. Therefore, in contrast to the mid and low latitudes, in the Arctic, clouds mostly warm the surface on annual average. However, the CRF will change as the sea ice retreats in a warming climate, which might be accelerated due to the enhanced warming of the Arctic, known as Arctic Amplification. Thus, to quantify the contrast of the CRF over sea ice-covered and sea ice-free ocean surfaces, several airborne campaigns have been conducted in the vicinity of Svalbard in the recent years. The measurements of cloud macrophysical and microphysical properties as well as radiative and turbulent fluxes cover different seasons (spring to early autumn).</p><p>Airborne broadband radiation measurements under all-sky conditions were used to calculate the surface CRF during low-level flight sections. In this study, observations from the concurrent campaigns Multidisciplinary drifting Observatory for the Study of Arctic Climate – Airborne observations in the Central Arctic (MOSAiC-ACA) and MOSAiC-Icebird, conducted in August/September 2020, are presented. First results of the CRF over open ocean and the marginal sea ice zone (MIZ) of late summer/early autumn conditions are assessed and compared to the previous airborne spring and early summer campaigns to analyse the seasonal variability of the CRF.</p>

Sir John Houghton (30 December 1931 — 15 April 2020) and radiation transfer

<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>

The sensitivity of cloud radiative forcing with respect to the macrophysical properties and organization of the trade-wind cloud field during EUREC4A

<p>The clouds in the Atlantic trade-wind region are known to have an important role in the global climate system, but the interactions between the microphysical, macrophysical and radiative properties of these clouds are complex. This work seeks to understand how the macrophysical properties and organization of the cloud field impact the large-scale cloud radiative forcing in order to provide the necessary information for the evaluation of the representation of these clouds in models. During the 2020 EUREC<sup>4</sup>A campaign, the German HALO aircraft was equipped for the first time with two instruments - the BACARDI instrument, a broadband radiometer that encompasses a set of pyrgeometers and pyranometers to measure the upward and downward solar and terrestrial radiation at flight level, and the VELOX Thermal IR imager. Simultaneously, one-minute resolution observations of the flight domain were obtained by the GOES-E satellite, thus providing information about the properties of the clouds on a spatial scale compatible with the large footprint of the BACARDI instrument. Using the products of these three instruments, we observe how the changing cloud field (e.g. cloud fraction, mean liquid water path (LWP), cloud top height, degree of clustering) in the EUREC<sup>4</sup>A domain impacts the radiation measured at flight level. We see that although cloud fraction plays a significant role as expected, it is not sufficient to parameterize the cloud radiative effects. Furthermore, the results indicate that the general organization of the cloud field as well as other properties describing the cloud population are necessary, but their relative importance varies between different cloud scenes.</p>