Revealing the widespread potential of forests to increase low level cloud cover

Forests play a key role in humanity’s current challenge to mitigate climate change thanks to their capacity to sequester carbon. Preserving and expanding forest cover is considered essential to enhance this carbon sink. However, changing the forest cover can further affect the climate system through biophysical effects. One such effect that is seldom studied is how afforestation can alter the cloud regime, which can potentially have repercussions on the hydrological cycle, the surface radiation budget and on planetary albedo itself. Here we provide a global scale assessment of this effect derived from satellite remote sensing observations. We show that for 67% of sampled areas across the world, afforestation would increase low level cloud cover, which should have a cooling effect on the planet. We further reveal a dependency of this effect on forest type, notably in Europe where needleleaf forests generate more clouds than broadleaf forests.

ESD Ideas: A Weak Positive Feedback Between Sea Level and the Planetary Albedo

Since the planetary albedo of Earth above ocean is typically lower than above land surface, increasing sea-level reduces the planetary albedo. This causes a feedback that is very weak on the global scale, but significantly positive. Its amplitude can be assumed to be higher locally and to grow with the considered time scale.

Libera – Observing and Understanding Earth’s Energy Budget

<p>The recently selected NASA mission Libera, named for the daughter of Ceres in Roman mythology, will provide continuity of the Clouds and the Earth’s Radiant Energy System (CERES) Earth radiation budget (ERB) observations from space.</p><p>Seamless extension of the ERB climate data record is achieved by acquiring integrated radiances over the CERES FM6-heritage broad spectral bands in the shortwave (0.3 to 5 μm), longwave (5 to 50 μm) and total (0.3 to beyond 100 μm). To gain deeper insight into shortwave energy deposition, Libera adds a split-shortwave band (0.7 to 5 μm) that allows to provide deeper insight into shortwave energy deposition.</p><p>Libera’s advanced detector technologies is based on vertically aligned black-carbon nanotubes with closed-loop electrical substitution radiometry to achieve radiometric uncertainty of approximately 0.2%. Additionally, a wide field-of-view camera is employed to provide scene context and explore pathways for separating future ERB missions from complex imagers.</p><p>This presentation will summarize Libera’s attributes and mission goals, as well as some of the applications of the camera radiances, and the role of the additional split-shortwave channel that splits the shortwave band into its visible and near-IR contributions. This split is vital for the better understanding of shortwave absorption, feedbacks, and planetary albedo variability. The hemispheric symmetry of planetary albedo, as observed by CERES, is not achieved by most state-of-the-art climate models and is associated with long-standing biases in circulation and cloud properties. We will exemplify the study of processes relevant to albedo symmetry by means of CMIP6 simulations that provide the visible and near-IR fluxes.</p>

Cloudiness and Earth's hemispheric albedo symmetry

<p>Radiation measurements at the top of the atmosphere show that the two hemispheres of Earth reflect the same amount of shortwave radiation in the long time average (so-called hemispheric albedo symmetry). Here we try to find the origin of this symmetry by analyzing radiation data directly, as well as cloud properties. The radiation data, while being mostly noise, hint that a hemispheric communication mechanism is likely but do not provide enough information to identify it. Cloud properties allow us to define an effective cloud albedo field, much more useful than the commonly used cloud area fraction. Based on that we first show that extra cloud albedo of the SH exactly compensates the extra surface albedo of the NH. We then identify that this this compensation comes almost exclusively from the storm tracks of the extratropics. We close discussing the importance of approaching planetary albedo as a whole and open questions that remain.</p>

Scattering ratio profiles retrieved from ALADIN/Aeolus and CALIOP/CALIPSO lidar observations: instantaneous overlaps, statistical comparison, and sensitivity to high clouds

<p>Clouds and aerosols play an important role in the Earth’s energy budget through a complex interaction with solar, atmospheric, and terrestrial radiation, and air humidity. Optically thick clouds efficiently reflect the incoming solar radiation and, globally, clouds are responsible for about two thirds of the planetary albedo. Thin cirrus trap the outgoing longwave radiation and keep the planet warm. Aerosols scatter or absorb sunlight depending on their size and shape and interact with clouds in various ways.</p><p>Due to the importance of clouds and aerosols for the Earth’s energy budget, global satellite observations of their properties are essential for climate studies, for constraining climate models, and for evaluating cloud parameterizations. Active sounding from space by lidars and radars is advantageous since it provides the vertically resolved information. This has been proven by CALIOP lidar which has been observing the Earth’s atmosphere since 2006. Another instrument of this kind, CATS lidar on-board ISS provided measurements for over 33 months starting from the beginning of 2015. The ALADIN lidar on-board ADM/Aeolus has been measuring horizontal winds and aerosols/clouds since August 2018. More lidars are planned – in 2022, the ATLID/EarthCare lidar will be launched and other space-borne lidars are in the development phase.</p><p>In this work, we compare the scattering ratio products retrieved from ALADIN and CALIOP observations. The former is aimed at 35 deg from nadir, it measures the atmospheric backscatter at 355nm from nadir, is capable of separating the molecular and particular components (HSRL), and provides the profiles with a vertical resolution of ~1km up to 20km altitude.  The latter, operating at 532nm is aimed at 3 deg from nadir and measures the total backscatter up to 40 km. Its natural vertical resolution is higher than that of ALADIN, but the scattering ratio product used in the comparison is provided at ~0.5km vertical grid.</p><p>We have performed a search of nearly simultaneous common volume observations of atmosphere by these two instruments for the period from 28/06/2019 through 31/12/2019 and analyzed the collocated data. We present the zonal averages of scattering ratios as well as the instantaneous profile comparisons and the statistical analysis of cloud detection, cloud height agreement, and temporal evolution of these characteristics.</p><p>The preliminary conclusion, which can be drawn from this analysis, is that the general agreement of scattering ratio profiles retrieved from ALADIN and CALIOP observations is good up to 6-7 km height whereas in the higher atmospheric layers ALADIN is less sensitive to clouds than the CALIOP. This lack of sensitivity might be compensated by further averaging of the input signals and/or by an updating of the retrieval algorithms using the collocated observations dataset provided in the present work.</p>

Aerosols enhance cloud lifetime and brightness along the stratus-to-cumulus transition

Anthropogenic aerosols are hypothesized to enhance planetary albedo and offset some of the warming due to the buildup of greenhouse gases in Earth’s atmosphere. Aerosols can enhance the coverage, reflectance, and lifetime of warm low-level clouds. However, the relationship between cloud lifetime and aerosol concentration has been challenging to measure from polar orbiting satellites. We estimate two timescales relating to the formation and persistence of low-level clouds over1○×1○spatial domains using multiple years of geostationary satellite observations provided by the Clouds and Earth’s Radiant Energy System (CERES) Synoptic (SYN) product. Lagrangian trajectories spanning several days along the classic stratus-to-cumulus transition zone are stratified by aerosol optical depth and meteorology. Clouds forming in relatively polluted trajectories tend to have lighter precipitation rates, longer average lifetime, and higher cloud albedo and cloud fraction compared with unpolluted trajectories. While liquid water path differences are found to be negligible, we find direct evidence of increased planetary albedo primarily through increased drop concentration (Nd) and cloud fraction, with the caveat that the aerosol influence on cloud fraction is positive only for stable atmospheric conditions. While the increase in cloud fraction can be large typically in the beginning of trajectories, the Twomey effect accounts for the bulk (roughly 3/4) of the total aerosol indirect radiative forcing estimate.

Global and Regional Entropy Production by Radiation Estimated from Satellite Observations

Vertical profiles of shortwave and longwave irradiances computed with satellite-derived cloud properties and temperature and humidity profiles from reanalysis are used to estimate entropy production. Entropy production by shortwave radiation is computed by the absorbed irradiance within layers in the atmosphere and by the surface divided by their temperatures. Similarly, entropy production by longwave radiation is computed by emitted irradiance to space from layers in the atmosphere and surface divided by their temperatures. Global annual mean entropy production by shortwave absorption and longwave emission to space are, respectively, 0.852 and 0.928 W m−2 K−1. With a steady-state assumption, entropy production by irreversible processes within the Earth system is estimated to be 0.076 W m−2 K−1 and by nonradiative irreversible processes to be 0.049 W m−2 K−1. Both global annual mean entropy productions by shortwave absorption and longwave emission to space increase with increasing shortwave absorption (i.e., with decreasing the planetary albedo). The increase of entropy production by shortwave absorption is, however, larger than the increase of entropy production by longwave emission to space. The result implies that global annual mean entropy production by irreversible processes decreases with increasing shortwave absorption. Input and output temperatures derived by dividing the absorbed shortwave irradiance and emitted longwave irradiance to space by respective entropy production are, respectively, 282 and 259 K, which give the Carnot efficiency of the Earth system of 8.5%.