Reply on RC1

<strong class="journal-contentHeaderColor">Abstract.</strong> Natural cirrus clouds and contrails cover about 30 % of the Earth's mid-latitudes and up to 70 % of its Tropics. Due to their widespread occurrence, cirrus have a considerable impact on the Earth energy budget, which, on average, leads to a warming net radiative effect (solar + thermal-infrared). However, whether the instantaneous radiative effect (RE) of natural cirrus or contrails is positive or negative depends on their microphysical, macrophysical, and optical, as well as radiative properties of the environment. This is further complicated by the fact that the actual ice crystal shape is often unknown and thus ice clouds remain one of the components that are least understood in the Earth's radiative budget. The present study aims to separate the effect on cirrus RE of eight parameters: solar zenith angle, ice water content, ice crystal effective radius, cirrus temperature, surface albedo, surface temperature, liquid water cloud optical thickness of an underlying cloud, and three ice crystal shapes. In total, 94,500 radiative transfer simulations have been performed, spanning the parameter ranges that are typically associated with natural cirrus and contrails. The multi-dimensionality and complexity of the 8-dimension parameter space makes it impractical to discuss all potential configurations in detail. Therefore, specific cases are selected and discussed. The ice crystal effective radius has the largest impact on solar, thermal-infrared (TIR), and net RE. The second most important parameter is the ice water content, which impacts the solar and terrestrial RE equally. Solar and TIR RE have opposite signs, meaning that the ice water content has a relatively small impact on net RE. Beyond the ice crystal effective radius and the ice water content, the solar RE of cirrus is determined by solar zenith angle, surface albedo, liquid cloud optical thickness, and the ice crystal shape in descending priority. RE in the TIR spectrum is dominated by the surface temperature, the ice cloud temperature, the liquid water cloud optical thickness, and the ice crystal shape. Net RE is controlled by the surface albedo, the solar zenith angle, and the surface albedo in decreasing importance. The relative importance of the studied parameters differs depending on the ambient conditions and during nighttime the net RE is equal to the TIR RE. The data set generated in this work is publicly available. It can be used to compute the radiative effect of cirrus clouds, contrails, and contrail cirrus instead of full radiative transfer calculations.

Abstract. Cirrus clouds play an important role in the radiation budget of the Earth; nonetheless, the radiative effect of ultra-thin cirrus clouds in the tropopause region and in the lowermost stratosphere remains poorly constrained. These clouds have a small vertical extent and optical depth and are frequently neither observed even by sensitive sensors nor considered in climate model simulations. In addition, their short-wave (cooling) and long-wave (warming) radiative effects are often in approximate balance, and their net effect strongly depends on the shape and size of the cirrus particles. However, the CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere instrument (CRISTA-2) allows ultra-thin cirrus clouds to be detected. Here we use CRISTA-2 observations in summer 1997 in the Northern Hemisphere midlatitudes together with the Suite Of Community RAdiative Transfer codes based on Edwards and Slingo (SOCRATES) radiative transfer model to calculate the radiative effect of observed ultra-thin cirrus. Using sensitivity simulations with different ice effective particle size and shape, we provide an estimate of the uncertainty in the radiative effect of ultra-thin cirrus in the extratropical lowermost stratosphere and tropopause region during summer and – by extrapolation of the summer results – for winter. Cloud top height and ice water content are based on CRISTA-2 measurements, while the cloud vertical thickness was predefined to be 0.5 or 2 km. Our results indicate that if the ice crystals of these thin cirrus clouds are assumed to be spherical, their net cloud radiative effect is generally positive (warming). In contrast, assuming aggregates or a hexagonal shape, their net radiative effect is generally negative (cooling) during summer months and very likely positive (warming) during winter. The radiative effect is in the order of ±(0.1–0.01) W m−2 for a realistic global cloud coverage of 10 %, similar to the magnitude of the contrail cirrus radiative forcing (of ∼ 0.1 W m−2). The radiative effect is also dependent on the cloud vertical extent and consequently the optically thickness and effective radius of the particle size distribution (e.g. effective radius increase from 5 to 30 µm results in a factor ∼ 6 smaller long- and short-wave effects, respectively). The properties of ultra-thin cirrus clouds in the lowermost stratosphere and tropopause region need to be better observed, and ultra-thin cirrus clouds need to be evaluated in climate model simulations.

Observations of the microphysical and radiative properties of two cirrus clouds are reported. The measurements were taken from the C-130 aircraft of the UK Meteorological Research Flight as part of the field campaign of the International Cirrus Experiment, ICE'89. Cirrus microphysical properties are analysed in terms of ice water content and a suitably defined ice crystal ‘effective radius’. Estimates of the ice water content are derived from both a standard Particle Measuring Systems 2D-C cloud probe and a Lyman-α total water content (TWC) hygrometer. With both flights large discrepancies are seen to occur between the two instruments near the cloud base and top, with the ice water contents derived from the TWC probe being consistently greater than those from the 2D-C probe. Evidence is presented which suggests that the differences near the cloud base are due to a significant number of large (>800 μm) crystals, which the 2D-C probe is unable to sample. Near the cloud top the differences are assumed to be due to small (<100μm) crystals, which the 2D-C probe is again unable to sample correctly. Thus more weight has been given to the TWC probe measurements of ice water content, with the measurements of ice crystal effective radius being adjusted accordingly. Model clouds, constructed from the microphysical measurements, are used as input to a number of radiation parametrizations, both in the long-wave and short-wave regions of the spectrum. In the long wave, model agreement with both narrow-band and broad-band measurements is reasonably good, although discrepancies exist near the cloud tops. Some evidence is presented which suggests that the measured effective radii from one of the flights are too large to explain the radiative characteristics of the cloud. The effect of long-wave scattering is shown to be small. The inferred long-wave broad-band mass-absorption coefficient varies between 0.002 m2g-1 and 0.087 m2g−1, and it shows a marked inverse dependence on the ice crystal effective radius, as anticipated by simple theoretical calculations. In the short wave, agreement between the measured irradiances and those computed using several parametrizations is again reasonably good, although limited by the natural variability of the clouds. The net radiation budgets of both cloud layers are analysed, and the possibility of the existence of a relationship between ice water content and effective radius is considered.

Contrail cirrus are a major component of the climate forcing due to air traffic. For a given contrail cirrus cover, ice water content and ice crystal shape, their impact on radiation is dependent on the number and size of ice crystals. Here we use a global climate model to study the impact of a reduction in initially formed ice crystal numbers, as may be caused by reduced soot number emissions. We find that for reduced initial ice crystal numbers the ice water content is decreased and ice crystal sizes increased, leading to a reduction in contrail cirrus optical depth and doubling the fraction of contrail cirrus that cannot be detected by satellite remote sensing. Contrail cirrus lifetimes and coverage are strongly reduced leading to significant reductions in contrail cirrus radiative forcing. The global climate impact of contrail cirrus is nonlinearly dependent on the reduction in initial ice crystal numbers. A reduction in the initial ice crystal number of 80% leads to a decrease in contrail cirrus radiative forcing by 50%, whereas a twofold reduction leads to a decrease in radiative forcing by approximately 20%. Only a few contrail cirrus outbreaks explain a large percentage of the climate impact. The contrail cirrus climate impact can be effectively mitigated by reducing initial ice crystal concentrations in such outbreak situations. Our results are important for assessments dealing with mitigating the climate impact of aviation and discussions about the use of alternative fuels or lean combustion in aviation.

The radiative energy budget in the Arctic undergoes a rapid transformation compared to global mean changes. Understanding the role of cirrus in this system is vital, as they interact with short- and long-wave radiation and the presence of cirrus can be decisive as to a net gain or loss of radiative energy in the polar atmosphere. In an effort to derive radiative properties of cirrus in a real scenario in this sensitive region, we use in-situ measurements of ice water content (IWC) performed during the POLSTRACC aircraft campaign in the boreal winter and spring 2015/2016 employing the German research aircraft HALO. A large dataset of IWC measurements of mostly thin cirrus at high northern latitudes was collected in the upper troposphere and also frequently in the lowermost stratosphere. From this dataset we selected vertical profiles that sampled the complete vertical extent of cirrus cloud layers. These profiles exhibit a vertical IWC structure that will be shown to control the instantaneous radiative effect both in the long and short wavelength regimes in the polar winter. We perform radiative transfer calculations with the UVSPEC model from the libRadtran program package in a one-dimensional column between the surface and the top of the atmosphere (TOA), taking as input the IWC profiles, as well as the state of the atmospheric column at the time of measurement, as given by weather forecast products. In parameter studies, we vary the surface albedo and solar zenith angle in ranges typical for the Arctic region. We find the strongest (positive) radiative forcing of cirrus over bright snow, whereas the forcing is mostly weaker and even ambiguous over the open ocean in winter and spring. The IWC structure over several kilometres in the vertical affects the irradiance at the TOA through the distribution of optical thickness. A strong heating rate profile within the cloud drives dynamical processes and contributes to the thermal stratification at the tropopause. Our case studies highlight the importance of a detailed resolution of cirrus clouds and consideration of surface albedo for estimations of the radiative energy budget in the Arctic.

The radiative energy budget in the Arctic undergoes a rapid transformation compared to global mean changes. Understanding the role of cirrus in this system is vital, as they interact with short- and long-wave radiation and the presence of cirrus can be decisive as to a net gain or loss of radiative energy in the polar atmosphere. In an effort to derive radiative properties of cirrus in a real scenario in this sensitive region, we use in-situ measurements of ice water content (IWC) performed during the POLSTRACC aircraft campaign in the boreal winter and spring 2015/2016 employing the German research aircraft HALO. A large dataset of IWC measurements of mostly thin cirrus at high northern latitudes was collected in the upper troposphere and also frequently in the lowermost stratosphere. From this dataset we selected vertical profiles that sampled the complete vertical extent of cirrus cloud layers. These profiles exhibit a vertical IWC structure that will be shown to control the instantaneous radiative effect both in the long and short wavelength regimes in the polar winter. We perform radiative transfer calculations with the UVSPEC model from the libRadtran program package in a one-dimensional column between the surface and the top of the atmosphere (TOA), taking as input the IWC profiles, as well as the state of the atmospheric column at the time of measurement, as given by weather forecast products. In parameter studies, we vary the surface albedo and solar zenith angle in ranges typical for the Arctic region. We find the strongest (positive) radiative forcing of cirrus over bright snow, whereas the forcing is mostly weaker and even ambiguous over the open ocean in winter and spring. The IWC structure over several kilometres in the vertical affects the irradiance at the TOA through the distribution of optical thickness. A strong heating rate profile within the cloud drives dynamical processes and contributes to the thermal stratification at the tropopause. Our case studies highlight the importance of a detailed resolution of cirrus clouds and consideration of surface albedo for estimations of the radiative energy budget in the Arctic.

The gas-aerosol composition of the atmosphere determines specific features of the radiative net irradiance in the atmosphere and the associated climatic changes. The study discusses the anthropogenic variations in the content of the most important greenhouse gases, such as carbon dioxide (CO2) and ozone (O3), as well as the atmospheric aerosol. The estimates of their radiative effects (forcing, RF) in various geophysical conditions are given. Based on the modern ECRAD radiation model, the RF values of considered anthropogenic trace gases and aerosols over the past 20 years were estimated for different seasons of the year relative to their preindustrial level for the Moscow region. We determined specific features of the vertical profiles of RF in the shortwave and longwave spectral ranges and their values at the bottom (BOA) and the top of the atmosphere (TOA) depending on solar elevation and surface albedo, both taking seasonal variations into account and without them. The total RF Total for the warm period with a low surface albedo is negative (–8,33 W/m2 at BOA and –1,35 W/m2 at TOA) if seasonal variations of all considering trace gases and aerosols are taken into account. A weakly absorbing aerosol typical of mid-latitudes plays a significant role in the negative RF. The RF Total for the cold season is –0,08 W/m2 at BOA and +0,38 W/m2 at TOA. The positive value of the total RF at the TOA is due to the positive RF of the aerosol with its absorbing properties and the influence of the high albedo of snow surface, as well as due to the positive RF of carbon dioxide. A nonlinear RF dependence of the trace gases and aerosol on the solar elevation and surface albedo was found. The radiative heating rate due to greenhouse gases at TOA is negative in all seasons of the year. At the same time, it is positive at BOA (about 0,45 K/day in the warm season and 0,25 K/day in the cold season), mainly due to the absorption by aerosol in the lower layers of the troposphere. The radiative heating rate due to CO2 is slightly positive.

This study investigates the relationship of cloud properties and radiative effects with air mass origin during the winter (November–February, 2016–2020) at Ny-Ålesund, Svalbard, through a combination of cloud radar, ceilometer, and microwave radiometer measurements. The liquid cloud fraction (CF) was less than 2%, whereas the ice CF predominantly exceeded 10% below 6 km. The liquid water content (LWC) of mixed-phase clouds (LWCmix), which predominantly exist in the boundary layer (CFmix: 10–30%), was approximately four times higher than that of liquid clouds (LWCliq). Warm air mass advection (warmadv) cases were closely linked with strong southerly/southwesterly winds, whereas northerly winds brought cold and dry air masses (coldadv) to the study area. Elevated values of LWC and ice water content (IWC) during warmadv cases can be explained by the presence of mixed-phase clouds in the boundary layer and ice clouds in the middle troposphere. Consistently, the re of ice particles in warmadv cases was approximately 5–10 μm larger than that in coldadv cases at all altitudes. A high CF and cloud water content in warmadv cases contributed to a 33% (69 W m−2) increase in downward longwave (LW) fluxes compared to cloud-free conditions.

Accurate measurements of snowfall in mid- and high-latitudes are particularly important, because snow provides a vital freshwater source, and impacts glacier mass balances as well as surface albedo. However, ice water content (IWC) and snowfall rates (SR) are hard to measure due to their high spatial variability and the remoteness of polar regions.Here, we present novel ice water content - equivalent radar reflectivity (IWC-Ze) and snowfall rate - equivalent radar reflectivity (SR-Ze) relations for 40&amp;#176; slanted and vertically pointing W-band cloud radar. The relations are derived from joint in situ snowfall and remote sensing (radar and radiometer) data from the SAIL site (Colorado, USA) and validated for sites in Hyyti&amp;#228;l&amp;#228; (Finland), Ny-&amp;#197;lesund (Svalbard, Norway), and Eriswil (Switzerland). In addition, gauge measurements from SAIL and Hyyti&amp;#228;l&amp;#228; are used as an independent reference for validation. We show the dependence of IWC-Ze and SR-Ze on riming, which we utilize to reduce the spread in the IWC-Ze and SR-Ze spaces. Normalized root mean square errors (NRMSE) are below 25% for IWC&gt;0.1 gm&amp;#8315;&amp;#179;. For SR, the NRMSE is below 70% over the whole SR range. We also present relations using liquid water path (LWP) as a proxy for the occurrence of riming, which can be applied to both ground-based and space-borne radar-radiometer instruments. The latter is demonstrated using the example of the proposed ESA Earth Explorer 11 candidate mission WIVERN, which consists of a conical scanning 94 GHz radar and a passive 94 GHz radiometer. With this approach, NRMSE are below 75% for IWC&gt;0.1 gm&amp;#8315;&amp;#179; and below 80% for SR&gt;0.2 mmhr&amp;#8315;&amp;#185;.The proposed IWC and SR relations provide a novel way to reduce uncertainties of IWC and SR estimates for W-band radar by accounting for particle riming. Advantages to current literature relations are the flexibility in terms of viewing angle and the inclusion of LWP, allowing the application to ground-based and space-borne radar-radiometer combinations like EarthCARE or the proposed WIVERN mission.

<strong class="journal-contentHeaderColor">Abstract.</strong> Globally emissions from aviation affect the Earth&rsquo;s climate via a complex set of processes. Contrail cirrus and carbon dioxide emissions are the largest factors from aviation in the radiative forcing on climate. Although contrail cirrus enhances the impact of natural clouds on the climate, there are still unresolved questions concerning their characteristics and life cycle. Despite extensive research in studying contrails, significant uncertainties persist. Contrail cirrus encompasses linear contrails and the associated cirrus clouds; these are characterized by ice particle properties, e.g., size, concentration, extinction, ice water content, optical depth, geometrical depth, and cloud coverage. The climate impact of contrails may intensify due to projected increases in air traffic. The radiative forcing from global contrail cirrus has the potential to triple and could reach as much as 160 mW m^&minus;2 by 2050. This projection is based on anticipated growth in air traffic and a potential shift to higher altitudes. The future climate impact of contrail cirrus is influenced by factors like the magnitude and geographical spread in air traffic, advancements in fuel efficiency, the effects from use of alternative fuels, and the effects of the changing climate on the background atmosphere. This study reviews the microphysical processes affecting contrail formation and the aging of contrails and contrail-cirrus. Furthermore, the study explores global observational datasets for contrails, current analyses, and future projections and will aid in evaluating the effectiveness and trade-offs associated with various mitigation strategies. The research highlights gaps in knowledge and uncertainties while outlining research priorities for the future.

The effects of sampling resolution on the surface albedos of dominant land cover types in the North American boreal region

On the relationship between clear-sky planetary and surface albedos: a parameterization for simple energy balance climate models

Radiative transfer simulations are performed to determine how water vapor and nonprecipitating cloud liquid water and ice particles within typical midlatitude atmospheres affect brightness temperatures TB's of moisture sounding channels used in the Advanced Microwave Sounding Unit (AMSU) and AMSU-like instruments. The purpose is to promote a general understanding of passive top-of-atmosphere TB's for window frequencies at 23.8, 89.0, and 157.0 GHz, and water vapor frequencies at 176.31, 180.3 1, and 182.31 GHz by documenting specific examples. This is accomplished through detailed analyses of TB's for idealized atmospheres, mostly representing temperate conditions over land. Cloud effects are considered in terms of five basic properties: droplet size distribution, phase, liquid or ice water content, altitude, and thickness. Effects on TB of changing surface emissivity also are addressed. The brightness temperature contribution functions are presented as an aid to physically interpreting AMSU TB's. Both liquid and ice clouds impact the TB's in a variety of ways. The TB's at 23.8 and 89 GHZ are more strongly affected by altostratus liquid clouds than by cirrus clouds for equivalent water paths. In contrast, channels near 157 and 183 GHz are more strongly affected by ice clouds. Higher clouds have a water impact on 157- and 183-GHz TB's than do lower clouds. Clouds depress TB's of the higher-frequency channels by suppressing, but not necessarily obscuring, radiance contributions from below. Thus, TB's are less closely associated with cloud-top temperatures than are IR radiometric temperatures. Water vapor alone accounts for up to 89% of the total attenuation by a midtropospheric liquid cloud for channels near 183 GHz. The Rayleigh approximation is found to be adequate for typical droplet size distributions; however, Mie scattering effects from liquid droplets become important for droplet size distribution functions with modal radii greater than 20 µm near 157 and 183 GHz, and greater than 30–40 µm at 89 GHz. This is due mainly to the relatively small concentrations of droplets much larger than the mode radius. Orographic clouds and tropical cumuli have been observed to contain droplet size distributions with mode radii in the 30–40-µm range. Thus, as new instruments bridge the gap between microwave and infrared to frequencies even higher than 183 GHz, radiative transfer modelers are cautioned to explicitly address scattering characteristics of such clouds.

Accurately determining and reducing the climate impact of aviation and its uncertainties is one of the pressing challenges of our times. Contrail cirrus are estimated to contribute more than half of the total effective radiative forcing from aviation, yet the uncertainties in their optical and radiative properties are large. In contrast to midlatitude cirrus, high-latitude cirrus are less anthropogenically influenced; thus, they are more pristine. However, little is known about Arctic cirrus properties and their role in the amplified warming of this region. The Cirrus in High Latitudes (CIRRUS-HL) mission using the High Altitude and Long Range Research Aircraft (HALO) provides measurements in mid- and high latitudes during summer (June/July) 2021, exploiting HALO’s capabilities and a comprehensive cloud–aerosol–trace gas and radiation instrumentation. The results of 24 HALO flights provide new insights into both natural cirrus and contrail cirrus properties in high (60°–76°N) and midlatitudes (38°–60°N). In particular, we find lower ice water content (−42%) and lower number concentrations (−88%) of cirrus particles with larger mean diameters (+22%) in high latitudes. Ice supersaturated regions were frequently observed in mid- and high latitudes, with median in-cloud relative humidity over ice between 105% and 122%. Mean aerosol number concentrations in the midlatitudes were reduced by up to 80% compared to pre-COVID-19 times. Less air traffic during the COVID-19 lockdowns, reduced contrail cirrus coverage, and lower ice nucleating particle concentrations in high latitudes help to explain the observed differences in cirrus properties. The extensive dataset will be used to improve weather and climate models. Significance Statement In contrast to Arctic cirrus, midlatitude cirrus are more often modified by human activities, of which air traffic is a significant contributor through the formation of contrails and contrail cirrus. These man-made cirrus warm Earth, but to constrain their effects on climate, in situ and remote sensing measurements were conducted with the German research aircraft High Altitude and Long Range Research Aircraft (HALO). During 24 flights, we used HALO’s exceptional altitude and distance range to sample and contrast different cirrus types from the dense air traffic regions to the remote Arctic regions. We find that microphysical properties of high- and midlatitude cirrus differ substantially, related to their formation pathway, the abundance of air traffic, and the availability of ice nucleating particles. The measurements will help to validate contrail cirrus and climate models.

Algorithm for retrieving surface pressure from hyper-spectral measurements in oxygen A-band