The article describes a new practical model for the infrared absorption of chlorofluorocarbons and other gases with dense spectra, based on HITRAN absorption cross-sections. The model is very simple, consisting of frequency-dependent polynomial coefficients describing the pressure and temperature dependence of absorption. Currently it is implemented for the halocarbon species required by the Radiative Forcing Model Intercomparison Project (RFMIP). This approach offers practical advantages compared to previously available options, and is traceable, since the polynomial coefficients follow directly from the laboratory spectra. The model is applied to the problem of computing instantaneous clear-sky halocarbon radiative efficiencies and present day radiative forcing. Results are in reasonable agreement with earlier assessments that were carried out with the less explicit Pinnock method, and thus broadly validate that method. Overall, halocarbons are responsible for a substantial share of the present-day forcing, 0.573 Watt/squaremeter (instantaneous clear-sky at the TOA), corresponding to approximately 20% of the total anthropogenic forcing, or 44% compared to anthropogenic CO2 forcing alone.

Recent advances in geostationary imaging have enabled the derivation of high spatiotemporal-resolution cloud-motion winds for the study of mesoscale unsteady flows. Due to the general absence of ground truth, the quality assessment of satellite winds is challenging. In the current limited practice, straightforward plausibility checks on the smoothness of the retrieved wind field or tests on aggregated trends such as the mean velocity components are applied for quality control. In this paper, we demonstrate additional diagnostic tools based on feature extraction from the retrieved velocity field. Lagrangian Coherent Structures (LCS), such as vortices and transport barriers, guide and constrain the emergence of cloud patterns. Evaluating the alignment of the extracted LCS with the observed cloud patterns can potentially serve as a test of the retrieved wind field to adequately explain the time-dependent dynamics. We discuss the suitability and expressiveness of direct, geometry-based, texture-based, and feature-based flow visualization methods for the quality assessment of high spatiotemporal-resolution winds through the real-world example of an atmospheric Kármán vortex street and its laboratory archetype, the 2D cylinder flow.

El Nino Southern Oscillation (ENSO) is one of the most important climate variabilities on an inter-annual time-scale. We aim to find out whether ENSO frequency will change in a changing climate. We analyse two ensembles of General Circulation Models that participated in the Coupled Model Intercomparison Project Phase 6 (CMIP6) and an initial-conditions ensemble of the MPI-ESM-LR model. We identify the uncertainty caused by natural variability by comparing 120-year time-series of the pre-industrial control and the 1-percent CO2 simulations for the CMIP6 ensembles. We found that the multi-member mean for all ensembles predicts almost no change in ENSO frequency, but the uncertainties are large, and that most of the inter-member variability can be attributed to natural variability. This means that the impact of inter-model differences might have been overstated in previous studies. This makes it impossible to make reliable predictions of changes in ENSO frequency based on 120-year simulations.

We quantify the temperature-dependence of the clear-sky climate sensitivity in a one-dimensional radiative-convective equilibrium model. The atmosphere is adjusted to fixed surface temperatures between 280 and 320 K while preserving other boundary conditions in particular the relative humidity and the CO2 concentration. We show that an out-of-bounds usage of the radiation scheme RRTMG can lead to an erroneous decrease of the feedback parameter and an associated “bump” in climate sensitivity as found in other modelling studies. Using a line-by-line radiative transfer model, we find an almost constant longwave radiative feedback at surface temperatures above 300 K. However, the line-by-line simulations also show a slight decrease in climate sensitivity when surface temperatures exceed 310 K. This decrease is caused by water-vapor masking the radiative forcing at the flanks of the CO2 absorption band, which reduces the total radiative forcing by about 18%.

Vortex streets formed in the stratocumulus-capped wake of mountainous islands are the atmospheric analogues of the classic Kármán vortex street observed in laboratory flows past bluff bodies. The quantitative analysis of these mesoscale unsteady atmospheric flows has been hampered by the lack of satellite wind retrievals of sufficiently high spatial and temporal resolution. Taking advantage of the cutting-edge Advanced Baseline Imager, we derived km-scale cloud-motion winds at 5-minute frequency for a vortex street in the lee of Guadalupe Island imaged by Geostationary Operational Environmental Satellite-16. Combined with Moderate Resolution Imaging Spectroradiometer data, the geostationary imagery also provided accurate stereo cloud-top heights. The time series of geostationary winds, supplemented with snapshots of ocean surface winds from the Advanced Scatterometer, allowed us to capture the wake oscillations and measure vortex shedding dynamics. The retrievals revealed a markedly asymmetric vortex decay, with cyclonic eddies having larger peak vorticities than anticyclonic eddies at the same downstream location. Drawing on the vast knowledge accumulated about laboratory bluff body flows, we argue that the asymmetric island wake arises due to the combined effects of Earth’s rotation and Guadalupe’s non-axisymmetric shape resembling an inclined flat plate at low angle of attack. The asymmetric vortex decay implies a three-dimensional wake structure, where centrifugal or elliptical instabilities selectively destabilize anticyclonic eddies by introducing edge-mode or core-mode vertical perturbations to the clockwise-rotating vortex tubes.

The science guiding the \EURECA campaign and its measurements are presented. \EURECA comprised roughly five weeks of measurements in the downstream winter trades of the North Atlantic — eastward and south-eastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, \EURECA marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or, or the life-cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso (200 km) and larger (500 km) scales, roughly four hundred hours of flight time by four heavily instrumented research aircraft, four global-ocean class research vessels, an advanced ground-based cloud observatory, a flotilla of autonomous or tethered measurement devices operating in the upper ocean (nearly 10000 profiles), lower atmosphere (continuous profiling), and along the air-sea interface, a network of water stable isotopologue measurements, complemented by special programmes of satellite remote sensing and modeling with a new generation of weather/climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that \EURECA explored — from Brazil Ring Current Eddies to turbulence induced clustering of cloud droplets and its influence on warm-rain formation — are presented along with an overview \EURECA’s outreach activities, environmental impact, and guidelines for scientific practice.

We quantify the temperature-dependence of clear-sky radiative feedbacks in a tropical radiative-convective equilibrium model. The longwave radiative fluxes are computed using a line-by-line radiative transfer model to ensure accuracy in very warm and moist climates. The one-dimensional model is tuned to surface temperatures between 285 and 313 K by modifying a surface enthalpy sink, which does not directly interfere with radiative fluxes in the atmosphere. The total climate feedback increases from -1.7 to -0.8 Wm^-2K^-1 for surface temperatures up to 305 K due to a strengthening of the water-vapor feedback. The temperature-dependence maximizes at surface temperatures around 297 K, which is close to the present-day tropical mean temperature. At surface temperatures above 305 K, the atmosphere becomes fully opaque and the radiative feedback is almost constant. This near-constancy is in agreement with a theoretical model of the water-vapor feedback presented by Ingram (2010), but in disagreement with other modeling studies.