Mid-tropospheric elevated moist layers (EMLs) near the melting level have been found in various regional observational studies in the tropics. Recently, a preponderance of EMLs in the presence of aggregated convection was found in cloud resolving simulations of radiative convective equilibrium (RCE), highlighting a significant circulation coupling. Here, we present global monthly EML occurrence rates based on reanalysis, yielding a broader view on where and when EMLs occur in the real world. Over the Atlantic, EML occurrence follows an annual cycle that maximizes in summer, aligning with maximized ITCZ intensity and organisation. Resembling the results in RCE, the large-scale circulation over the Atlantic shifts from a deep overturning in January to a bottom-heavy circulation in July. While EMLs embedded in the July cross-equatorial Hadley cell are found to be sourced from the ITCZ, EMLs north of the ITCZ emerge from the strongly sheared zonal flow over West Africa.

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A document by Florian Elias Roemer. Click on the document to view its contents.

A document by Akos Horvath. Click on the document to view its contents.

The new capabilities of global storm-resolving models to resolve individual clouds allow for a more physical perspective on the tropical high-cloud radiative effect and how it might change with warming. In this study, we develop a conceptual model of the high-cloud radiative effect as a function of cloud thickness measured by ice water path. We use atmospheric profiles from a global ICON simulation with 5 km horizontal grid spacing to calculate the radiation offline with the ARTS line-by-line radiative transfer model. The conceptual model of the high-cloud radiative effect reveals that it is sufficient to approximate high clouds as a single layer characterised by an albedo, emissivity and temperature, which vary with ice water path. The increase of the short-wave high-cloud radiative effect with ice water path is solely explained by the high-cloud albedo. The increase of the long-wave high-cloud radiative effect with ice water path is governed by an increase of emissivity for ice water path below 10-1 kg m-2, and by a decrease of high-cloud temperature with increasing ice water path above this threshold. The total high-cloud radiative effect from the ARTS simulations for the chosen day of this ICON model run is 2.59 W m-2, which is closely matched by our conceptual model with 2.56 W m-2. Because the high-cloud radiative effect depends on the assumed radiative alternative, assumptions on low clouds make a substantial difference. The conceptual model predicts that doubling the fraction of low clouds causes a doubling of the high-cloud radiative effect.

We investigated various aspects of the in-orbit performance of SEVIRI on Meteosat-10 (launch: 05 Jul 2012) and -11 (launch: 15 Jul 2015) with images, where Mercury or Venus appeared in a corner. These objects are of similar or smaller size than the instantaneous field of view, and therefore they are well suited for checks of geometric requirements. From comparing the position of Venus in different channels we conclude that the North-South distance between the two focal planes is shorter than the nominal value by 0.66 km at SSP (Sub-Satellite Point) with Meteosat-10 and longer by 1.44 km at SSP with Meteosat-11. The tilt of the detector array against the equator is less than 0.0037° for SEVIRI on Metosat-10. The sampling with narrow channels is 3.0016 km, with a one-sigma uncertainty of 30 cm at sub-satellite point. The tests we carried out to check the geometric performance of the instrument confirmed that SEVIRI is compliant with the requirements. The Point Spread Function as determined from the image of a planet agrees well with the expectations based on its combination with the finite impulse response. Finally we determined the stability of the calibration coefficients from the counts obtained on the planetary targets and found the reproducibility of the measurements of planetary fluxes similar to those of vicarious calibration targets. Hence planets are a promising alternative to established methods of in-flight characterisation and validation of imagers.

Reducing the model spread in free-tropospheric relative humidity (RH) and its response to warming is a crucial step towards reducing the uncertainty in clear-sky climate sensitivity, a step that is hoped to be taken with recently developed global storm-resolving models (GSRMs). In this study we quantify the inter-model differences in tropical present-day RH across GSRMs, making use of DYAMOND, a first 40-day intercomparison. We find that the inter-model spread in tropical mean free-tropospheric RH is reduced compared to conventional atmospheric models, except from the the tropopause region and the transition to the boundary layer. We estimate the reduction to approximately 50-70% in the upper troposphere and 25-50% in the mid troposphere. However, the remaining RH differences still result in a spread of 1.2 Wm-2 in tropical mean clear-sky outgoing longwave radiation (OLR). This spread is mainly caused by RH differences in the lower and mid free troposphere, whereas RH differences in the upper troposphere have a minor impact. By examining model differences in moisture space we identify two regimes with a particularly large contribution to the spread in tropical mean clear-sky OLR: rather moist regimes at the transition from deep convective to subsidence regimes and very dry subsidence regimes. Particularly for these regimes a better understanding of the processes controlling the RH biases is needed.

Changes in the concentration of greenhouse gases within the atmosphere lead to changes in radiative fluxes throughout the atmosphere. The value of this change, called the instantaneous radiative forcing, varies across climate models, due partly to differences in the distribution of clouds, humidity, and temperature across models, and partly due to errors introduced by approximate treatments of radiative transfer. This paper describes an experiment within the Radiative Forcing Model Intercomparision Project that uses benchmark calculations made with line-by-line models to identify parameterization error in the representation of absorption and emission by greenhouse gases. The clear-sky instantaneous forcing by greenhouse gases to which the world has been subject is computed using a set of 100 profiles, selected from a re-analysis of present-day conditions, that represent the global annual mean forcing with sampling errors of less than 0.01 \si{\watt\per\square\meter}. Six contributing line-by-line models agree in their estimate of this forcing to within 0.025 \si{\watt\per\square\meter} while even recently-developed parameterizations have typical errors four or more times larger, suggesting both that the samples reveal true differences among line-by-line models and that parameterization error will be readily resolved. Agreement among line-by-line models is better in the longwave than in the shortwave where differing treatments of the water vapor vapor continuum affect estimates of forcing by carbon dioxide and methane. The impacts of clouds on instantaneous radiative forcing are roughly estimated, as are adjustments due to stratospheric temperature change. Adjustments are large only for ozone and for carbon dioxide, for which stratospheric cooling introduces modest non-linearity.

Meteorological research satellites on polar orbits observe occasionally the Moon, when it moves through their deep space view. Over the last few decades, a large data set built up, which allows to determine the lunar flux with unprecedented accuracy especially at wavelengths, for which the Earth’s atmosphere is opaque. We determined the disk-integrated brightness temperature of the Moon at 19 wavelengths between 4 and 15 µm and at five frequencies between 89 and 190 GHz for phase angles between -80° and +70° with HIRS, AMSU-B, MHS, and ATMS on NOAA and MetOp satellites

We conduct a series of eight 45-day experiments with a global storm-resolving model (GSRM) to test the sensitivity of relative humidity R in the tropics to changes in model resolution and parameterizations. These changes include changes in horizontal and vertical grid spacing as well as in the parameterizations of microphysics and turbulence, and are chosen to capture currently existing differences among GSRMs. To link the R distribution in the tropical free troposphere with processes in the deep convective regions, we adopt a trajectory-based assessment of the last-saturation paradigm. The perturbations we apply to the model result in tropical mean R changes ranging from 0.5% to 8% (absolute) in the mid troposphere. The generated R spread is similar to that in a multi-model ensemble of GSRMs and smaller than the spread across conventional general circulation models, supporting that an explicit representation of deep convection reduces the uncertainty in tropical R. The largest R changes result from changes in parameterizations, suggesting that model physics represent a major source of humidity spread across GSRMs. The R in the moist tropical regions is disproportionately sensitive to vertical mixing processes within the tropics, which impact R through their effect on the last-saturation temperature rather than their effect on the evolution of the humidity since last-saturation. In our analysis the R of the dry tropical regions strongly depends on the exchange with the extra-tropics. The interaction between tropics and extratropics could change with warming and presage changes in the radiatively sensitive dry regions.

The total lunar eclipse from October 28, 2004, was observed by AMSU-B (Advanced Microwave Sounding Unit - B) on NOAA-15 during a couple of minutes of each orbit around the Earth. From this unique vantage point in space it could provide disk-integrated, effective lunar temperatures at lower frequencies than employed in most previous observations of eclipses, in intervals of 100 min. The relative changes in brightness temperature were 6.4±2.1% at 89 GHz and 16.6±2.1% at 183 GHz. This trend of stronger temperature drop at higher frequency was expected on the basis of a new radiative transfer model simulating the global brightness temperatures. Our measurements disprove two results published in the past, but they confirm the claim made before that the changes are similar at the center and for the whole disc. In terms of precision they are comparable to those carried out in the past with radio telescopes on ground.

A major problem with calculating the uncertainties of measurements with weather satellites is the fact that a full characterisation and calibration of their instruments can only be carried out before launch. The Moon, however, makes at least some of these activities possible in flight as well by providing a reliable flux reference at a well-defined position. We used serendipitous observations of the Moon with AMSU-B and MHS on eight different satellites to measure pointing accuracy, spectral channels coregistration, and beamwidth with unprecedented accuracy in flight. In addition we compared these findings with the corresponding values obtained on ground. By analysing more than a hundred Moon intrusions in the deep space view we could determine the radiance of the Moon as a function of its phase angle and distance from the Sun. The difference in average brightness temperature of the lunar disk between perihelion and aphelion amounts to $4.4 \pm 2.3$ K at 183 GHz. We compare the measured brightness temperature of the Moon as a function of phase angle between $-85^{\circ}$ (waxing) and $+76^{\circ}$ (waning) with the predictions from two models and find that one of them reproduces the shape of this function very well.