Clouds and Water Vapour

We use the method of the direct inversion of the continuity equation to infer 2D-fields of middle atmospheric circulation vectors (von Clarmann and Grabowski, Atmos. Chem. Phys., 16, 14563-14584, 2016) from MIPAS measurements of SF₆, CFC-11, CFC-12, HCFC-22, CCl₄, CH₄, N₂O, CO and H₂O. This novel method avoids the well-known problems associated with the use of the age of stratospheric air as a diagnostic of the intensity of the Brewer-Dobson circulation, because no age spectra are needed; subsidence of mesospheric air depleted in the target species is properly accounted for by an observational upper boundary condition; and the analysis of the circulation is well resolved in space and time. Resulting circulation patterns show the variability of the Brewer Dobson Circulation and the mesospheric overturning circulation in unprecedented detail. We present multi-annual monthly mean circulation patterns along with their inter-annual variabilities and discuss prominent features of the circulation.

In this study, the Shortwave Infrared CO Retrieval (SICOR) algorithm is deployed on reflectance measurements in the 2.3 μm spectral range from the Tropospheric Monitoring Instrument (TROPOMI) on ESA's Sentinel 5-P mission to simultaneously retrieve vertical column abundances of H₂O and HDO. Information about the isotopic fractionation in the atmosphere allows to conclude on the atmospheric transport of air parcels and by that is highly relevant, for example, for investigations of the hydrological cycle. We present first results of TROPOMI H₂O/HDO retrievals and a comparison with ground based measurements of the Total Carbon Column Observing Network (TCCON). Furthermore, we discuss the relevance of recent developments in spectroscopical databases for water vapour isotopologues. This contribution is the first step towards a scientific H2O/HDO TROPOMI data product which will enable new research possibilities.

Water vapour is a key component of the hydrological cycle. It is also the most important natural greenhouse gas and plays a key role in tropospheric chemistry, as source of the hydroxyl radical, OH, and as a third body in key reactions of hydroperoxyl radical, HO₂. Its amount is highly variable and is also affected by anthropogenic global warming. To investigate these effects, long time series of global water vapour amount and distribution are required.

The Air Mass Corrected Differential Optical Absorption Spectroscopy (AMC-DOAS) approach to derive global water vapour vertical columns was originally developed for Global Ozone Monitoring Experiment (GOME) on ERS-2, but has been applied also to measurements of the SCIAMACHY instrument on ENVISAT and the GOME-2 instruments on METOP-A and METOP-B. An application of the AMC-DOAS method to TROPOMI data on Sentinel-5P is currently under development.

In this presentation, we show first promising results from our research. These include comparisons with independent data sets to assess the quality of the derived data. Since there is currently no operational water vapour product from Sentinel-5P, the new AMC-DOAS product will provide a valuable addition to the Sentinel 5P project. In combination with GOME, SCIAMACHY and GOME-2 data it will be possible to produce a consistent long term time series of AMC-DOAS global water vapour amounts from which e.g. global and local trends can be derived.

Atmospheric water plays a key role for the Earth's energy budget and temperature distribution via radiative effects (clouds and vapour) and latent heat transport and the distribution and transport of water vapour is closely linked to atmospheric dynamics on all scales. In this context, global monitoring of the water vapour distribution is essential for numerical weather prediction, climate modeling and the understanding of climate feedbacks.
Total column water vapour (TCWV) was first retrieved using Differential Optical Absorption Spectroscopy (DOAS) analysis in the „red“ spectral range (620-670nm) as implemented in the GOME Data Processor GDP, because of the relatively strong water vapour absorption in that spectral range. However, this method has some limitations: several post-corrections e.g. due to non-linear effects (for strong water vapour absorptions) have to be applied and the low ocean surface albedo leads to low sensitivity for near-surface layers. In addition some of the new satellite sensors do not cover the red spectral range, e.g. TROPOMI on board ESA's Sentinel 5 Precursor.
Here, we apply a new approach using the spectral absorption structures of H₂O in the „blue“ spectral range (430-450nm). It has the advantage that nonlinear effects are negligible. Also, this spectral window shows a higher sensitivity for near-surface layers over ocean and in general a smoother spatial distribution of the surface albedo compared to the red spectral range.
The retrieval uses a linear least-squares fit and is performed on a single spectral window, which enables a fast and robust processing of large data sets. In the retrieval spectral absorption by NO₂, O₃, O₄ and the Ring effect are considered in addition to H₂O. Furthermore, changes of the instrument spectral resolution function (ISRF) along the satellite‘s orbit are accounted for using a linearized treatment of ISRF parameters as pseudo-absorbers.
We use this new algorithm for retrieving TCWV from TROPOMI spectra and compare these results with TCWVs retrieved from OMI and GOME-2 spectra. In addition we show first results of a validation study using a variety of different reference data sets.

After ten years of CALIPSO lidar record, there are now no doubt that lidar is one of the most powerful tool to study cloud physics. Recent study (Chepfer et al. 2018) also shows that providing a long enough time series, lidar allows studying cloud trends. CALIPSO which works at 532 nm is at the end of life, but other lidar mission with 355-nm laser are planned or just launched: ADM-Aeolus, while not designed for clouds (only few vertical levels) could be very useful (among others) to make junction between CALIPSO and future other lidar missions, EarthCare, and other future lidar missions that are under study in the agencies and institutes. It is then necessary to study how continuity can be ensured between CALIPSO cloud products and upcoming space lidars that have different wavelengths, and what is the added value of this new wavelength for clouds.

Here, the objective is then to study how leveraging upcoming 355-nm channels in space lidars for cloud science.

To address this question, we use IPRAL lidar located in the SIRTA ground-based observatory (near Paris, France): IPRAL is a high-performance backscatter lidar that works at three wavelengths including 532- and 355-nm channels. It crosses atmosphere from ground to 20 km at a 15-m vertical resolution. We will present work on: (i) the creation of the equivalent of GOCCP CALIPSO cloud product from IPRAL 532-nm channel, and the adaptation of the method for the IPRAL 355-nm channel in order to make them as consistent as possible; (ii) comparison to CALIPSO for some collocated cases study in order to qualify and quantify the limits of a possible 532/355 continuity cloud product; (iii) new diagnostics relevant for cloud science based on the 355-nm IPRAL signal analysis.

Total Column Water Vapour (TCWV) retrievals from measurements of the polar orbiting, sun-synchronous satellite spectrometers OLCI (Ocean and Land Colour Instrumenton board Sentinel-3) enable observations of high spatial resolution and accuracy over land surfaces on a global scale. We have validated the TCWV product as retrieved from OLCI measurements by means of GNSS (PGS), MW (Micro-Wave), and AERONET observations of TCWV.

A high agreement could be found between ground-based and satellite measurements, however, a wet-bias of up to 10% of the OLCI TCWV retrievals have been identified. This finding has to be further analysed with respect to failures in the spectroscopy and stray-light correction.

We document, for the first time, how detailed vertical profiles of cloud fraction change diurnally between 51°S and 51°N, by taking advantage of 15 months of measurements from the Cloud-Aerosol Transport System (CATS) lidar on the non-sun-synchronous International Space Station (ISS). In the Tropics, we compare CATS observations to the diurnal cycles of water vapor profiles and top-of-the-atmosphere radiation revealed by the several sensors onboard the Megha-Tropiques low-orbiting platform.

We explore the diurnal cycles of low-level and high-level clouds over the course of the day globally and in several large regions during the boreal summer. We find distinct behaviours for high clouds over the tropical ocean, for mid-level clouds over tropical land, and for the omnipresent low and high level clouds over the Southern Ocean. Over all continental areas, we see boundary layer clouds develop upwards following sunlight activation and reach maximum occurrence at about 2.5km a.s.l. early in the afternoon.

We find that the cloud profiles derived from CATS measurements at local times of 01:30 and 13:30 are consistent with those observed from CALIPSO at similar times. Our results suggest that CALIPSO measurements, always sampled at local times of 01:30 and 13:30, document the daily extremes of the cloud fraction profiles, most accurately over ocean. These findings are applicable to other instruments with similar local overpass times, including all the other A-Train instruments and the upcoming EarthCARE mission.

Finally, two robust behaviours in tropical regions dominated by subsidence are explained by comparison with Megha-Tropiques measurements: 1) over ocean, a positive anomaly of opaque clouds in the lower atmosphere grows from sunset to sunrise, dampening the diurnal variation of oceanic surface temperature. 2) over land, a positive relative moisture anomaly near the surface at sunrise turns into a positive anomaly of opaque clouds in the free troposphere in the early afternoon, and into a near-tropopause thin clouds positive anomaly early at night, before vanishing with sunrise. This results in a strong surface temperature diurnal variation (17K).