European Facility For Airborne Research

SAVEX-D

Sunphotometer Airborne Validation Experiment in Dust

Scientific project

TA-006. Applications of atmospheric in-situ measurements

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ESTELLES Victor

Atmospheric aerosols, primarily related to columnar aerosol optical and radiative characterisation by ground based sunphotometry. Secondary experimental experience include solar radiation measurements and other in situ aerosol retrieval techniques (measured at ground level). I consider myself to be mostly inexperienced in the use of research aircraft, and particularly with the BAe-146, although have previously participated on an airborne experiment by supporting the flight with ground based measurements.

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BAe146 - FAAM

This experiment addresses the following areas of study indicated in the TA-006 call: Remote-sensing of atmospheric properties, Radiative transfer in the atmosphere, Aerosol physical properties and Aerosol direct radiative impacts.

The main objective of this experiment is the validation of AERONET and SKYNET/ESR ground based sunphotometer retrievals of columnar aerosols properties such as volume size-distribution, single scattering albedo, refractive index, and phase function, by direct comparison with aircraft measurements. These properties, retrieved by sunphotometers in many stations across the world, are vital for climatological studies, aerosol model verification and satellite retrieval validation. Comparisons between in situ airborne aerosol measurements and ground based sunphotometers have been previously performed (Haywood et al., 2003; Esteve et al., 2012; Müller et al., 2012). However, previous comparisons between both methodologies have been only performed at ground level, and significant differences were found (Estellés et al., 2012). Thus we propose a thorough comparison between airborne and both AERONET and SKYNET/ESR ground based measurements.

Two different ground based sunphotometers (Cimel CE318 and Prede POM) will be operated continuously from Cape Verde, beneath the Saharan dust plume, where high aerosol loadings are expected. The aircraft will overfly the ground station in order to provide high quality measurements of in-situ and remote dust aerosol properties, such as size distribution, optical properties, chemical composition and radiative effect (broadband and spectrally resolved in both shortwave and longwave). Horizontal and vertical variability will be measured by a series of stacked horizontal runs and vertical profiles, and from onboard lidar measurements. This will then allow validation of retrievals from the sun photometers, discriminating the most reasonable combination of instrument and inversion algorithm used at ground level from both Cimel-AERONET and Prede-SKYNET/ESR networks.

The conclusions will be employed to improve methods of inversion for sunphotometer datasets, and to improve the estimate of uncertainties, thus providing better observational datasets for climatological studies. In addition, by understanding and quantifying the differences between sunphotometer makes and algorithms, the database of much-needed measurements available to aerosol climate studies will be greatly expanded. The results obtained from this analysis will be published in international journals, presented in international conferences and in the annual meetings of the SKYNET organized workshops. Future PhD theses in the framework of the Spanish Ministry call will also focus in the analysis of the produced data. The dissemination plan includes sharing the campaign results with the developers of the SKYRAD algorithm to assist on the improvement of the inversion algorithms for the benefit of ESR/SKYNET and AERONET members in particular, and for the aerosol scientific community in general. The processed data for the ground instruments will be make publicly available through the ESR network website by setting up a FTP service.

References:
- Campanelli, M. et al., 2012. Monitoring of Eyjafjallajökull volcanic aerosol by the new European Skynet Radiometers (ESR) network. Atmos. Environ. 48, 33-45.
- Estellés V. et al., 2012. Comparison of AERONET and SKYRAD4.2 inversion products retrieved from a Cimel CE318 sunphotometer. Atmos. Meas. Tech., 5, 569–579.
- Esteve, A. R., et al., 2012: Sources of discrepancy between aerosol optical depth obtained from AERONET and in-situ 10 aircraft profiles, Atmos. Chem. Phys., 12, 2987–3003, doi:10.5194/acp-12-2987-2012.
- Haywood, J. et al., 2003: Comparison of aerosol size distributions, radiative properties, and optical depths determined by aircraft observations and Sun photometers during SAFARI 2000. 108, D13, 8471, doi:10.1029/2002JD002250.
- Müller, D. et al., 2012: Comparison of optical and microphysical properties of pure Saharan mineral dust observed with AERONET Sun photometer, Raman lidar, and in situ instruments during SAMUM 2006. J. Geophys. Res. 117, D07211, doi:10.1029/2011JD016825.

Dust is the preferred aerosol type for this experiment, because of the challenges it poses for the sunphotometer retrievals (non spherical particles and coarse mode) and due to typical high loadings over the ocean where performing dedicated flight patterns is possible. In order to retrieve the whole set of products from both ground instruments involved (Prede and Cimel) the sun must be unobscured. Conditions with a significant optical depth are necessary in order to decrease the uncertainty of both ground and airborne sampling techniques. The flights will require medium-high dust loadings, to ensure that the measurement accuracy is satisfactory, and cloudless sky to ensure that the ground based observations of the sky radiance are free of cloud contamination. The optimum scenario would be a completely cloudless sky with a high turbidity due to an intrusion of mineral dust from the Sahara.

In order to increase the probability of cloudless skies and dust intrusions, the field campaign should be organized in the summer season. The best times of the day would be 8:00 – 11:30 UTC in the morning or 15:30 – 19:00 UTC in the afternoon (times are estimated for summer in Cape Verde) in order to maximize the range of scattering angles measured at the ground by the sun sky radiometers in the almucantar geometry (maximum scattering angle from 60 to 160º). This ensures the best retrievals for the sunphotometer data.

The chosen study area is Cape Verde archipelago, because it is an area usually affected by air masses originating over the Sahara region, carrying high loadings of mineral dust.

The archipelago is located in the tropical Atlantic Ocean, in the western outflow of the Saharan dust. Ground based instrumentation already exist in the archipelago, including currently operative stations such as the Cape Verde AERONET site (Sal Island) established in 1994, and the Calhau site in Sao Vicente (CVAO), established in 2012. Previous campaigns have already taken place in this area, specifically at Praia site in Santiago Island (SAMUM campaign). Therefore, previous experiences make the archipelago an ideal location for the proposed experiment. Our ground-based site will be located in one of these three locations.

The three islands mentioned in the text are highlighted in the figure.

The optimum plan would consist of two-three 5-hour flights under different dust load conditions, totaling around 10-15 flight hours

Flight pattern
1.Overfly ground-based site at high altitude, to retrieve the aerosol profile with the lidar and measure radiation above aerosol layer. Release drop sondes.
2.2 banked orbits above aerosol layer (almucantar scans with SWS).
3.Descend to minimum safe altitude; acquire vertical profile of dust using nephelometer, PSAP and size distribution instruments.
4.4 banked orbits at the minimum permitted altitude (almucantar scans with SWS).
5.Series of stacked straight level runs at different heights to sample the aerosol properties in situ (altitudes as determined earlier by lidar) and make radiation measurements.

A Cimel/AERONET and a Prede/SKYNET ground-based instrument pair would be necessary for this experiment. The ground team would operate from one of the existing Cimel sunphotometer stations or in coordination with other ground teams from clustered projects to avoid the cost of delivering an extra Cimel sunphotometer. The Prede POM radiometer would be provided by the University of Valencia team, and its transport, operations and data processing funded by current projects.

For a complete validation and comparison of the ground based determined parameters, the following airborne measurements would be required (ordered by priority):
1) Aerosol size distributions. Key Instruments: PCASP and CDP.
2) Aerosol size distributions. Desirable instruments: CAS, CIP-15, SID2, GRIMM, SMPS.
3) Vertical structure and homogeneity of aerosols: LIDAR
4) In situ scattering and absorption: Nephelometer and PSAP.
5) Net fluxes of short and long wave radiation for aerosol radiative forcing estimation: broad-band radiometers.
6) Thermodynamic structure of the atmosphere: dropsondes (2-4 per flight).
7) Determination of Black Carbon in case of biomass burning aerosols present: SP2 (*)
8) Aerosol composition: filters
9) Spectral sky radiance and irradiance: SWS, SHIMS and ARIES (*)

(*) if the instruments are already supported by the ICE-D campaign

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- PCASP, CDP, CAS, CIP-15, SID2, GRIMM, SMPS.
- Airborne lidar.
- Nephelometer and PSAP
- Airborne BBRs (pyranometers and pyrgeometers).
- Dropsondes
- SP2 (*)
- Filter sampling
- Spectral radiation measurements: SWS and SHIMS (shortwave) and ARIES (longwave) (*)

(*) if the instruments are already supported by the ICE-D campaign

Not applicable for airborne deployment.

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A ground site will be operated in a suitable location, equipped with a Prede POM01L sun-sky radiometer and a Cimel CE318 sun-sky photometer (the latter can be a pre-existing AERONET instrument). The co-location of both instruments will allow the compared validation of both AERONET and SKYNET/ESR retrievals, permitting the evaluation of both instruments and inversion algorithms. Other radiometers such as pyranometers and pyrgeometers could be also deployed, in order to measure the down welling solar irradiance at ground level for radiative forcing estimation. The pre and post calibration of the ground instruments will be provided by AERONET (for the Cimel sunphotometer) and ESR (for the Prede radiometer).
Suitable ground-based sites in Cape Verde are being considered. For example Capo Verde station at Sal Island, CVAO at Calhau, Sao Vicente, or any other convenient site where clustered projects would plan the deployment of a ground station, if an AERONET Cimel CE318 sunphotometer is also deployed. In the latter case, Praia site would a feasible site.

During the field campaign, a daily quick and preliminary processing will be applied on the ground data in order to make available preliminary products for all the scientists in the clustered projects. These products will be useful for an adequate understanding of the aerosol properties evolution during the campaign, and will be combined with the daily outputs of atmospheric models and satellite imagery in order to report the expected atmospheric and aerosol conditions during the day/s following. This information is crucial for choosing the best opportunities for a successful airborne experiment. The University of Valencia and the ISAC-CNR members will provide a quick analysis of the ground based sunphotometer data, and will retrieve the atmospheric models outputs and satellite imagery. The whole SAVEX-D team will responsible for combining and analyzing the information to help in the decision for a possible next day flight. This will be also an opportunity for the UV and ISAC-CNR members to get experience on the design of the flight plans.

During the post-campaign analysis, the workload distribution will be as follows:
- Victor Estellés from the University of Valencia will obtain the aerosol inversions by applying the sunrad and skyrad algorithms to the Cimel CE318 sunphotometer data. Calibrations for this instrument will be supplied by AERONET (with the permission of the site operator and responsible).
- The aerosol inversions from the Prede POM radiometer will be performed by Monica Campanelli (ISAC-CNR). The calibration for the Prede POM radiometer will be provided by the ESR network by the application of the Improve Langley Plot technique.
- The airborne LIDAR profiles will be analyzed by Franco Marenco, and will be very useful to assess the homogeneity of the aerosol layers during the experiment and interpret the validation results.
- The airborne in situ scattering and absorption coefficients will be processed by Claire Ryder.
- Anna Esteve will be involved in the analysis of the airborne aerosol size distributions and will support the analysis of the in situ scattering coefficients.
- Sara Segura (current PhD student at the University of Valencia) is expected to support the analysis of in situ data (absorption/scattering coefficients and size distributions) during her training and after the campaign.
- The comparison of the airborne and ground-based aerosol size distributions and other aerosol properties will be performed by Victor Estellés and Monica Campanelli. Vertical integration and normalization of the in situ airborne properties will be performed and the relative comparison with AERONET and SKYNET/ESR retrievals will be analyzed, including an analysis of sensitivity with the input atmospheric parameters.
- Measurements from the airborne broadband radiometers will be processed by the University of Reading and University of Valencia members within the scope of current and future project proposals, and will be used to estimate the forcing induced by the mineral aerosol layers at different heights. Upwelling radiance will be used to infer the surface albedo, an important parameter for the inversion of the sunphotometer inversions and the estimation of the radiative forcing.
- Spectra obtained with SWS and SHIMS during aircraft orbits (almucantar scans) can be evaluated by comparison with the more stable ground-based measurements, and combined with model simulations. By applying the current inversion algorithms used with the ground radiometer to the airborne radiances will yield information on the aerosol properties at different heights.
- The analysis of the filter samples will be used to obtain the chemical composition of the aerosols and determine the refractive index expected for such an aerosol sample, to be compared to the effective refractive index obtained by inversion of sky radiances. The chemical analysis of filter samples will be incorporated into ongoing filter analysis from the BAe146. This dataset and the conclusions drawn from them will be very useful for the improvement of the skyrad inversion strategies. The results will be made available to the SKYRAD developers (University of Tokyo) so they can further improve the inversion techniques implemented in the SKYRAD algorithm.

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The FAAM aircraft is scheduled to take part in the Ice in Clouds Experiment – Dust (ICE-D) in August and September 2015 in Cape Verde. ICE-D is aimed at determining the influence of Saharan dust on the production of ice particles and precipitation in convective clouds. This measurement period is ideal for our experiment, and enables the possibility of clustering. The FAAM aircraft will already be equipped to measure dust particle size, composition and optical properties as well as take radiation measurements as part of this remit, so it would be an ideal opportunity for this project.

Therefore, for clustering reasons and in order to increase the chance of finding deep layers of mineral dust advected from the Sahara, the summer season (June to September) would be best for our experiments.

Yes

During the pre-campaign stage, the team members who have no previous active experience in airborne research (Victor Estellés, Monica Campanelli and Sara Segura) will have training opportunities in planning airborne experiments and measuring techniques. This experience will be very useful for their own development in this field of research.
The campaign and data analysis stages will be also potentially important for the training of current and future PhD students involved with the participant groups. At least one current PhD student (Sara Segura) from the University of Valencia group is expected to attend the campaign in support of ground and airborne activities and is also expected to contribute in the data analysis stage. Claire Ryder will support and provide training for Sara Segura in the analysis of in-situ airborne measurement data. Sara Segura is currently finishing her thesis on the analysis of ground absorption and scattering coefficients from the ground based instruments available at the University of Valencia site (nephelometer, aethalometer, Multi Angle Absorption Photometer and very recently, Aerodynamic Particle Sizer). Therefore, the training on the equivalent airborne in situ techniques will be very useful to improve her skills within a relatively short time, and she will contribute to the evaluation of the comparison between ground and airborne retrievals.
Further opportunities of collaboration on the analysis of the campaign data will be possible through the application for a postdoctoral fellowship to the annual call from the Spanish Ministry (case of Sara Segura) and through an application for a 1-3 month stay to the annual call from the University of Valencia (case of Victor Estellés). Both stays would permit a further training on the analysis of airborne data.
Finally, a 4 year PhD grant request will be included in the project proposal for the Spanish Ministry call in 2015/16, in order to work on the analysis of the ground retrievals and its comparison with airborne retrieved data.

Several opportunities to support the ground segment of the SAVEX-D experiment have been already described in section 4:
1) Applications for partially funding the ground segment of the experiment (transport of instrumentation, etc), and later analysis and publication of the campaign results have been recently placed for small local government and private calls (status: pendent).
2) Support and cooperation with MOSCATEL project (Methodology for the retrieval of ground particles properties from remote sensing columnar measurements; IP José Antonio Martínez Lozano, University of Valencia) funded by the Spanish Ministry of Economy (years 2013 – 2015) is expected. Although no funding is allocated in this project for the flights, MOSCATEL would partially support the transport of instrumentation to Cape Verde and first stage of data analysis.
3) It is also expected that the University of Valencia group will submit a project proposal to the 2016 – 2018 Spanish Ministry call. This proposal would continue the MOSCATEL project and it would include tasks for the data analysis, publication of the SAVEX-D results and a 4 year grant for a PhD student to work on the analysis of the experiment.

We propose clustering the SAVEX-D proposal with the Ice in Clouds Experiment-Dust (ICE-D) led by the Met Office and already planned for the summer of 2015. The ICE-D experiment will take advantage of the formation of cumulus congestus clouds during the advection of Saharan dust over the Cape Verde archipelago to study the influence of Saharan dust on the production of ice particles and precipitation in convective clouds. Therefore, the flight plans for the ICE-D campaign are aimed at the study of a variable cloudy weather even during precipitation regimes. On the contrary, the SAVEX-D experiment targets cloud-free skies during Saharan advection cases. These experiments have complementary objectives and could optimize the use of the aircraft in different weather regimes during the campaign period. Furthermore, the information retrieved during the SAVEX-D flight could be also useful for the ICE-D project to understand the how properties of the dust aerosols before the dust particles, acting as CCN, are activated and the convection clouds grow. We are willing to collaborate with the ICE-D team on the general aerosol science during the experiment. Ground data will be accessible via the ESR website and later archived in an ftp service.

Yes

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• A study on the sensitivities of simulated aerosol optical properties to composition and size distribution using airborne measurements
• Comparison of AERONET and SKYRAD4.2 inversion products retrieved from a Cimel CE318 sunphotometer
• Evaluation of the new ESR network software for the retrieval of direct sun products from CIMEL CE318 and PREDE POM01 sun-sky radiometers
• Monitoring of Eyjafjallajökull volcanic aerosol by the new European Skynet Radiometers (ESR) network
• Optical properties of Saharan dust aerosol and contribution from the coarse mode as measured during the Fennec 2011 aircraft campaign
• Project SAVEX-D - Map of general area
• Project SAVEX-D - Objectives project for clustering