European Facility For Airborne Research

OLACTA-2

Observing the Low-level Atmospheric Circulation in Tropical Atlantic

Scientific project

TA-015. Applications of atmospheric in-situ measurements.

Boundary layer

ROSENBERG Phil

First degree in physics and astronomy, PhD in Planetary Science, 3 years experience working at FAAM. Currently working at University of Leeds on Fennec, DIAMET and COPE aircraft related projects involving dynamics, dust and cloud physics.

de Coetlogon G., M. Leduc-Leballeur, R. Meynadier, S. Bastin, M. Diakhate, L. Eymard, H. Giordani, S. Janicot, A Lazar, 2013 : Air-Sea interaction in the eastern Equatorial Atlantic at intraseasonal timescales in boreal spring and summer : a quasi-biweekly equatorial oscillation. QJRMS, DOI : 10.1002/qj.2250

Knippertz, P.; Fink, A. H.; Schuster, R.; Trentmann, J.; Schrage, J. M.; Yorke, C., 2011: Ultra-low clouds over the southern West African monsoon region. Geophys. Res. Lett., 38, L21808, doi:10.1029/2011GL049278

Leduc-Leballeur, M., G. De Coëtlogon, et L. Eymard, 2013 : Air-sea interaction in the gulf of Guinea at intraseasonal time-scales : wind bursts and coastal precipitation in boreal spring. Quart. J. Roy. Meteor. Soc., 139, 671, 387-400

S. Crumeyrolle, P. Tulet, L. Garcia-Carreras, C. Flamant, D.J. Parker, A. Matsuki, A. Schwarzenboeck, P. Formenti and L. Gomes, 2011 : Transport of dust particles from the Bodele region to the monsoon layer. Case study of the 9-14 June 2006 period, Atmos. Chem. Phys. 11, 479–494, doi:10.5194/acp-11-479-2011

Bodele region to the monsoon layer. Case study of the 9-14 June 2006 period, Atmos. Chem. Phys. 11, 479–494, doi:10.5194/acp-11-479-2011

The low-level atmospheric circulation (LLAC) over the Gulf of Guinea (GG), which develops in response to an equatorial upwelling and resulting sea-surface temperature (SST) gradients, is most intense around the time of the monsoon onset (June/July). This has strong implications on air-sea interactions, moisture transport, cloud development, dust aerosols re-circulation and pollution ventilation in the coastal areas of southern West Africa (SWA). Most of the knowledge on this circulation has been gained through NWP and mesoscale models. To date, there exist no comprehensive observational dataset against which the models simulations can be challenged. Yet, most climate and regional models show large biases in simulated surface winds and SST in the Eastern Equatorial Atlantic. It is likely due to a mis-representation of the regional ocean-atmosphere couplings. The OLACTA-2 project aims at obtaining a definitive dataset on the LLAC in the GG, based on a suite of state-of-the-art in situ and remote sensing instruments intended to document the dynamics, thermodynamics and composition of the LLAC together with sea surface properties and near surface turbulent fluxes. Two identical meridional flights between coastal SWA and 2°N are planned (once the equatorial cold tongue is established) for a total of 10 hours of flying time. This project is intended to take place in the framework of the EU funded DACCIWA project planned in June/July 2016 in SWA. The project will also serve the purpose of the EU funded PREFACE project which aims at advancing knowledge on Tropical Atlantic climate.

ATR42 - SAFIRE

The ATR 42 is the most suitable aircraft for the project as it has the longest endurance (~3h30), thereby enabling the documentation of the low-level atmospheric circulation between 2°N and the Guinean coast. It is also the ONLY aircraft involved in DACCIWA that will enable documenting the dynamics and thermodynamics of the LLAC. In addition, an airborne backscatter LIDAR will be used to document the aerosol and cloud fields in the low-troposphere thereby providing continuous information on the marine PBL structure and elevated temperature inversion layers associated with the LLAC return flow. Air-sea interactions will be document via turbulent sensible and latent heat fluxes as well as momentum fluxes. The ATR 42 also has unique capabilities in terms of remote sensing of the sea surface with several downlooking radiometers: a CLIMAT IR thermometer (SST measurements), as well as a broadband pyranometer and a broadband pyrgeometer (upwelling visible and IR radiation).

Alternative aircraft: DLR FALCON 20
The DLR FALCON 20 which is also involved in DACCIWA, could be an alternative. The FALCON will have dropsonde launching capability thereby enabling the documentation of the low-level atmospheric circulation. Some chemistry (O3,CO2,CO) could be measured to track airmasses documenting the aerosol and cloud fields in the low-troposphere thereby providing continuous information on the marine PBL structure and elevated temperature inversion layers associated with the LLAC return flow.

A strong ocean-atmosphere coupling exists in the Tropical Atlantic at intraseasonal timescales: low-level winds are strongly controlled by the SST in the GG (de Coetlogon et al 2010). The GG is characterized in April-May by the set-up of an equatorial oceanic cold tongue that remains active until the end of August. This cold tongue leads to a peculiar regional atmospheric circulation, which plays a major a role in the setting of precipitation along the Guinean coast (Leduc-Leballeur et al. 2013): this low level meridional circulation, also called LLAC, feeds and sustains moist convection at the coast, discouraging convection near the equator (Thorncroft et al. 2010). It is also of primary importance for transporting natural and anthropogenic aerosols from West Africa toward the ocean. The LLAC is most intense in June and July, and its vertical structure and position are dependent on the location of the northern edge of the cold front associated with the Equatorial upwelling. For instance, the average position of the LLAC southern edge has been shown to experience an abrupt northward displacement about two weeks before the so-called monsoon onset. Most of the knowledge on the vertical structure of this circulation (e.g. the southerly flow in the low-level and the return flow around 700 hPa) has been gained through NWP and mesoscale models.
In spite of its potential large impact on regional climate, there exist no comprehensive observational dataset of the LLAC to date. The OLACTA-2 project aims at filling this gap. The experimental work proposed here will be developed within the EU funded DACCIWA project. DACCIWA (Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa) will help to improve the understanding of interactions between emissions, clouds, radiation, precipitation and regional circulation in SWA, a region under massive economic and population growths. The proposed EUFAR TA OLACTA-2 project is focused on the role of the air-sea interaction on the SWA climate dynamics, a scientific topic presently not covered by DACCIWA.

The DACCIWA field campaign is planned for 27 June to 17 July 2016 and will involve three research aircraft including the ATR 42 and the FALCON 20. The aircraft detachment base will be in Lomé. All aircraft-related expenses will be paid for by the EU project, except for the flight hours requested to achieve the objectives of OLACTA-2.

The overarching objectives of the TA proposal is to acquire a definitive dataset, against which the models simulations can be challenged, to advance knowledge of (i) SST gradients and (ii) background winds impact on the LLAC vertical structure in the GG, as well as (iii) the influence of the LLAC return flow on the transport of natural and anthropogenic aerosols over the Atlantic Ocean.

The dataset will be acquired from a suite of state-of-the-art in situ and remote sensing instruments intended to document the dynamics, thermodynamics and composition of the LLAC together with sea surface properties and near surface turbulent fluxes.

Two identical meridional flights between SWA coast and 2°N are planned for a total of 10 hours of flight time. As OLACTA-2 dedicated flights need to be conducted once the equatorial cold tongue is well established and after the so-called monsoon onset, the proposed time frame for DACCIWA (27 June - 17 July 2016) is ideal. From a climatological point of view, the monsoon onset is determined to be around 24 June (plus or minus 8 days). As one of the objectives is to assess the impact of background winds on the LLAC structure, one of the two proposed flights will be scheduled during a large-scale wind burst event in the GG, remotely driven by the Saint-Helena anticyclone fluctuations. These wind bursts are very frequent at this time of the year, and exhibit a robust bi-weekly periodicity (de Coëtlogon et al. 2010, 2013) that can be forecasted a few days to one week in advance. Both flights will likely allow observing at that time the tropospheric dry intrusions that suppress convection over the Guinean coast. These dry intrusions, linked to the Indian monsoon system, have been shown to play a major role in the West African monsoon onset (Flaounas et al. 2011).

Each flight consists of two long, leveled legs oriented along the main low-level flow (see Figure 1). The first leg (SWA coastline to Equator) will be performed at mid-level (5000 m) in order to fully sample the LLAC by doing aircraft sounding and using the LIDAR. In addition to providing continuous information on the marine PBL structure and clouds, the nadir pointing LIDAR will allow to observe the aerosol plumes (natural or anthropogenic) transported above the marine PBL with the return circulation of the LLAC. The second leg (2°N to SWA coastline) will be made at the lowest possible flight level to observed sea surface properties and sample the marine boundary layer dynamics, thermodynamics and composition as well as turbulent fluxes under the influence of strong SST meridional gradients.

This project thus aims at advancing knowledge on the low-level atmospheric circulation in the Eastern Equatorial Atlantic in relationship with air-sea interactions in the presence of significant SST gradients and highly variable background wind conditions (wind bursts observed in response to the St Helena anticyclone synoptic variability). Advancing knowledge on Tropical Atlantic Climate is also the primary goal of the ongoing EU-funded PREFACE project. Indeed, in spite of significant improvements in the global models between the CMIP3 and CMIP5 exercise, strong biases are still found in the Eastern Tropical Atlantic region. PREFACE aims at understanding the origin of these flaws, such as the mis-representation of low and mid-levels clouds (Knippertz et al. 2011), (by conducting ship-borne campaigns among other things) in order to correct them. The TA OLACTA-2 project will therefore contribute linking the DACCIWA and PREFACE projects, which will considerably enrich both projects by involving scientists from both projects.

de Coetlogon G., M. Leduc-Leballeur, R. Meynadier, S. Bastin, M. Diakhate, L. Eymard, H. Giordani, S. Janicot, A Lazar, 2013 : Air-Sea interaction in the eastern Equatorial Atlantic at intraseasonal timescales in boreal spring and summer : a quasi-biweekly equatorial oscillation. QJRMS, DOI : 10.1002/qj.2250.
Flaounas E., S. Janicot, S. Bastin, R. Roca and E. Mohino, 2011 : The role of Indian monsoon onset in the West African monsoon onset : observations and AGCM nudged simulations. Clim. Dyn, DOI 10.1007/s00382-011-1045.
Knippertz, P.; Fink, A. H.; Schuster, R.; Trentmann, J.; Schrage, J. M.; Yorke, C., 2011: Ultra-low clouds over the southern West African monsoon region. Geophys. Res. Lett., 38, L21808, doi:10.1029/2011GL049278.
Leduc-Leballeur, M., G. De Coëtlogon, et L. Eymard, 2013 : Air-sea interaction in the gulf of Guinea at intraseasonal time-scales : wind bursts and coastal precipitation in boreal spring. Quart. J. Roy. Meteor. Soc., 139, 671, 387-400.
S. Crumeyrolle, P. Tulet, L. Garcia-Carreras, C. Flamant, D.J. Parker, A. Matsuki, A. Schwarzenboeck, P. Formenti and L. Gomes, 2011 : Transport of dust particles from the Bodele region to the monsoon layer. Case study of the 9-14 June 2006 period, Atmos. Chem. Phys. 11, 479–494, doi:10.5194/acp-11-479-2011.
Thorncroft C. D., Nguyen H., Zhang C. et Peyrille P. 2011. Annual Cycle of the West African Monsoon : Regional Circulations and associated Water Vapour Transport. Q. J. R. Meteorol. Soc., 137 : 129–147.

The two flights are planned to be operated once the equatorial cold tongue is well established and the monsoon onset has taken place, i.e. end of June to early July (as derived from climatologies). The GG is generally observed to be less cloudy once the deep convection has transitioned from the coastal SWA (before the onset) to the Sahel (after the onset). High and mid level clouds (above 5500 m) are not an issue has the aircraft will fly below them. Low-level clouds will also not be an issue and will be used as tracers of the low-level temperature inversion layers in the LIDAR data to retrieve information on the marine PBL structure and LLAC return flow. The wind conditions that we are looking for will be present during the whole DACCIWA aircraft detachment, and the decision to fly different background wind conditions (associated with the wind bursts linked to the St Helena high) will made on reliable forecast, several days in advance.

We shall seek coordination, whenever possible, with CALIPSO daytime overpasses as well as imaging radiometers and scatterometers (SSM/I, SSMIS, AMSR-E, AMSR2, WindSat) overpasses. The former will give a more regional context for the cloud and aerosol layers over SWA and the GG, the later will be used to provide surface winds at the regional scale.

Listed in the table below are the ascending and descending equatorial crossing times for currently functioning instruments. Some satellite orbits are more stable than others and little change occurs over the years of operation.

Platform Insrument Ascending Descending Orbit Stability
DMSP F15 SSM/I 14:53 02:53 Unstable
DMSP F16 SSMIS 16:42 04:42 Unstable
DMSP F17 SSMIS 18:03 06 :03 Stable
Corolis WindSat 18:03 06:03 Stable
GCOM-W1 AMSR2 13:31 01:31 Stable
MetOp A ASCAT 21 :00 09 :00 Stable
CALIPSO CALIOP Lidar 13:30 01:30 Stable
Table 1: Equatorial crossing times (local solar time) as of September 2014 (source: http://www.remss.com/support/crossing-times)

Flights may occur on the weekend. We target the month of July 2016 to conduct our flights.

Location is the Gulf of Guinea in SWA as this proposal is piggybacking on the EU-funded DACCIWA project (for which the aircraft detachment base will be in Lomé) and aims at observing the LLAC which develops over the GG.

Two identical meridional flights between SWA coast and 2°N are planned for a total of 10 hours of flight time. Each flight consists of two long, leveled legs oriented along the main low-level flow (see Figure 1). The first leg (SWA coastline to 2°N) will be performed at mid-level (5000 m) in order to fully sample the LLAC by doing aircraft sounding and using the LIDAR. In addition to providing continuous information on the marine PBL structure and clouds, the nadir pointing LIDAR will allow to observe the aerosol plumes (natural or anthropogenic) transported above the marine PBL with the return circulation of the LLAC. The second leg (Equator to SWA coastline) will be made at the lowest possible flight level to observed sea surface properties and sample the marine boundary layer dynamics, thermodynamics and composition as well as turbulent fluxes under the influence of strong SST meridional gradients.

It should be noted that OLACTA-2 is a resubmission of a successfully funded OLACTA proposal to make adjustments based on the fact that the BAe-146 aircraft will no longer be involved in the DACCIWA project and is hence unavailable.

• Nadir pointing backscatter LIDAR
• Turbulence probes
• In situ dynamics and thermodynamics probes
• CLIMAT infrared thermometer (SST gradients)
• Downward facing broad band (pyranometer and pyrgeometer) radiometers.

None

All instruments listed above that are key for the OLACTA-2 science will be part of the ATR 42 fit for DACCIWA.

None

There is a possibility that a ship-borne campaign will be organized in the framework of PREFACE in July 2016, in which case the ATR 42 flights will be coordinated with the ship cruise.

Aircraft data (LIDAR, turbulence, in situ measurements of dynamics and thermodynamics, radiometers) will be processed and provided by SAFIRE.

Preliminary data analysis will be performed at the DACCIWA Operation Center, to foster interactions with the instrument scientists as well as the DACCIWA aircraft scientists. After the campaign, data interpretation will be further enhanced via combined analysis with dedicated coupled ocean-atmosphere regional climate simulations performed at LATMOS in Paris.

The OLACTA-2 project will benefit from all the resources available at the DACCIWA Operation Center during the ATR 42 detachment, including modeling tools dedicated for aircraft planning and operations. The project will also benefit from observations acquired by DACCIWA instrumented platforms during the course of the 2 OLACTA-2 flights. The project will also benefit from the LATMOS computer cluster to be used to perform dedicated coupled ocean-atmosphere regional climate simulations to enhance data interpretation.

The data acquired during the 2 OLACTA-2 flights will be made available to the DACCIWA and PREFACE EU project scientists. The data will be available through the SEDOO DACCIWA portal.

The DACCIWA ATR 42 detachment will take place from 27 June to 17 July 2016. The OLACTA-2 flights will be scheduled for the later part of the ATR 42 detachment, i.e. July 2016.

Yes

This proposal will allow the principal coordinator, Philip Rosenberg, to acquire expertise with leading part of a collaborative airborne project. He will learn from more experienced aircraft principle investigators involved in the OLACTA-2 and DACCIWA projects how a multi-aircraft detachment is run in an international cooperative project, as well as learn about the collaborative decision making process. The experience gained in the first week of the project, will be highly beneficial when his turn comes to make decision about when to fly the ATR 42 to achieve the science proposed for the OLACTA-2 project.

The project will also allow Moussa Diakhate, PhD student at the University of Dakar, to participate for the first time in a field campaign and to learn about measuring techniques and data analysis. It will allow as well Rory Fitzpatrick, PhD student at the University of Leeds, to get involved into a field campaign closely related to his research program.

Finally, mutual benefits for both the DACCIWA/OLACTA-2 and PREFACE EU projects are expected from the participation of Remi Meynadier and Gaëlle de Coëtlogon, experienced ship-borne researcher. It will allow Remi Meynadier to gain experience with airborne experimental research.

Jean-Luc Redelsperger
Guy Caniaux
Chris Thorncroft

A proposal has already been funded by the CNRS LEFE program to buy 12 dropsondes for the OLACTA-2 project.

Yes

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