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Observing the Low-level Atmospheric Circulation in Tropical Atlantic

EUFAR-supported flight campaign, July 2016

Clustered with the DACCIWA campaign, the OLACTA-2 project is focused on the role of the air-sea interaction on the southern West-African climate dynamics, a scientific topic presently not covered by DACCIWA. This project thus aims at advancing knowledge on the low-level atmospheric circulation (LLAC) in the Eastern Equatorial Atlantic in connection with air-sea interactions in the presence of significant sea-surface temperature (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 exercises, strong biases are still found in the Eastern Tropical Atlantic region. PREFACE aims at understanding the origin of these flaws, such as the misrepresentation of low and mid-levels clouds (by conducting ship-borne campaigns among other things) in order to correct them. With 10 fully funded flight hours on board SAFIRE’s ATR42 funded by EUFAR’s transnational access activity, OLACTA-2 will therefore contribute to and enrich the findings of both the DACCIWA and PREFACE projects.

The overarching objectives of OLACTA-2 are to acquire a definitive dataset, against which the models simulations can be challenged, to advance knowledge of (i) sea-surface temperature (SST) gradients and (ii) background winds impact on the low-level atmospheric circulation (LLAC) vertical structure in the Gulf of Guinea, as well as (iii) the influence of the LLAC return flow on the transport of natural and anthropogenic aerosols over the Atlantic Ocean.

Given that the OLACTA-2 dedicated flights needed to be conducted once the equatorial cold tongue was well established and after the so-called monsoon onset, the proposed time frame for DACCIWA (27 June - 17 July 2016) was 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 proposed flights was scheduled during a large-scale wind burst event in the Gulf of Guinea, 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 that can be forecasted a few days to one week in advance. The planned flights also aimed to observe at the same 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.

During the OLACTA-2 field campaign three flights took place on 2, 7 and 14 of July on board SAFIRE’s ATR 42 with roughly 10 hours of flying time in total. The flights concentrated on measuring sea surface gradients north of 2 N as the cold tongue near the equator was too far to reach.

The development of the Atlantic Cold Tongue was unusually late this season compared to climatology. The regions measured included temperature changes of around 3 degrees as measured by satellites and the warm sea regions were often observed to be collocated with regions of offshore afternoon convection, with the SST gradients being a potential source of instability. Interestingly, the measurements from the aircraft indicated near constant SSTs over the region, however this may be a limitation of the accuracy of the raw data and fully processed data is expected to reveal more detail.

The field campaign allowed for airborne measurements of sea surface temperatures that can be compared with satellite observations and observations of boundary layer structure with lidar. These will be further analysed and compared to satellite observations to examine the role of SST in the boundary layer structure and convection initiation. As part of the DACCIWA project we also ran the Met Office Unified Model (UM) as a forecast tool at convective permitting 4km resolution. Further runs with the same domain will be possible and these will be used as a tool to understand the data.

The only major aircraft related difficulty was the lack of lidar for some part of the project as described above. This was dealt with as rapidly as possible and was missing for only one flight. This problem has therefore not unduly affected the ability to achieve the project aims. The fact that the SST cold tongue was further south than predicted was also a difficulty. However, the SST gradients closest to shore were strong and their impact on the boundary layer structure was measured, hence the aims of OLACTA in measuring air sea interactions at temperature gradients should be largely unaffected. This data will allow us to tie into the DACCIWA dataset more closely and make use of model runs and other analyses. Part of this data can also be fed into DACCIWA sea breeze studies.

The research team was provided with the SST data from the CLIMAT instrument in real-time, but as a raw product during the flights, and are currently awaiting fully processed data from the SAFIRE team. So far, this fully processed data is only available for the flight and initial analysis shows that the temperature gradients were observed. The rest of the data is expected to be fine. The data from the other radiometers (CIMEL and Kipp & Zonen broadband radiometer) have been checked for the first flight and look as expected.

Unfortunately due to a technical fault, lidar data for the second flight will not be available. This was unavoidable and repairs were made after instrument parts were shipped out to the field, ensuring we had the instrument back online for the third flight. All other data is fine as far as we can tell at this early stage.

The flight data will be available in the EUFAR/CEDA archive as soon as possible. For more information, contact the project lead researcher, Phil Rosenberg (University of Leeds). 

Originally published on Sept. 6, 2016
Last update on April 21, 2017

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