Why Do Global Climate Models Struggle to Represent Low-Level Clouds in the West African Summer Monsoon?

Hannak, Lisa; Knippertz, Peter; Fink, Andreas H.; Kniffka, Anke; Pante, Gregor

doi:10.1175/jcli-d-16-0451.1

Cited by 67 publications

(96 citation statements)

References 62 publications

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“…Hannak et al [40] showed that in the region of West Africa there are large and persistent low cloud fields. They postulated that the self-calibrating method of the Heliosat algorithm leads to an underestimated effective cloud albedo in the region with persistent cloud cover, since the scenes with minimum number of counts are still contaminated with clouds [40]. This leads to a strong overestimation of surface radiation and SDU in this area.…”

Section: Discussionmentioning

confidence: 99%

A Satellite-Based Sunshine Duration Climate Data Record for Europe and Africa

et al. 2017

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Besides 2 m temperature and precipitation, sunshine duration is one of the most important and commonly used parameter in climatology, with measured time series of partly more than 100 years in length. EUMETSAT's Satellite Application Facility on Climate Monitoring (CM SAF) presents a climate data record for daily and monthly sunshine duration (SDU) for Europe and Africa. Basis for the advanced retrieval is a highly resolved satellite product of the direct solar radiation from measurements by Meteosat satellites 2 to 10. The data record covers the time period 1983 to 2015 with a spatial resolution of 0.05 • × 0.05 • . The comparison against ground-based data shows high agreement but also some regional differences. Sunshine duration is overestimated by the satellite-based data in many regions, compared to surface data. In West and Central Africa, low clouds seem to be the reason for a stronger overestimation of sunshine duration in this region (up to 20% for monthly sums). For most stations, the overestimation is low, with a bias below 7.5 h for monthly sums and below 0.4 h for daily sums. A high correlation of 0.91 for daily SDU and 0.96 for monthly SDU also proved the high agreement with station data. As SDU is based on a stable and homogeneous climate data record of more than 30 years length, it is highly suitable for climate applications, such as trend estimates.

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Section: Discussionmentioning

confidence: 99%

A Satellite-Based Sunshine Duration Climate Data Record for Europe and Africa

et al. 2017

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“…3). These diagrams also indicate different phases of the monsoon season, which were distinguished by Knippertz et al (2017) mainly based on the north-south precipitation difference between the coastal and the Soudanian-Sahelian zones. These are: the pre-onset phase characterized by a rainfall maximum near the coast (before 21 June, phase 1); the post-onset phase during which the rainfall maximum occurred inland 30 (22 June -20 July, phase 2); the wet westerly regime when the rainfall maximum shifted back to the coast (21 -26 July, phase 3); and the recovery of the monsoon with a shift of the rainfall maximum inland (27 July until the end of the campaign, phase 4).…”

Section: Measurement Sites and Available Observationsmentioning

confidence: 97%

“…According to van der Linden et al (2015), their extent covers an area of about 800 000 km 2 . As the clouds often persist long into the following day, they control the daytime 5 radiative energy supply at the Earth's surface and, hence, influence the diurnal cycle of the atmospheric boundary layer (ABL) and, by this, they considerably affect the regional climate (Knippertz et al, 2011;Hannak et al, 2017). Up to now, high-quality data in this region are rare and only a few model investigations have been performed to investigate the temporal evolution and spatial distribution of LLCs in this region (e.g.…”

Section: Introductionmentioning

confidence: 99%

“…Brooks et al (2017) provides a detailed report about these measurements at the ground-based supersites and the available data. A description of the synoptic conditions that prevailed during the period of the DACCIWA campaign is given in Knippertz et al (2017). In this paper, we give an overview of the meteorological conditions at the three ground-based supersites.…”

Section: Introductionmentioning

confidence: 99%

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An overview of the diurnal cycle of the atmospheric boundary layer during the West African monsoon season: Results from the 2016 observational campaign

Kalthoff¹,

Lohou²,

Brooks³

et al. 2017

Preprint

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Abstract.A ground-based field campaign was conducted in southern West Africa from mid June to the end of July 2016 within the framework of the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) project. It aimed to provide a high-quality comprehensive data set for process studies, in particular into interactions between low-level clouds (LLCs) and boundary-layer conditions. In this region missing observations are still a major issue. During the campaign, 20 extensive remote sensing and in-situ measurements were conducted at three supersites: Kumasi (Ghana), Savè (Benin) and Ile-Ife (Nigeria). Daily radiosoundings were performed at 0600 UTC and 15 intensive observation periods (IOPs) were performed during which additional radiosondes were launched every 1.5 to 3 hours. Remotely piloted aerial systems were also operated during the IOPs. Extended stratiform LLCs form frequently in southern West Africa during the night time and persist long into the following day. They affect the radiation budget and hence the evolution of the atmospheric boundary 25 layer and regional climate. The relevant parameters and processes governing the formation and dissolution of the LLCs are still not fully understood. This paper gives an overview of the diurnal cycles of the energy-balance components, near-surface temperature, humidity, wind speed and direction as well as the conditions (LLCs, low-level jet) in the boundary layer at the supersites and relates them to synoptic-scale conditions (monsoon layer, Harmattan layer, African easterly jet, tropospheric stratification) in the DACCIWA operational area. The characteristics of LLCs vary considerably from day to day, including 30 a few almost cloud-free nights. During cloudy nights we found large differences in the LLC's formation and dissolution times as well as in the cloud-base height. The differences exist at individual sites and also between the sites. The synoptic conditions are characterised by a monsoon layer with south-westerly wind, on average about 1.9 km deep, and easterly wind above; the depth and strength of the monsoon flow show great day-to-day variability. Within the monsoon layer, a nocturnal low-level jet forms in approximately the same layer as the LLC. Its strength and duration is highly variable from night to 35 night. This unique data set will allow us to test some new hypotheses about the processes involved in the development of LLCs and their interaction with the boundary layer and can also be used for model evaluation.

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“…Cloud cover over West Africa is an important feature of the African monsoon but is poorly represented by global models (Hannak et al, 2017). This could lead to too low simulated clouds and too high incoming radiation at the surface that imply too high diurnal cycles of temperature and relative humidity over this region.…”

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confidence: 99%

LES study of the impact of moist thermals on the oxidative capacity of the atmosphere in southern West Africa

Brosse¹,

Leriche²,

Mari³

et al. 2017

Preprint

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Abstract. The hydroxyl radical (OH) is a highly reactive specie and plays a key role in the oxidative capacity of the atmosphere. The total OH reactivity, corresponding to the inverse of OH lifetime, may have a significant fraction non-attributable to commonly measured compounds. The turbulence-driven segregation of OH and its reactants can cause substantial modification of averaged reaction rates, and thus of the total OH reactivity, when compared to a perfectly mixed assumption. We study the impact of turbulent mixing on the OH reactivity with Large-Eddy Simulations from the Meso-NH model coupled on-line with a 5 detailed chemistry mechanism in two contrasted regimes. Our findings show that the non-mixing of isoprene (resp. aldehydes) and OH leads to 30% decrease (resp. 16% increase) of the mean reaction rate at the top of the boundary layer and consequently to 9% decrease (resp. 5% increase) of the OH total reactivity in a biogenic (resp. anthropogenic) environment. Moreover, the total OH reactivity is highest inside thermals in both cases.

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Why Do Global Climate Models Struggle to Represent Low-Level Clouds in the West African Summer Monsoon?

Cited by 67 publications

References 62 publications

A Satellite-Based Sunshine Duration Climate Data Record for Europe and Africa

A Satellite-Based Sunshine Duration Climate Data Record for Europe and Africa

An overview of the diurnal cycle of the atmospheric boundary layer during the West African monsoon season: Results from the 2016 observational campaign

LES study of the impact of moist thermals on the oxidative capacity of the atmosphere in southern West Africa

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