A seamless assessment of the role of
convection in the water cycle of the
West African Monsoon
Cathryn E. Birch1, D. J. Parker2, J. H, Marsham2, D. Copsey3, L. Garcia-Carreras2
1MetOffice@Leeds,UK; 2University
of Leeds, UK; 3MetOffice, UK
2. Simulations
1. Introduction
• Many NWP and climate models are
unable to reproduce the position of
the West African Monsoon rainbelt
(there is a dry bias in the Sahel).
• Previous work by Marsham et al.
(2013) suggested that biases in the
diurnal cycle of convection and a
lack of cold pool outflows from
storms play a significant role.
• Simulations from the ‘Cascade’
consortium project are used to
understand the role of convection
in the regional water budget.
Figure 1 Schematic illustrating the conclusions of this
study and those of Marsham et al. (2013). The figure
compares simulations with (right) and without (left)
convective parameterisation,
• 40-day Met Office Unified Model
simulations are run over West Africa
• 25 July – 3 September 2006, to
coincide with AMMA observations
• Limited-area model domains with
parameterised (40 and 12 km) and
explicit (12, 4 and 1.5 km)
convection.
• Additional data: global NWP
analyses and a 10-year atmosphereonly climate simulation.
Figure 2 Model domains (black boxes) and 40day mean TRMM rainfall (shading). North, mid
and south analysis domains (red boxes).
3. Results
• Analysis is through the vertically
integrated atmospheric water budget:
𝛿 1
𝛿𝑡 𝑔
1
𝑞𝑑𝑝 = 𝐸 − 𝑃 − 𝛻.
𝑔
𝑞𝑉𝑑𝑝 ,
𝑃𝑊𝑉𝑡 = 𝐸 − 𝑃 − 𝑀𝐹𝐷.
• where PWVt is the precipitable water
vapour tendency, E is evapotranspiration,
P is precipitation and MFD is the moisture
flux divergence
Figure 3 Mean diurnal cycle of P. Observations
are an average of 4 different products.
Figure 5 40-day mean E minus P (left) and MFD (right)
• There is a step change in the diurnal cycle of convection when
the parameterisation is switched off (Fig. 3).
• The incorrect diurnal cycle in the parameterised simulations
changes the meridional pressure gradient (Fig. 1). Too much
water is transported from the Sahel to the Sahara and this
moisture is not replaced from the south (Fig. 4).
Northern box
Figure 4
Mean
diurnal cycle
of meridional
moisture flux
at 400m
above
ground level,
averaged
8°W-6°E.
• The dry bias in 12kmParam is illustrated by the red regions in
Fig. 5; it is evaporating more than it is raining and water
vapour is advected away from the region.
• The bias becomes large when the first major African Easterly
Wave of the simulation crosses the domain (day 10, Fig. 6).
• The water budget is better represented to the north and south
of the Sahel (Fig. 6).
Mid box
Southern box
Figure 6 Cumulative plots the water budget terms for the northern (left), mid (centre) and southern (right) boxes.
4. Conclusions
• Errors in the diurnal cycle increase the northward advection of moisture out of the Sahel but decrease the advection into the
Sahel from further south. This limits the availability of moisture for Sahelian rainfall.
• The biases occur within the first 24h of the simulations, indicating that they originate from the representation of fast physical
processes (i.e. the convection scheme).
• Once the bias is established the water budget terms act to reinforce the bias, locking the rainbelt’s latitude.
• Improving the diurnal cycle will improve the ability of models to represent Sahelian rainfall
Met Office FitzRoy Road, Exeter, Devon, EX1 3PB United Kingdom
Tel: 01392 885680 Fax: 01392 885681
Email: cathryn.birch@metoffice.gov.uk
5. References
•
Birch, C. E., D. J. Parker, J. H. Marsham, D. Copsey, L. Garcia-Carreras, 2014: A
seamless assessment of the role of convection in the water cycle of the West
African Monsoon, J. Geophys. Res., doi:10.1002/2013JD020887.
•
Marsham, J. H., N. Dixon, L. Garcia-Carreras, G. M. S. Lister, D. J. Parker, P.
Knippertz, C. E. Birch, 2013: The role of moist convection in the West African
monsoon system - insights from continental-scale convection-permitting
simulations, Geophys. Res. Lett., 40, 1843-1849, doi:10.1002/grl.50347.
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