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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

1. Introduction

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,

• 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.

2. Simulations• 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 atmosphere-only climate simulation.

Figure 5 40-day mean E minus P (left) and MFD (right)

3. Results

Figure 6 Cumulative plots the water budget terms for the northern (left), mid (centre) and southern (right) boxes.

Figure 3 Mean diurnal cycle of P. Observations are an average of 4 different products.

4. Conclusions

Figure 2 Model domains (black boxes) and 40-day mean TRMM rainfall (shading). North, mid and south analysis domains (red boxes).

Figure 4 Mean diurnal cycle of meridional moisture flux at 400m above ground level, averaged 8°W-6°E.

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.

• Analysis is through the vertically integrated atmospheric water budget:

• where PWVt is the precipitable water vapour tendency, E is evapotranspiration, P is precipitation and MFD is the moisture flux divergence

• 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).

𝛿𝛿𝑡

1𝑔∫𝑞𝑑𝑝=𝐸−𝑃 −𝛻 .

1𝑔∫𝑞𝑉𝑑𝑝 ,

𝑃𝑊𝑉𝑡=𝐸−𝑃 −𝑀𝐹𝐷 .

Northern box Mid box Southern box

• 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).

• 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