West African Monsoon water cycle: 2. Assessment of numerical weather prediction water budgets

Abstract

[1] Water budgets from European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA)-Interim and National Centers for Environmental Prediction (NCEP) Reanalysis I and II are intercompared and compared to GPS precipitable water and to the 6 year hybrid budget data set described in part 1 of this study. Deficiencies are evidenced in the reanalyses which are most pronounced over the Sahel.Results from operational models (ECMWF Integrated Forecast System, NCEP Global Forecast System, and ARPEGE-Tropiques) and the special ECMWF African Monsoon Multidisciplinary Analyses reanalysis confirm and help understanding these findings. A bias (∼1-2 mm d −1 ) in precipitation and evapotranspiration leads to an unrealistic view of West Africa as a moisture source during the summer. North of the rainband (13°N-16°N), moisture flux convergence (MFC) shows a minimum in the NCEP models and divergence in the ECMWF models not consistent with the hybrid data set. This feature, added to presence of a deep layer of northerly dry air advected at midlevels (800-400 hPa), is thought to block the development of deep convection in the models and the northward propagation of the monsoonal rainband. The northerly flow is part of a shallow meridional circulation that is driven by the Saharan heat low. This circulation appears too strong in some of the models, a possible consequence of the too-approximate representation of physical processes and land surface properties over the Sahel. In most of the models, evapotranspiration shows poor connection with precipitation. This is linked with large analysis increments in precipitable water, soil moisture, and MFC. Despite the large biases affecting the water budget components in the models, temporal variations (seasonal and interannual) might nevertheless be recovered with reasonable accuracy.

Annual cycle of the West African monsoon: regional circulations and associated water vapour transport

“…ERA-Interim has half the grid-length of ERA40 and has 16 pressure-levels between 500 hPa and 1000 hPa compared to just 7 in ERA40. Several of the peak meridional and zonal fluxes highlighted in section 4 of this paper were around 50% stronger in ERA-Interim, which is consistent with the resolution dependence discussed by Meynadier et al (2010). It should be recognized however that since the ERA40 and ERA-Interim model and analysis systems are also different, the differences may not be just due to resolution.…”

“…The errors associated with data assimilation and time truncation error require a detailed comparison with independent in situ measurements (cf. Meynadier et al, 2010) which is beyond the scope of our study. It should be stressed here, however, that while the magnitudes of moisture fluxes and their convergence are important, the most important aspects that will be emphasized here are the time evolution of the patterns and how these relate to the regional circulations.…”

“…It should be noted however that different re-analysis products exhibit different characteristics in the West African region (Zhang et al, 2008;Meynadier et al, 2010) and so caution is needed in focusing on the details of one analysis in isolation. Rather than showing results from all available reanalyses, the pragmatic approach taken here is to consider results based on one re-analysis dataset, highlighting the aspects of the annual evolution we have most confidence in because they are reproduced in other re-analysis datasets.…”

“…As discussed in Meynadier et al (2010) it is important to be aware of the potential errors in such analyses. Several potential sources of error exist including most importantly those associated with: (i) horizontal and vertical grid resolution, (ii) misrepresentation of assimilated observations, and (iii) time truncation error due to the 6-hour time resolution.…”

Impact of soil moisture and convectively generated waves on the initiation of a West African mesoscale convective system

“…The Sahel has suffered serious droughts during recent decades (Janicot et al, 2008) and therefore it is important that rainfall variability can be predicted on various time-scales. There are currently problems with rainfall prediction over West Africa, both at numerical weather prediction (Meynadier et al, 2010;Agustí-Panareda et al, 2010c) and at climate scales (Hourdin et al, 2010). MCSs are a particular problem in global models because accurate representation of their trajectories, propagation speed and precipitation distribution requires the explicit treatment of moist convection.…”