West African Monsoon observed with ground‐based GPS receivers during African Monsoon Multidisciplinary Analysis (AMMA)

Abstract

[1] A ground-based GPS network has been established over West Africa in the framework of African Monsoon Multidisciplinary Analysis (AMMA) in tight cooperation between French and African institutes. The experimental setup is described and preliminary highlights are given for different applications using these data. Precipitable water vapor (PWV) estimates from GPS are used for evaluating numerical weather prediction (NWP) models and radiosonde humidity data. Systematic tendency errors in model forecasts are evidenced. Correlated biases in NWP model analyses and radiosonde data are evidenced also, which emphasize the importance of radiosonde humidity data in this region. PWV and precipitation are tightly correlated at seasonal and intraseasonal timescales. Almost no precipitation occurs when PWV is smaller than 30 kg m À2 . This limit in PWV also coincides well with the location of the intertropical discontinuity. Five distinct phases in the monsoon season are determined from the GPS PWV, which correspond either to transition or stationary periods of the West African Monsoon system. They may serve as a basis for characterizing interannual variability. Significant oscillations in PWV are observed with 10-to 15-day and 15-to 20-day periods, which suggest a strong impact of atmospheric circulation on moisture and precipitation. The presence of a diurnal cycle oscillation in PWV with marked seasonal evolutions is found. This oscillation involves namely different phasing of moisture fluxes in different layers implying the low-level jet, the return flow, and the African Easterly Jet. The broad range of timescales observed with the GPS systems shows a high potential for investigating many atmospheric processes of the West African Monsoon.

Observations of precipitable water vapour over complex topography of Ethiopia from ground-based GPS, FTIR, radiosonde and ERA-Interim reanalysis

“…Inspection of the estimation error shows an increase in scatter at 00:00 UTC for all eight GPS sites. This has also been reported by Bock et al (2008). The estimation error for Addis Ababa (Fig.…”

West African monsoon intraseasonal activity and its daily precipitation indices in regional climate models: diagnostics and challenges

“…2k, RCA4 is the closest model to ERAI in representing the position and the amplitude of the baroclinic zone. In particular, the 30 mm contour of PW, which is the marker of the Inter Tropical Discontinuity (ITD) according to Bock et al (2008), is quite well reproduced. The JJAS-mean precipitation of RCA4 is also consistent with the observations, except near the western coast, where it is overestimated These locations (varying in longitude "Long") have been sampled at 15°W (coast), 0°E (Greenwich) and 15°E (Central Africa).…”

“…Bock et al (2007) showed high correlations between the WAM rainfall and PW at 5 ground-based GPS stations, both at seasonal and intraseasonal scales. It was also inferred by Bock et al (2008) that no rainfall occurs when PW is smaller than 30 mm. At synoptic scale, Poan et al (2013) have shown that PW is particularly well suited for the detection and monitoring of the AEW dynamics, as well as the associated precipitation.…”

2D PWV monitoring of a wide and orographically complex area with a low dense GNSS network

“…The ZTD can be computed in the zenith direction above the GNSS station using an appropriate mapping function. The estimation of the ZTD helps to enhance the positioning precision and interpret severe meteorological events, as demonstrated in previous studies (Bouma and Stoew 2001;De Pondeca and Zou 2001;Basili et al 2003;Bock et al 2008;Boniface et al 2009;Bennitt and Jupp 2012;Sguerso et al 2013Sguerso et al , 2015Tsuda et al 2013).…”