The north-south gradient in surface temperature and rainfall in West Africa leads to the summertime monsoon circulation. Here, the full extent of the relationship between the land surface and the atmosphere is discussed with particular reference to the impact that soil moisture has on the atmosphere at different spatial scales. Observations from the AMMA field campaign in 2005 and 2006 are combined with satellite analysis and model simulations to discuss various interactions between the land surface and the atmosphere. Tethered balloon observations from Mali in August 2005 are used to assess the characteristics of the nocturnal boundary layer. It is observed that a stronger surface temperature inversion after sunset leads to a faster nocturnal jet, and these findings are further investigated using surface station data. Case studies of two nights of observations are used to discuss the variation of observed boundary layer structures. It is found that on nights where the nocturnal jet is weaker, the winds align with African Easterly Wave (AEW) circulations on the larger scale. . Following this, the impact that AEWs had on sl1rface properties is examined. Flux data from Niamey showed little statistical correlation with wave passage. It is suggested this could be partly due to the study year having more westerly initiating waves than climatology. The inducement of circulations by soil moisture inhomogeneities are discussed in regard to previous literature, where a moist cool surface leads to high pressure and anticyclonic circulation. The relation of this theory to the synoptic scale is investigated using a case study from 25-29 July 2006. During this time, an unusually-structured AEW left a distinct synoptic 'wave' pattern of soil moisture in the Sahel region due to its modulation of convection. The structure of this wave and the initial conditions which lead to the soil moisture pattern are discussed. The atmospheric impact of the soil moisture wave is investigated using the Met Office Unified Model. It is found that th~ enhanced soil moisture leads to a cooler, moister, . thinner boundary layer. This leads to divergent winds at low levels and a reduction in the monsoon flow due to the reduction in the north-south pressure gradient. There is indication that low-level anticyclonic circulations are enhanced. The enhanced soil moisture wave also leads to an increase in easterly winds at the African Easterly Jet level: it is shown that this is due to a decrease in boundary layer height and a reduction in turbulence. Inspection of wave energetics shows the case study wave appears to be in a decaying phase. There is evidence that the soil moisture wave increases the thermal decay by decreasing the temperature behind the trough in the warm region, reducing the temperature eddies and re-establishing the zonal temperature gradient. This study has implications for weather forecasting as the results suggest that patterns in soil moisture on the large scale are able to alter atmospheric dynamics at the synoptic scale within the time frame of a few days. This leads to further questions as to whether a realistic representation of soil moisture in mo.dels would lead to an improvement in the simulation 'of tropical synoptic dynamics.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:491661 |
| Date | January 2008 |
| Creators | Bain, Caroline Louise |
| Publisher | University of Leeds |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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