Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Variability in boundary layer structure during HAPEX-Sahel wet-dry season transition
    Wai, M.M.K. ; Smith, E.A. ; Bessemoulin, P. ; Culf, A.D. ; Dolman, A.J. ; Lebel, T. - \ 1997
    Journal of Hydrology 188-189 (1997)1-4. - ISSN 0022-1694 - p. 965 - 997.
    klimaatverandering - paleoklimatologie - neerslag - hydrologie - evapotranspiratie - microklimaat - planten - grenzen - Sahel - climatic change - palaeoclimatology - precipitation - hydrology - evapotranspiration - microclimate - plants - boundaries - Sahel
    The variability of the Sahelian boundary layer has been studied with streamline analyses, rainfall measurements, and upper air soundings during its transition from wet to dry season. The 1992 rainy season ended prematurely because of the early arrival of westerly troughs over West Africa. The change in the circulation is related to global-scale atmospheric circulations as successive westerly troughs over this region can be traced back upstream on a planetary scale. Once the upper level easterlies changed to westerlies, the large-scale circulation brought the surface northeasterly flow southward, which led to the retreat of the Southwest Monsoon in Niger. The boundary layer responded quickly to this transition of synoptic events from wet to dry seasons. During the wet period, the boundary layer was relatively cool and moist because evapotranspiration dominated, keeping the surface cool and preventing significant direct sensible heating of the boundary layer. During the transition period, less extensive showery weather allowed the boundary layer to have more time to recover from rainfall episodes, leading to a warming and drying trend. During the dry period, soil moisture contents dropped rapidly. With more sensible heat flux made available for boundary layer heating and energetics, the boundary layer reached its maximum temperatures and minimum moisture contents during the Hydrological Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel) intensive observational period. Budget calculations indicate that the horizontal advection and vertical flux divergence terms were most important during the wet period, whereas during the dry period, the subsidence and vertical flux divergence terms were most important. From wet to dry seasons, the vertical wind shear of the zonal wind was reduced from 23 m s−1 to 16 m s−1, consistent with vertical wind shear differences between wet and dry years as reported in the literature. Similarities and differences with the First ISLSCP Field Experiment (FIFE-89) boundary layer are also examined. It is hypothesized that the retreat of the southwesterly monsoon could be upheld by a sustained secondary circulation if the wet season rainfall pattern imprints an organized south to north soil moisture gradient maintaining a concurrent reverse gradient in surface sensible heating. The boundary layer circulation that would be established in response to the heating gradient would reinforce surface southwesterlies, as well as reinforcing mid-level easterlies of which the African Easterly Jet is a part, and thus help uphold the intrusion of westerlies and the monsoon retreat. Such a mechanism, whose effectiveness would be a function of how distinct the south-north soil moisture gradient develops from the wet season precipitation pattern, could help explain the large interannual variability of rainfall over the Sahel.
    Variability in boundary layer structure during HAPEX-Sahel wet-dry season transition
    Wai, M.M.K. ; Smith, E.A. ; Bessemoulin, P. ; Culf, A.D. ; Dolman, A.J. ; Lebel, T. - \ 1997
    In: HAPEX-Sahel / Goutorbe, J.P., Dolman, A.J., Gash, J.H.C., Kerr, Y.H., Lebel, T., Prince, S.D., Stricker, J.N.M., Amsterdam : Elsevier - p. 965 - 997.
    A comparison of surface fluxes at the HAPEX-Sahel fallow bush sites
    Lloyd, C.R. ; Bessemoulin, P. ; Cropley, F.D. ; Culf, A.D. ; Dolman, A.J. ; Elbers, J.A. ; Heusinkveld, B. ; Moncrieff, J.B. ; Monteny, B. ; Verhoef, A. - \ 1997
    In: HAPEX-Sahel / Goutorbe, J.P., Dolman, A.J., Gash, J.H.C., Kerr, Y.H., Lebel, T., Prince, S.D., Stricker, J.N.M., Amsterdam : Elsevier - p. 400 - 425.
    Observations of boundary layer development during the HAPEX-Sahel intensive observation period
    Dolman, A.J. ; Culf, A.D. ; Bessemoulin, P. - \ 1997
    In: HAPEX-Sahel / Goutorbe, J.P., Dolman, A.J., Gash, J.H.C., Kerr, Y.H., Lebel, T., Prince, S.D., Stricker, J.N.M., Amsterdam : Elsevier - p. 998 - 1016.
    Observations of boundary layer development during the HAPEX-Sahel intensive observation period
    Dolman, A.J. ; Culf, A.D. ; Bessemoulin, P. - \ 1997
    Journal of Hydrology 188/189 (1997). - ISSN 0022-1694 - p. 998 - 1016.
    microklimaat - planten - grenzen - hydrologie - evapotranspiratie - neerslag - sahel - microclimate - plants - boundaries - hydrology - evapotranspiration - precipitation - sahel
    The radiosonde measurement programme of the boundary layer during the Hydrologic Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel) is described. Typical examples of boundary layer evolution at two sites for wet and dry periods are shown. The Central Site was consistently dryer and warmer than the Southern Site and also reached higher boundary layer depths in the dry period in October. A brief analysis of surface flux variability is used to determine the errors associated with using a single average value for boundary layer input. Budget calculations using a mixed layer model show that advection plays an important role in the early morning budgets of heat and moisture. In the dry period subsidence also contributed to boundary layer heating.
    A comparison of surface fluxes at the HAPEX-Sahel fallow bush sites.
    Lloyd, C.R. ; Bessemoulin, P. ; Cropley, F.D. ; Culf, A.D. ; Dolman, A.J. ; Elbers, J. ; Moncrieff, J. ; Monteny, B. ; Verhoef, A. - \ 1997
    Journal of Hydrology 188-189 (1997)1-4. - ISSN 0022-1694 - p. 400 - 425.
    microklimaat - planten - grenzen - plantengemeenschappen - vegetatie - onderzoek - sahel - microclimate - plants - boundaries - plant communities - vegetation - research - sahel
    The variability between surface flux measurements at the fallow sites of the three HAPEX-Sahel supersites is examined over periods of three or four consecutive days. A roving eddy correlation instrument provided a common base for comparison at each supersite. The inhomogeneity of the surface and the instrumental layout did not provide the conditions to allow the separation of the effects of instrument error from those due to the spatial variability of vegetation cover and soil moisture. Surface fluxes of sensible and latent heat and energy balance terms were intercompared at each supersite over summation timescales of 1 hour and 3 days. It is shown that, generally, HAPEX-Sahel hourly sensible heat flux and latent heat values have confidence limits of 15% and 20% respectively. The three-day period energy balance shows the combined sensible and latent heat fluxes to have a confidence limit of 3%. It is concluded that, due to the averaging effect of longer time periods and larger flux footprints on spatial inhomogeneity, confidence in the surface flux measurements increases with longer summation periods and with neutral atmospheric surface layers which characterise the rainy period of the Intensive Observation Period.
    The variability of evaporation during the HAPEX-Sahel Intensive Observation Period.
    Gash, J.H.C. ; Kabat, P. ; Monteny, B. ; Amadou, M. ; Bessemoulin, P. ; Billing, H. ; Blyth, E.M. ; DeBruin, H.A.R. ; Elbers, J.A. ; Friborg, T. ; Harrison, G. ; Holwill, C.J. - \ 1997
    Journal of Hydrology 188/189 (1997). - ISSN 0022-1694 - p. 385 - 399.
    hydrologie - evapotranspiratie - gegevens verzamelen - sahel - hydrology - evapotranspiration - data collection - sahel
    The variation in evaporative fraction and actual evaporation is examined for three sample days in the HAPEX-Sahel Intensive Observation Period (IOP), including data from all the vegetation types and sites. The trends in evaporative fraction over the IOP are also presented for eight sites. The high rate of evaporation from bare soil in the days following rainfall produces a variability in evaporation which makes differences between sites difficult to interpret on a day-to-day basis, but over the whole IOP it is shown that the millet uses a smaller proportion of the available energy for evaporation than the tiger bush or fallow savannah. The combined effect of differences in the total energy used and its partitioning into evaporation and sensible heat flux is demonstrated from the trends in cumulative total energy use and evaporation at the three southern sites, where it is shown that there is systematically less evaporation from the millet than from the savannah or tiger bush sites
    Geographical, biological and remote sensing aspects of the Hydrologic Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel)
    Prince, S.D. ; Kerr, Y.H. ; Goutorbe, J.P. ; Lebel, T. ; Tinga, A. ; Bessemoulin, P. ; Brouwer, J. ; Dolman, A.J. ; Engman, E.T. ; Gash, J.H.C. ; Hoepffner, M. ; Kabat, P. ; Monteny, B. ; Said, F. ; Sellers, P. ; Wallace, J. - \ 1995
    Remote Sensing of Environment 51 (1995). - ISSN 0034-4257 - p. 215 - 234.
    toepassingen - microklimatologie - remote sensing - sahel - bodemwaterbalans - statistiek - applications - microclimatology - remote sensing - sahel - soil water balance - statistics
    HAPEX-Sahel was an international programme focused on the soil-plant-atmosphere energy, water and carbon balances in the West African Sahel. It was aimed at improving our understanding of the interaction between the Sahel and the general atmospheric circulation, providing a baseline for studies of climate change. It was carried out in West Niger during three to four years with an intensive observation period from August to October 1992. An interdisciplinary approach was adopted with contributed studies in hydrology and soil moisture, surface fluxes and vegetation, remote sensing science, and meteorology and mesoscale modelling. An information system has been established for data dissemination.
    HAPEX-Sahel: a large-scale study of land-atmosphere interactions in the semi-arid tropics.
    Goutorbe, J.P. ; Lebel, T. ; Tinga, A. ; Bessemoulin, P. ; Brouwer, J. - \ 1994
    Annales geophysicae 12 (1994). - ISSN 0755-0685 - p. 53 - 64.
    A large-scale study of land-atmosphere interactions in the semi-arid tropics (HAPEX-Sahel).
    Goutorbe, J.P. ; Lebel, T. ; Tinga, A. ; Bessemoulin, P. ; Brouwer, J. - \ 1993
    In: Proc. Yokohama Symp. Exchange processes at the land surface for a range of space and time scales. AHS Publ. 212 - p. 357 - 361.
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