@article{fdi:010048238,
  title = {{S}urface thermodynamics and radiative budget in the {S}ahelian {G}ourma : seasonal and diurnal cycles},
  author = {{G}uichard, {F}. and {K}ergoat, {L}. and {M}ougin, {E}ric and {T}imouk, {F}ranck and {B}aup, {F}. and {H}iernaux, {P}. and {L}avenu, {F}ran{\c{c}}ois},
  editor = {},
  language = {{ENG}},
  abstract = {{O}ur understanding of the role of surface-atmosphere interactions in the {W}est {A}frican monsoon has been particularly limited by the scarcity of measurements. {T}he present study provides a quantitative analysis of the very pronounced seasonal and diurnal cycles of surface thermodynamics and radiative fluxes in the {C}entral {S}ahel. {I}t makes use of data collected from 2002 to 2007 in the {M}alian {G}ourma, close to {A}goufou, at 1.5 degrees {W}-15.3 degrees {N} and sounding data collected during the {AMMA} field campaign. {T}he seasonal cycle is characterized by a broad maximum of temperature in {M}ay, following the first minimum of the solar zenith angle ({SZA}) by a few weeks, when {A}goufou lies within the {W}est {A}frican {H}eat {L}ow, and a late summer maximum of equivalent potential temperature (0e) within the core of the monsoon season, around the second yearly maximum of {SZA}. {D}istinct temperature and moisture seasonal and diurnal dynamics lead to a sharpening of the early (late) monsoon increase (decrease), more steadiness of 0e and larger changes of relative humidity in between. {R}ainfall starts after the establishment of the monsoon flow, once temperature already started to decrease slowly, typically during {J}une. {S}pecific humidity increases progressively from {M}ay until {A}ugust, while the monsoon flow weakens during the same period. {S}urface net radiation ({R}-net) increases from around 10-day mean values of 20 {W} m(-2) in {W}inter to 120-160 {W} m(-2) in late {S}ummer, {T}he increase is sharper during the monsoon than before, and the decrease fast. {T}he seasonal cycle of {R}-net arises from distinct shortwave and longwave fluctuations that are both strongly shaped by modifications of surface properties related to rainfall events and vegetation phenology (with a decrease of both surface longwave emission and albedo). {D}uring the monsoon, clouds and aerosols reduce the incoming solar radiation by 20-25% (about 70 {W} m(-2)). {T}hey also significantly enhance the day-to-day variability of {R}-net. {N}evertheless, the surface incoming longwave radiative flux ({LW}in) is observed to decrease from {J}une to {S}eptember. {A}s higher cloud covers and larger precipitable water amounts are typically expected to enhance {LW}in, this feature points to the significance of changes in atmospheric temperature and aerosols during the monsoon season. {T}he strong dynamics associated with the transition from a drier hot {S}pring to a brief cooler moist tropical {S}ummer climate involves large transformations of the diurnal cycle, even within the monsoon season, which significantly affect both thermodynamical, dynamical and radiative fields (and low-level dynamics). {I}n particular, for all moist {S}ummer months except {A}ugust, specific humidity decreases in such a way during daytime that it prevents an afternoon increase of 0e. {I}n agreement with some previous studies, strong links are found between moisture and {LW}net all year long and a positive correlation is identified between {R}-net and 0e during the monsoon. {T}he observational results presented in this study further provide valuable ground truth for assessing models over an area displaying a rich variety of surface-atmosphere regimes.},
  keywords = {{S}ahel ; {M}onsoon ; {S}urface ; {R}adiative flux ; {L}ongwave ; {S}hortwave},
  booktitle = {},
  journal = {{J}ournal of {H}ydrology},
  volume = {375},
  numero = {1-2},
  pages = {161--177},
  ISSN = {0022-1694},
  year = {2009},
  DOI = {10.1016/j.jhydrol.2008.09.007},
  URL = {https://www.documentation.ird.fr/hor/fdi:010048238},
}