Surface Temperature and Surface-Layer Turbulence in a Convective Boundary Layer
Previous laboratory and atmospheric experiments have shown that turbulence influences the surface temperature in a convective boundary layer. The main objective of this study is to examine land-atmosphere coupled heat transport mechanism for different stability conditions. High frequency infrared imagery and sonic anemometer measurements were obtained during the boundary layer late afternoon and sunset turbulence (BLLAST) experimental campaign. Temporal turbulence data in the surface-layer are then analyzed jointly with spatial surface-temperature imagery. The surface-temperature structures (identified using surface-temperature fluctuations) are strongly linked to atmospheric turbulence as manifested in several findings. The surface-temperature coherent structures move at an advection speed similar to the upper surface-layer or mixed-layer wind speed, with a decreasing trend with increase in stability. Also, with increasing instability the streamwise surface-temperature structure size decreases and the structures become more circular. The sequencing of surface- and air-temperature patterns is further examined through conditional averaging. Surface heating causes the initiation of warm ejection events followed by cold sweep events that result in surface cooling. The ejection events occur about 25 % of the time, but account for 60–70 % of the total sensible heat flux and cause fluctuations of up to 30 % in the ground heat flux. Cross-correlation analysis between air and surface temperature confirms the validity of a scalar footprint model.
KeywordsAtmospheric surface layer Convective boundary layer Infra-red imagery Surface-layer plumes Surface temperature
We thank (i) Daniel Alexander from University of Utah, USA; Dr. Marie Lothon, Dr. Fabienne Lohou, Solene Derrien from Laboratoire d’Aérologie, Université de Toulouse, France; Dr. Arnold Moene, Dr. Oscar Hartogensis, Anneke Van de Boer from Wageningen University, Netherlands for field assistance, data sharing and discussion; (ii) Peter Cottle and Anders Nottrott from University of California, San Diego for pre-experimental laboratory assistance and discussion about the data analysis respectively; (iii) BLLAST organizers for their hospitality during the experiment; (iv) funding from a NASA New Investigator Program award for AG and JK, and from INSU-CNRS (Institut National des Sciences de l’Univers, Centre national de la Recherche Scientifique, LEFE-IDAO program), Météo-France, Observatoire Midi-Pyrénées (University of Toulouse), EUFAR (EUropean Facility for Airborne Research) and COST ES0802 (European Cooperation in the field of Scientific and Technical) for the BLLAST field experiment.
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