Characterization of the Cloud-Topped Boundary Layer at the Synoptic Scale Using AVHRR Observations during the SEMAPHORE Experiment

• Main Authors: Mathieu, A., Seze, G., Lahellec, A., Guerin, C., Weill, Alain
• Other Authors: Centre d'étude des environnements terrestre et planétaires (CETP), Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Laboratoire de Météorologie Dynamique (UMR 8539) (LMD), Institut national des sciences de l'Univers (INSU - CNRS)-École polytechnique (X), Institut Polytechnique de Paris (IP Paris)-Institut Polytechnique de Paris (IP Paris)-École des Ponts ParisTech (ENPC)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Département des Géosciences - ENS Paris, École normale supérieure - Paris (ENS-PSL), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-École normale supérieure - Paris (ENS-PSL), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2003
• Subjects: [SDU]Sciences of the Universe [physics]; North Atlantic
• Online Access: https://hal.science/hal-03183044; https://hal.science/hal-03183044/document; https://hal.science/hal-03183044/file/1520-0450_2003_042_1720_cotcbl_2.0.co_2.pdf; https://doi.org/10.1175/1520-0450(2003)042<1720:COTCBL>2.0.CO;2

International audience Satellite platforms NOAA-11 and-12 Advanced Very High Resolution Radiometer (AVHRR) data are used during the daytime to study large sheets of stratocumulus over the North Atlantic Ocean. The application concerns an anticyclonic period of the Structure des Echanges Mer-Atmosphère, Propriétés des Hétérogénéités Océaniques: Recherché Expérimentale (SEMAPHORE) campaign (10-17 November 1993). In the region of interest, the satellite images are recorded under large solar zenith angles. Extending the SEMAPHORE area, a region of about 3000 ϫ 3000 km 2 is studied to characterize the atmospheric boundary layer. A statistical cloud classification method is applied to discriminate for low-level and optically thick clouds. For AVHRR pixels covered with thick clouds, brightness temperatures are used to evaluate the boundary layer cloud-top temperature (CTT). The objective is to obtain accurate CTT maps for evaluation of a global model. In this application, the full-resolution fields are reduced to match model grid size. An estimate of overall temperature uncertainty associated with each grid point is also derived, which incorporates subgrid variability of the fields and quality of the temperature retrieval. Results are compared with the SEMAPHORE campaign measurements. A comparison with ''DX'' products obtained with the same dataset, but at lower resolution, is also presented. The authors claim that such instantaneous CTT maps could be as intensively used as classical SST maps, and both could be efficiently complemented with gridpoint error-bar maps. They may be used for multiple applications: (i) to provide a means to improve numerical weather prediction and climatological reanalyses, (ii) to represent a boundary layer global characterization to analyze the synoptic situation of field experiments, and (iii) to allow validation and to test development of large-scale and mesoscale models.