Coherent structures and flux contribution over an inhomogeneously irrigated cotton field
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The turbulence data measured at two levels (i.e., 8.7 and 2.7 m) in the Energy Balance Experiment (EBEX), which was conducted in San Joaquin Valley in California during the period from July 20 to August 24, 2000, are used to study the characteristics of coherent structures over an irrigated cotton field. Patch-to-patch irrigation in the field generated the dry-to-wet horizontal advection and the oasis effects, leading to the development of a stably internal boundary layer (SIBL) in the late mornings or the early afternoons. The SIBL persisted in the rest of the afternoons. Under this circumstance, a near-neutral atmospheric surface layer (ASL) developed during the period with a stratification transition from the unstable to stable conditions during the daytime. Therefore, EBEX provides us with unique datasets to investigate the features of coherent structures that were generated over the patches upstream and passed by our site in the unstable ASL, the near-neutral ASL, and the SIBL. We use an objective detection technique and the conditional average method that is developed based on the wavelet analysis. Our data reveal some consistencies and inconsistencies in the characteristics of coherent structures as compared with previous studies. Ramp-like structures and sweep–ejection cycles under the daytime SIBL have similar patterns to those under the nocturnal stable ASL. However, some features (i.e., intermittence) are different from those under the nocturnal stable ASL. Under the three stratifications, thermal and mechanical factors in the ASL perform differently in affecting the ramp intensity for different quantities (i.e., velocity components, temperature, and specific humidity), leading to coherent structures that modulate turbulence flow and alter turbulent transfer differently. It is also found that coherent structures contribute about 10–20% to the total fluxes in our case with different flux contributions under three stratifications and with higher transporting efficiency in sensible heat flux than latent heat and momentum fluxes.
KeywordsCoherent Structure Convective Boundary Layer Atmospheric Surface Layer Cotton Field Internal Boundary Layer
Each participant in EBEX was funded primarily through his or her own institution, and some contributed personal resources. Funding for the deployment of NCAR facilities was provided by the National Science Foundation. Arrangement for use of the field site was facilitated by Bruce Roberts, Director of the University of California Cooperative Extension, Kings County. Westlake Farms generously provided both use of the land for this experiment and helped with logistical support. We are grateful to all of these people and organizations. Steven Oncley is especially acknowledged for his great efforts in organizing EBEX. We acknowledge support from NSF under AGS0847549. Yu Zhang’s Post-doc work was partly supported by NOAA Howard-NCAS under grant no. NA06OAR4810172. Heping Liu’s participation in EBEX was partly supported by City University of Hong Kong (grant 8780046 and SRG 7001038). We thank two anonymous reviewers for their valuable comments.
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