A Study of Near-Surface Boundary Layer Characteristics During the 2015 Chennai Flood in the Context of Urban-Induced Land Use Changes
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Chennai metropolitan region along with the northern part of Tamil Nadu and southern part of Andhra Pradesh witnessed extreme rainfall events leading to urban flooding during November–December 2015. In order to understand the near-surface and boundary layer (BL) characteristics during the event in the context of land use and land cover (LULC) change, three decades of satellite images are analysed. In addition, the Weather Research Forecasting (WRF) model is used to perform finer-scale simulations, considering different land use (LU) data sets as input. For this purpose, LU datasets from the United Sates Geological Survey (USGS), Moderate Resolution Imaging Spectro-radiometer (MODIS), and Indian Space Research Organization (ISRO) are considered along with the Noah and Noah multi-physics (NMP) land surface model (LSM). Impact of Noah-based LSMs on the near-surface and BL characteristics over Chennai are examined besides the change in LULC during the flood event. Significant improvement of 1–2 °C is obtained in case of near-surface temperature in the simulation considering recent LU and the NMP LSM. Some WRF-simulated variables like near-surface temperature, relative humidity (RH) and convective available potential energy (CAPE) are compared with available observations for qualitative and quantitative analysis. The distorted variations of the near-surface and boundary layer parameters including temperature, BL height, sensible heat flux and CAPE, are mostly observed during phases with prevalent low-pressure systems due to the presence of large-scale forcing. In other phases, (where low-pressure systems are absent), with dominance of localised effects, noticeably higher values of the variables viz. near-surface air temperature, wind speed, RH and moisture flux, CAPE and BL height are attributed to the increased impervious layers inside the city boundary due to urbanization and its growth.
KeywordsUrban flood Chennai urbanization WRF boundary layer
The authors would like to acknowledge the IMD (http://www.imd.gov.in), ECMWF (http://apps.ecmwf.int/datasets), NCAR-UCAR (http://rda.ucar.edu) and University of Wyoming weather website (http://weather.uwyo.edu) for providing the necessary data sets to carry out the research work. We would also like to thank Mr. Bijay Kumar Guha of NIT, Rourkela, for his technical help and Mr. Falguni Muduli for helping in data collection. The authors are thankful to the anonymous reviewers for their valuable feedback, which helped in the overall improvement of the manuscript.
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