Spatiotemporal Dynamics Of Surface Urban Heat Island In Coastal Mega Cities Along The Belt And Road And The Impact Of Urbanization On The Suhi In Karachi

Urbanization is the most significant human activity process since the 20^th century,and with the urbanization process,the unique environmental problem,urban heat ialand,not only affects the local and regional climate of the city,but also has a significant negative effect on the living comfort and energy consumption of the urban residents.The study of urban heat island effect is of great significance for monitoring local,reginoal and even global climate change and guiding urban development planning.Firstly,taking the coastal mega cities along the Belt and Road as study areas,and based on the MODIS land surface temperature products from 2001 to 2017 and land use classification data from Landsat,the spatiotemporal changes of the surface urban heat island effect were analyzed from the seasonal and inter-annual perspectives in the coastal mega cities by using the Surface Urban Heat Island Intensity?SUHII?as an indicator.Then,taking a typical mega city,Karachi,as an example,based on the Landsat land use classification data of 1990,2000,2010 and 2019,RS and GIS technologies were used to analyze the land use cover changes and urban expansion in Karachi in the past 30 years.Finally,based on the land surface temperature data and land use classification data from Landsat in 2000,2010 and 2019,the influence of land use cover characteristics on surface urban heat island in Karachi was studied.The analysis results showed that:?1?During 2001-2017,the high intensity urban heat island of mega cities were mainly distributed in the densely populated core areas of the cities;Among the 10megacities,the annual average SUHII of Karachi was strongest with the value of3.02?,and the SUHII of Chennai showed a significant upward trend?0.07?/year,P<0.1?;There were seasonal differences in the urban heat island among the mega cities.In summer,the average SUHII of Istanbul was strongest with the value of2.88?.In winter,the average SUHII of Karachi was the strongest with the value of4.45?.?2?The areas of building land in Karachi showed a continuous upward trend,and it mainly expanded by occupying the bare land/bare rock;During 1990 to 2019,the total expansion areas of Karachi's building land was approximately 254.50 km²,which mainly occurred in the periods of 1990-2000 and 2010-2019,with the values of151.00 and 99.92 km²,respectively,accounting for 98.60%of the total expansion areas.The fastest period of urban expansion occurred in 1990-2000,with an expansion rate of 15.10 km²/a,and the expansion dynamic degree was 7.46%.?3?The shoreline length of Karachi showed a significant extension trend?9.51km/a,P<0.05?,and the average values of the End Point Rate?EPR?and the Linear Regression Rate?LRR?of the shorelines were 4.97 and 4.63 m/a,respectively;During1990-2019,the total areas of land accretion and erosion in Karachi coast were about31.55 km²,among which the land accretion areas were 30.10 km²,accouting for 95%of the total changed areas.The land accretion of the coast in Karachi mainly occurred in 1990-2000 and 2010-2019,with the areas of 16.11 and 13.50 km²,respectively.?4?The sequence of land use cover types with the land surface temperature from high to low in Karachi was:bare land/bare rock>building land>terrestrial vegetation>beach/sand land>mangrove foreast;There was a significant positive correlation between the Land Surface Temperature?LST?and the Normalized Difference Built-up Index?NDBI?in Karachi;In 2000 and 2010,the LST of Karachi showed a positive correlation relationship with the Normalized Difference Vegetation Index?NDVI?,this maybe because the impervious surface of Karachi has played a controlling role in the impact on the LST,thereby weakening the impact of the natural surface on the LST.