Influence of Soil Moisture Deficit on Local and Regional Climate

Currently most people in the US reside in large metropolitan areas that consist of a dense urban core with high concentration of built spaces (mostly impermeable) surrounded by vast suburban enclaves that are a mix of built and natural surfaces (permeable). In large metropolitan areas this creates a gradient in water retention and heat storage capacity along the urban-rural transect. Both these factors combine to create an urban microclimate that is distinctly different compared to natural ecosystems. While the implications of increased thermal storage capacity is well understood, the role played by soil moisture deficit in shaping the climate at local and regional levels is poorly understood. To quantify this variability, we have instituted a dense surface observatory network in the New York Metropolitan area. This unique urban micro-network consists instrumentation to monitor soil moisture at multiple depths along with air temperature, relative humidity and precipitation (this is ongoing work which began very recently). We are also conducting numerical experiments to understand the impact of urban soil moisture deficit on local climate. In one such simulation conducted to partition the influence of storage flux, wind pattern and circulation and soil moisture deficit on urban heat island intensity (UHI), we found that the daily variability in UHI in NYC was sensitive to available energy and wind pattern. The long-term trend in UHI was however related to soil moisture deficit. In fact a prolonged heat wave period witnessed during summer 2006 correlated well with an extended dry period and the daily UHI in NYC almost doubled. Moreover, the urban soils also suffered from high degree of dessication, owing to drier urban boundary layer.