Abstract

Using Weather Research and Forecasting model (WRF) simulations with different initial soil moisture (ISM) conditions, we investigate the sensitivity to ISM for the three severe heat wave events dominating eastern China in 2003, 2007 and 2013. The control simulations are able to reproduce the spatial distributions and the daily evolutions for each of the three heat waves but apparently underestimate their amplitudes, intensities and spatial extensions. The decreased ISM could cause an enhancement on heat waves with increased amplitudes, extents and intensities, while it has insignificant influence on the spatial distributions and temporal variations. The responses of heat waves are generally decreasing with the increasing ISM, controlled by different regimes in the surface soil moisture-temperature relationship. Through enhanced sensible flux as well as reduced latent cooling, the initial soil dryness locally strengthens the surface warming and the further drying of the soil. The three heat waves were all dominated by high-pressure systems in the mid-troposphere. The reduced ISM forces positive anomalies of geopotential height at mid-troposphere and negative anomalies at lower levels, leading to an enhanced thickness of the atmosphere. Such a thickened atmosphere can strengthen the anomalous high-pressure systems, favoring the maintenance of severe heat waves. This acts as a positive feedback between atmospheric circulation, surface warming and soil dryness.

Highlights

  • Characterized by a period of consecutive days with abnormally high temperatures, heat waves have been known as one of the typical extreme weather events across the world (Perkins, 2015)

  • We investigated the impacts of initial soil moisture on the three severe observed heat wave events in 2003, 2007, and 2013 through WRF simulations

  • Results show that the CTL simulation is able to reproduce the spatial distributions and the daily evolutions for each of the three heat waves but apparently underestimates their amplitudes, intensities, and spatial extensions

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Summary

Introduction

Characterized by a period of consecutive days with abnormally high temperatures, heat waves have been known as one of the typical extreme weather events across the world (Perkins, 2015). The impacts of soil moisture on surface air temperature are mostly expected to be induced by its role for evapotranspiration in soil moisture-limited regimes, depicted in the relationship between soil moisture and evaporative fraction (EF), the ratio of latent heat flux to the total available energy (Seneviratne et al, 2010; Berg et al, 2014). In the transitional regime ( θWILT ≤ θ ≤ θCRIT), EF reacts effectively to the changes of soil moisture In this case, the dryness of the soil results in very low EF, with the constrained total energy used by the latent heat flux and more energy available for sensible heating. Dry soil conditions along with persistent high-pressure systems would amplify the soil moisture-temperature feedback and enhance surface warming (Rohini et al, 2016)

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