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Interaction of Surface-Atmosphere Processes Relating Soil-Moisture Conditions to Summer Rainfall Chung, Sung-Rae Dept. o

Interaction of Surface-Atmosphere Processes Relating Soil-Moisture Conditions to Summer Rainfall Chung, Sung-Rae Dept. of Atmospheric Science Yonsei University. Wheat field under clouded sky by Vincent van Gogh. Motivation.

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Interaction of Surface-Atmosphere Processes Relating Soil-Moisture Conditions to Summer Rainfall Chung, Sung-Rae Dept. o

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  1. Interaction of Surface-Atmosphere Processes Relating Soil-Moisture Conditions to Summer Rainfall Chung, Sung-Rae Dept. of Atmospheric Science Yonsei University Wheat field under clouded skyby Vincent van Gogh

  2. Motivation • The land surface characteristic such as soil moisture is associated with the flood due to the extreme rainfall. • It is need to understand the feedback mechanism of soil moisture-rainfall to explain whether the anomalous soil moisture conditions played a role in initiating or enhancing the extremes in rainfall or whether they are simply a by-product of these extremes. Effects of soil moisture on future rainfall by performing a series of numerical experiments where initial soil moisture conditions are varied. Key Reference: Pathways Relating Soil Moisture to Future Summer Rainfall within a Model of the Land-Atmosphere System (J.S. Pal and E.A.B. Eltahir 2001, J. Climate)

  3. Wet soil moisture conditions Decrease Bowen ratio Decreases sfc albedo Increase WV in PBL Decrease skin & sfc air temp Increase net sfc SW Increase net sfc LW Increase net sfc radiation Increase total sfc heat flux into PBL Decrease PBL depth Increase MSE per unit depth of PBL Decrease LCL & wet-bulb depression Increase moist convection (rainfall) Increase cloud amount Feedback mechanism of soil moisture-rainfall Clear sky

  4. Numerical Model • NCAR Regional Climate Model (RegCM) • Radiation scheme: CCM3-based • PBL scheme: nonlocal (Holtslag et al. 1990) • Resolvable cloud & precipitation scheme: Hsie et al. (1984) • Cumulus convection scheme: modified Kuo • Land-surface scheme: Biosphere–Atmosphere Transfer Scheme (BATS) • Initial & Boundary conditions: NCEP reanalysis data • SST: UKMO data • Soil moisture: Illinois State Water Survey; Hydrogeology; and a vegetation-based climatology datasets (HDG/ISWS)

  5. Design of Experiments • Model domain • center: 40.5N, 90W • size: 2050 km X 2500 km • grid point spacing: 50 km • Simulations • simulation months: May, Jun, Jul, Aug, and Sep of 1988 (drought year) and 1993 (flood year) • simulation duration: 1 month • control run: HDG/ISWS soil moisture • sensitivity runs: initial soil saturations of 10%, 25%, 50%, 75%, and 90% uniformly over entire domain and depth • initialized on the 1st of each month

  6. Sensitivity of future rainfall to initial soil moisture Rainfall(mm/day) 50-60% increase virtually remain ~250% increase Initial soil saturation The response of rainfall to change in soil moisture is largely due to changes in bare soil evaporation, some interception loss, and some transpirationfrom the upper soil layer.

  7. Model response to the timing of the soil moisture anomaly Rainfall % change /soil saturation % change Rainfall (mm/day) / unit soil saturation change

  8. Soil moisture-rainfall pathways Surface Radiation (W/m*m) Initial soil saturation

  9. Heat flux (W/m*m)

  10. deg C KJ/kg g/kg

  11. Summary and Conclusions • Soil moisture’s impact on both the energy and water budgets proves to be crucial in determining the strength of the soil moisture–rainfall feedback. • Soil moisture rainfall–feedback is more responsive to a negative soil moisture perturbation than to a positive perturbation. This suggests that the soil moisture–rainfall feedback favors droughts in comparison with floods over the experiment region. • The simulations indicate that the soil moisture–rainfall feedback remains strong when the model is initialized at observed extremes in soil saturation. Based on these model results, one would conclude that soil moisture did play a significant role in maintaining the persistence patterns of the drought of 1988 and the flood of 1993. • During the late spring and summer, the strength of the soil moisture–rainfall feedback displays little dependence on the timing of the soil moisture anomaly. This suggests that knowledge of the soil moisture conditions during any of these months can improve the predictability of rainfall.

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