Interaction of Surface-Atmosphere Processes Relating Soil-Moisture Conditions to Summer Rainfall
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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. of Atmospheric Science

Yonsei University

Wheat field under clouded skyby Vincent van Gogh


Motivation l.jpg
Motivation Soil-Moisture Conditions to Summer Rainfall

  • 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)


Feedback mechanism of soil moisture rainfall l.jpg

Wet soil moisture conditions Soil-Moisture Conditions to Summer Rainfall

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


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Numerical Model Soil-Moisture Conditions to Summer Rainfall

  • 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)


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Design of Experiments Soil-Moisture Conditions to Summer Rainfall

  • 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


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Sensitivity of future rainfall to initial soil moisture Soil-Moisture Conditions to Summer Rainfall

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.


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Model response to the timing of the soil moisture anomaly Soil-Moisture Conditions to Summer Rainfall

Rainfall % change /soil saturation % change

Rainfall (mm/day) / unit soil saturation change


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Soil moisture-rainfall pathways Soil-Moisture Conditions to Summer Rainfall

Surface Radiation (W/m*m)

Initial soil saturation


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Heat flux (W/m*m) Soil-Moisture Conditions to Summer Rainfall


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deg C Soil-Moisture Conditions to Summer Rainfall

KJ/kg

g/kg


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Summary and Conclusions Soil-Moisture Conditions to Summer Rainfall

  • 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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