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Convective Triggering Potential

Using morning vertical profiles (e.g. atmospheric soundings), the convective triggering potential and low-level dew point depression (CTP-Hilow) identify whether convection is more likely to be initiated over dry versus wet soils.

Quantifies how primed the atmosphere is to moist convection. The CTP measures the stability, and Hilow measures saturation.

When constrained by observations or model estimates, the CTP-Hilow spaces can be divided into several regions that favor moist convection: 1) Atmospherically controlled regions that is not sensitive to surface flux changes, 2) a region favoring moist convection over wet soils, 3) a region favoring convection over dry soils, and 4) a transition region that is not clearly in the dry or wet region. See Figure 15 in Part I.

When evaporative fraction available and CTP-Hilow is calculated, a statistical technique can be applied to quantify a triggering feedback strength (TFS) and amplified feedback strength (AFS). These two metrics measure how afternoon rainfall frequency and intensity vary with evaporative fraction, respectively (Findell et al. 2011).

After delineating between dry, wet, transitional, and atmospherically controlled regimes using the CTP-Hilow space (CTP being on the x-axis and Hilow on the y-axis) and soil moisture, a time series of regimes can be constructed. This time series of preferential regimes is then used to calculate a Coupling Drought Index by subtracting the number wet advantage days from the number of dry advantage days and dividing by the total number of days analyzed. This metric is a way of relating drought to land-atmosphere coupling and could potentially be used to improve drought forecasting going foward (Roundy et al. 2013).

Required Input Data:

Needs vertical profiles of early morning soundings only.

temperature

specific humidity

pressure

surface sensible heat flux

surface pressure

Distinctions between wet and dry soil moisture advantages may be dependent on the particular model used to determine when convection occurs. The levels used to integrate over for CTP may need to vary depending on location. Same goes for the Hilow variable.

Prototype Subroutine Call

subroutine ctp_hi_low ( nlev_in , tlev_in, qlev_in, plev_in, &
t2m_in , q2m , psfc_in, &
CTP , HILOW ,
missing )

Required Input

Output

How to Calculate

1
Integrate between the moist adiabat and the environmental lapse rate between 100mb and 300mb above the ground.
2
Calculate the sum of dew point depression from 50mb and 150mb above the ground.
3
Delineations between wet and dry soil advantages for triggering convection are returned from idealized 1-D boundary layer model simulations.

Relevant Citations

Method Description 

Findell, K. L., and E. A. B. Eltahir, 2003: Atmospheric controls on soil moisture-boundary layer interactions. Part I: Framework development. J. Hydrometeor., 4, 552-569., doi:10.1175

Findell, K. L., and E. A. B. Eltahir, 2003: Atmospheric controls on soil moisture-boundary layer interactions. Part II: Feedbacks within the continental United States. J. Hydrometeor., 4, 570-583., doi:10.1175

  Applications Beyond 

Joshua K. Roundy, Craig R. Ferguson, and Eric F. Wood, 2013: Temporal Variability of Land–Atmosphere Coupling and Its Implications for Drought over the Southeast United States. J. Hydrometeor, 14, 622–635. doi:10.1175/JHM-D-12-090.1

Findell, K. L., Gentine, P., Lintner, B. R. & Kerr, C. 2011: Probability of afternoon precipitation in eastern United States and Mexico enhanced by high evaporation. Nature Geosci. 4, 434–439. doi:10.1038/ngeo1174

Trent W. Ford, Anita D. Rapp, and Steven M. Quiring, 2015: Does Afternoon Precipitation Occur Preferentially over Dry or Wet Soils in Oklahoma?. J. Hydrometeor, 16, 874–888. doi: 10.1175/JHM-D-14-0005.1