Enables the separation of factors contributing to changes in relative humidity at the top of the boundary layer, and therefore the factors relating to cumulus formation. Specifically, non-evaporative terms (dry entrainment, boundary layer growth, and boundary layer heating) can be separated from the evaporative fraction contribution to cloud formation. See Figure 10 in Ek and Holtslag (2004).
The physical pathway leading to increasing relative humidity at the top of the convective boundary layer can also be better understood by looking at the time evolution of each contributing factor during hours prior to cumulus formation. See Table 1 from Ek and Holtslag (2004).
Sensitivity tests that vary soil moisture content can be performed to explore whether drier or wetter soils tend to favor cumulus formation. See Table 2 from Ek and Holtslag (2004).
Can also be calculated from observations where atmospheric soundings and surface fluxes are available at sub-daily timescales. Westra et al. (2012)
Needs vertical profiles of
surface latent heat flux
surface sensible heat flux
boundary layer height
Best suited for well-mixed convective boundary layers.
Prototype Subroutine Call
How to Calculate
M. Ek and L. Mahrt, 1994: Daytime Evolution of Relative Humidity at the Boundary Layer Top. Mon. Wea. Rev., 122, 2709–2721. doi: 10.1175/1520-0493
M. B. Ek and A. A. M. Holtslag, 2004: Influence of Soil Moisture on Boundary Layer Cloud Development. J. Hydrometeor, 5, 86–99. doi: 10.1175/1525-7541  Detailed Evaluation
D. Westra, G. J. Steeneveld, and A. A. M. Holtslag, 2012: Some Observational Evidence for Dry Soils Supporting Enhanced Relative Humidity at the Convective Boundary Layer Top. J. Hydrometeor, 13, 1347–1358. doi: 10.1175/JHM-D-11-0136.1