Description

The Hybrid EDMF scheme is a first-order turbulent transport scheme used for subgrid-scale vertical turbulent mixing in the PBL and above. It blends the traditional first-order approach that has been used and improved over the last several years with a more recent scheme that uses a mass-flux approach to calculate the countergradient diffusion terms.

The PBL scheme's main task is to calculate tendencies of temperature, moisture, and momentum due to vertical diffusion throughout the column (not just the PBL). The scheme is an amalgamation of decades of work, starting from the initial first-order PBL scheme of Troen and Mahrt (1986) [93], implemented according to Hong and Pan (1996) [41] and modified by Han and Pan (2011) [34] and Han et al.(2015) [35] to include top-down mixing due to stratocumulus layers from Lock et al. (2000) [63] and replacement of counter-gradient terms with a mass flux scheme according to Siebesma et al.(2007) [89] and Soares et al. (2004) [90]. Recently, heating due to TKE dissipation was also added according to Han et al.(2015) [35].

The scheme works on a basic level by calculating background diffusion coefficients and updating them according to which processes are occurring in the column. The most important difference in diffusion coefficients occurs between those levels in the PBL and those above the PBL, so the PBL height calculation is of utmost importance. An initial estimate is calculated in a "predictor" step in order to calculate Monin-Obukhov similarity values and a corrector step recalculates the PBL height based on updated surface thermal characteristics. Using the PBL height and the similarity parameters, the diffusion coefficients are updated below the PBL top based on Hong and Pan (1996) [41] (including counter-gradient terms). Diffusion coefficients in the free troposphere (above the PBL top) are calculated according to Louis (1979) [68] with updated Richardson number-dependent functions. If it is diagnosed that PBL top-down mixing is occurring according to Lock et al. (2000) [63] , then then diffusion coefficients are updated accordingly. Finally, for convective boundary layers (defined as when the Obukhov length exceeds a threshold), the counter-gradient terms are replaced using the mass flux scheme of Siebesma et al. (2007) [89] . In order to return time tendencies, a fully implicit solution is found using tridiagonal matrices, and time tendencies are "backed out." Before returning, the time tendency of temperature is updated to reflect heating due to TKE dissipation following Han et al. (2015) [35] .

Intraphysics Communication

Argument Table

General Algorithm

Hybrid EDMF General Algorithm