🔎Review of Thompson Microphysics Scheme

A review of the Thompson aerosol-aware scheme and the original Thompson paper used in WRF. This is to get a better understanding of the scheme used to study marine fog and do some research on it.

Explicit Forecasts of Winter Precipitation Using an Improved Bulk Microphysics Scheme. Part II: Implementation of a New Snow ParameterizationAMETSOC

Reference of scientific paper by AMETSOC

A Study of Aerosol Impacts on Clouds and Precipitation Development in a Large Winter CycloneAMETSOC

Reference of scientific paper by AMETSOC

Documentation

Introduction

For the Thompson microphysics scheme, it is assumed that each aerosol (except for snow) follows a generalized gamma function instead of the exponential distribution.

$$N(D) = \frac{N_t}{\Gamma(\mu +1)} \lambda^{\mu +1} D^{\mu} e^{-\lambda D}$$

N_t is the total number of particles in the distribution, D is the particle diameter, alpha is the distribution’s slope, and mu is the shape parameter. When mu 0, the distribution becomes the classic exponential (or Marshall–Palmer) distribution. [1]

Aerosols play a role in cloud formation and microphysics through heterogeneous nucleation of cloud and ice particles. When aerosol concentration increases, liquid water content increases and will often lead to a larger number of liquid droplets, which are smaller in case, resulting in an increase in cloud albedo, called the first indirect effect. The second indirect effect of having smaller cloud droplets is that rain droplets are reduced or even take more time to develop, causing a delay in precipitation [2].

Important notes from the Thompson Paper

• The idea behind the Thompson microphysics is that instead of doing double moment schemes, they concentrated their efforts on understanding the fault in the other previous single moment schemes and improving on them. The Thompson scheme then, being a single moment, can behave as accurately as a double-moment scheme.

• Cloud water droplet concentration is set up as a constant value at the start of the model run. The value is 100 cm-3, which is for relatively clean air or maritime conditions but it is strongly recommended that the value is changed for ocean cases, which has a range of 75-100 cm-3.

Important notes from the Thompson Aerosol-Aware Paper

• Different regions will have different aerosol types and distribution, which can impact some weather phenomena. For example, continental regions have relatively dry air while maritime regions have relatively moist air.

• Aerosols are important for different cloud properties such as radiation, precipitation, and dynamics. Also, aerosols are important for weather applications such as fog formation and other applications sensitive to LWC or ice content.

• Compared to the Thompson scheme, cloud droplet number concentration (N_c) is solved prognostically. The Thompson scheme had a generalized gamma function, where the shape parameter was made to vary with the preset N_c number, while the Thompson Aerosol-Aware scheme N_c is predicted explicitly. The equation is given below.

• The scheme has 3 more variables to the 8 microphysics species which increases the computational cost by 16%.

• In this scheme, the N_c is explicitly predicted by the activation of aerosols as cloud condensation nuclei (CCN) and ice nuclei (IN) which are the 3 different new variables.

• The cloud droplet number concentration varies with time as follows:

$$\frac{dN_c}{dt} = -(rain, snow, graupel \text{ collecting droplets}) \\ -(freezing \text{ into cloud ice}) -(collide/coalesce \text{ into rain}) \\ -(evaporation) + (CCN \text{ activation}) + (cloud \text{ ice melting})$$

• Cloud droplets nucleate from explicit aerosol number concentration (N_wfa) using a lookup table of activated fraction determined by the model’s predicted temperature, vertical velocity, number of available aerosols, and predetermined values of hygroscopicity parameter (0.4 in experiments performed in this research) and aerosol mean radius (0.04 um).

• The activation of aerosols as droplets in the new scheme is done at cloud base as well as anywhere inside a cloud where the lookup table value is greater than the existing droplet concentration.

• The water-friendly aerosol category was designed to be a combination of sulfates, sea salts, and organic matter because these aerosols represent a significant fraction of known CCN and are found in abundance in clouds worldwide.

• The scheme is capable of representing different aerosol populations by altering the hygroscopicity and aerosol mean radius variables, although, for this study, these variables were held constant throughout.

• Subgrid turbulence on vertical velocity and nucleation of water droplets or ice were neglected.

References

• Gregory Thompson et al., “ Explicit forecasts of winter precipitation using an improved bulk microphysics scheme. part ii: Implementation of a new snow parameterization”, AMETSOC, vol. 136, no. 12, p. 5095–5115, 2008. [Online]. DOI: https://doi.org/10.1175/2008MWR2387.1

• Twomey, S., 1974: Pollution and the planetary albedo. Atmos. Environ., 8, 1251–1256, doi:10.1016/0004-6981(74)90004-3.