2.2 Model Physics

options chosen for the study –

The

physics categories are (1) microphysics, (2) cumulus parameterization, (3)

planetary boundary layer (PBL), (4) land-surface model, and (5) radiation.

1.

Microphysics – Microphysics includes water

vapor (that is explicitly resolved), cloud, and precipitation processes. The model

is general enough to accommodate any number of mass mixing-ratio variables, and

other quantities such as number concentrations. Four-dimensional arrays with

three spatial indices and one species index are used to carry such scalars.

Memory, i.e., the size of the fourth dimension in these arrays, is allocated

depending on the needs of the scheme chosen, and advection of the species also

applies to all those required by the microphysics option. In the current

version of the ARW, microphysics is carried out at the end of the time-step as

an adjustment process, and so does not provide tendencies. The rationale for

this is that adjustment for condensation should be at the end of the time-step

to guarantee that the final saturation balance is accurate for the updated

temperature and moisture. However, it is also important to have the latent

heating forcing for potential temperature during the following dynamical steps,

and this is done by saving microphysical heating as an approximation for the

next time-step.

The

following scheme is implemented to include moisture variables, and whether

ice-phase and mixed-phase processes are included (mixed-phase processes are

those that result from the interaction of ice and water particles, such as

riming that produces graupel or hail) – WRF

Single-Moment (WSM) 5-class scheme

2.

Cumulus Parametrization – These schemes

are responsible for the sub-grid-scale effects of convective and/or shallow

clouds. The schemes are intended to represent vertical fluxes due to unresolved

updrafts and downdrafts and compensating motion outside the clouds. They

operate only on individual columns where the scheme is triggered and provide

vertical heating and moistening profiles. Some schemes additionally provide

cloud and precipitation field tendencies in the column, and future schemes may

provide momentum tendencies due to convective transport of momentum. The

schemes all provide the convective component of surface rainfall.

Cumulus

parameterizations are theoretically only valid for coarser grid sizes, (e.g.,

greater than 10 km), where they are necessary to properly release latent heat

on a realistic time scale in the convective columns. While the assumptions

about the convective eddies being entirely sub-grid-scale break down for finer

grid sizes, sometimes these schemes have been found to be helpful in triggering

convection in applications with a 5-10 km grid size. Generally, they should not

be used when the convective eddies are resolvable by the model (e.g., ? 5 km

grid).

Because

of the domain size being less than 5 km, cumulus parametrization is not

included in this study.

3.

Surface Layer – The surface layer

schemes calculate “friction velocities” and exchange coefficients that enable the

calculation of surface heat and moisture fluxes by the land-surface models and

surface stress in the planetary boundary layer scheme. Over water surfaces, the

surface fluxes and surface diagnostic fields are computed within the surface

layer scheme. The schemes provide no tendencies, only the stability-dependent

information about the surface layer for the land-surface and PBL schemes.

Currently, every surface layer option is tied with a given specific PBL options.

Note

that some boundary layer schemes (YSU and MRF) require the thickness of the

surface layer in the model to be representative of the actual surface layer

(e.g. 50-100 meters).

The

scheme implemented in this study is the Monin-Obukhov (Janjic eta) scheme, in

order to compound the effect of the “viscous sub-layer” due to the variable

roughness height (due to BEP) for temperature profiling.

4.

Land-Surface Model (LSM) – The

land-surface models (LSMs) combine the forcing generated from the surface layer

scheme, the radiative forcing from the radiation scheme, and the precipitation

forcing from the microphysics and convective schemes. This is then put together

with the land surface variables and properties to estimate the corresponding

heat and moisture fluxes over the land points of interest. These fluxes provide

a lower boundary condition for the vertical transport done in the PBL schemes. The

land-surface models have various degrees of sophistication in dealing with

thermal and moisture fluxes in multiple layers of the soil and also may handle

vegetation, root, and canopy effects and surface snow-cover prediction. The

land surface model provides no tendencies, but does update the land’s state

variables which include the ground (skin) temperature, soil temperature

profile, soil moisture profile, snow cover, and possibly canopy properties.

The

land-surface model implemented in this study is the Noah LSM which is a unified

code for research and operational purposes, being almost identical to the code

used in the NCEP North American Mesoscale Model (NAM). This has the benefit of

being consistent with the time-dependent soil fields provided in the analysis

datasets. This is a 4-layer soil temperature and moisture model with canopy

moisture and snow cover prediction. The scheme provides sensible and latent

heat fluxes to the boundary-layer scheme. The Noah LSM additionally predicts

soil ice, and fractional snow cover effects, has an improved urban treatment,

and considers surface emissivity properties.

5.

Planetary Boundary Layer (PBL) – The

planetary boundary layer (PBL) is responsible for vertical sub-grid-scale

fluxes due to eddy transports in the whole atmospheric column, not just the

boundary layer. Thus, when a PBL scheme is activated, explicit vertical

diffusion is de-activated with the assumption that the PBL scheme will handle

this process. The most appropriate horizontal diffusion choices are made such

that horizontal and vertical mixing are treated independently. The surface

fluxes are provided by the surface layer and land-surface schemes. The PBL

schemes determine the flux profiles within the well-mixed boundary layer and

the stable layer, and thus provide atmospheric tendencies of temperature, moisture

(including clouds), and horizontal momentum in the entire atmospheric column.

Most PBL schemes consider dry mixing, but can also include saturation effects

in the vertical stability that determines the mixing. The schemes are

one-dimensional, and assume that there is a clear scale separation between

sub-grid eddies and resolved eddies. This assumption will become less clear at

grid sizes below a few hundred meters, where boundary layer eddies may start to

be resolved, and in these situations the scheme should be replaced by a fully

three-dimensional local sub-grid turbulence scheme such as the TKE diffusion

scheme.

The

PBL scheme implemented in this study is the Mellor-Yamada-Janjic (Eta) TKE

scheme. In this implementation, an upper limit is imposed on the master length

scale. This upper limit depends on the TKE as well as the buoyancy and shear of

the driving flow. In the unstable range, the functional form of the upper limit

is derived from the requirement that the TKE production be nonsingular in the

case of growing turbulence. In the stable range, the upper limit is derived

from the requirement that the ratio of the variance of the vertical velocity

deviation and TKE cannot be smaller than that corresponding to the regime of

vanishing turbulence. The TKE production/dissipation differential equation is

solved iteratively.

2.3 Model parameters

and field specifics

The

control runs and sensitivity runs were performed for two different days

corresponding to 2 different seasonal configurations – 08/04/2017 (during the mid-summers)

and 11/17/2017 (during early winter). The 2 days were chosen for this study

because of the noticeably aberrant weather conditions observed on both the days

in terms of net solar input and cloud cover. Both the runs were performed over

a same set of domains on a 2-way nesting basis; centered over the College Park

area (38.9897 N, 76.9378 W).

The

outer domain was of a grid size of 3.6 km X 3.6 km. The inner domains are of a

reduced grid size, by a factor of 3 from the outer domain containing them. The 2m

air temperature was estimated by accounting for the urban heat flux (with the

ground flux) in the net flux equation. The skin temperature was estimated from

the heat budget at the surface.