The are included (mixed-phase processes are those

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
explicitly resolved water vapor, 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.

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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 condensation adjustment 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 dynamical sub-steps, and
this is done by saving the 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 5–10 km grid applications. Generally, they should not be used
when the model can resolve the convective eddies itself (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 in the surface layer
scheme itself. The schemes provide no tendencies, only the stability-dependent
information about the surface layer for the land-surface and PBL schemes.

Currently, each surface layer option is tied to particular boundary-layer options,
but in the future more interchangeability and options may become available.

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) use atmospheric information from the surface layer
scheme, radiative forcing from the radiation scheme, and precipitation forcing
from the microphysics and convective schemes, together with internal
information on the land’s state variables and land-surface properties, to
provide heat and moisture fluxes over land points and sea-ice points. These
fluxes provide a lower boundary condition for the vertical transport done in
the PBL schemes (or the vertical diffusion scheme in the case where a PBL
scheme is not run, such as in large-eddy mode). Note that large-eddy mode with
interactive surface fluxes is not yet available in the ARW, but is planned for
the near future. 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. There is no horizontal interaction between neighboring points in
the LSM, so it can be regarded as a one-dimensional column model for each WRF
land grid-point, and many LSMs can be run in a stand-alone mode.

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.