2.2 Model Physicsoptions chosen for the study – Thephysics categories are (1) microphysics, (2) cumulus parameterization, (3)planetary boundary layer (PBL), (4) land-surface model, and (5) radiation.1. Microphysics – Microphysics includes watervapor (that is explicitly resolved), cloud, and precipitation processes. The modelis general enough to accommodate any number of mass mixing-ratio variables, andother quantities such as number concentrations.
Four-dimensional arrays withthree 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 allocateddepending on the needs of the scheme chosen, and advection of the species alsoapplies to all those required by the microphysics option. In the currentversion of the ARW, microphysics is carried out at the end of the time-step asan adjustment process, and so does not provide tendencies. The rationale forthis is that adjustment for condensation should be at the end of the time-stepto guarantee that the final saturation balance is accurate for the updatedtemperature and moisture. However, it is also important to have the latentheating forcing for potential temperature during the following dynamical steps,and this is done by saving microphysical heating as an approximation for thenext time-step.Thefollowing scheme is implemented to include moisture variables, and whetherice-phase and mixed-phase processes are included (mixed-phase processes arethose that result from the interaction of ice and water particles, such asriming that produces graupel or hail) – WRFSingle-Moment (WSM) 5-class scheme2. Cumulus Parametrization – These schemesare responsible for the sub-grid-scale effects of convective and/or shallowclouds. The schemes are intended to represent vertical fluxes due to unresolvedupdrafts and downdrafts and compensating motion outside the clouds.
Theyoperate only on individual columns where the scheme is triggered and providevertical heating and moistening profiles. Some schemes additionally providecloud and precipitation field tendencies in the column, and future schemes mayprovide momentum tendencies due to convective transport of momentum. Theschemes all provide the convective component of surface rainfall. Cumulusparameterizations are theoretically only valid for coarser grid sizes, (e.g.
,greater than 10 km), where they are necessary to properly release latent heaton a realistic time scale in the convective columns. While the assumptionsabout the convective eddies being entirely sub-grid-scale break down for finergrid sizes, sometimes these schemes have been found to be helpful in triggeringconvection in applications with a 5-10 km grid size. Generally, they should notbe used when the convective eddies are resolvable by the model (e.g.
, ? 5 kmgrid).Becauseof the domain size being less than 5 km, cumulus parametrization is notincluded in this study.3.
Surface Layer – The surface layerschemes calculate “friction velocities” and exchange coefficients that enable thecalculation of surface heat and moisture fluxes by the land-surface models andsurface stress in the planetary boundary layer scheme. Over water surfaces, thesurface fluxes and surface diagnostic fields are computed within the surfacelayer scheme. The schemes provide no tendencies, only the stability-dependentinformation about the surface layer for the land-surface and PBL schemes.
Currently, every surface layer option is tied with a given specific PBL options.Notethat some boundary layer schemes (YSU and MRF) require the thickness of thesurface layer in the model to be representative of the actual surface layer(e.g. 50-100 meters).Thescheme implemented in this study is the Monin-Obukhov (Janjic eta) scheme, inorder to compound the effect of the “viscous sub-layer” due to the variableroughness height (due to BEP) for temperature profiling.4. Land-Surface Model (LSM) – Theland-surface models (LSMs) combine the forcing generated from the surface layerscheme, the radiative forcing from the radiation scheme, and the precipitationforcing from the microphysics and convective schemes.
This is then put togetherwith the land surface variables and properties to estimate the correspondingheat and moisture fluxes over the land points of interest. These fluxes providea lower boundary condition for the vertical transport done in the PBL schemes. Theland-surface models have various degrees of sophistication in dealing withthermal and moisture fluxes in multiple layers of the soil and also may handlevegetation, root, and canopy effects and surface snow-cover prediction. Theland surface model provides no tendencies, but does update the land’s statevariables which include the ground (skin) temperature, soil temperatureprofile, soil moisture profile, snow cover, and possibly canopy properties. Theland-surface model implemented in this study is the Noah LSM which is a unifiedcode for research and operational purposes, being almost identical to the codeused in the NCEP North American Mesoscale Model (NAM). This has the benefit ofbeing consistent with the time-dependent soil fields provided in the analysisdatasets.
This is a 4-layer soil temperature and moisture model with canopymoisture and snow cover prediction. The scheme provides sensible and latentheat fluxes to the boundary-layer scheme. The Noah LSM additionally predictssoil ice, and fractional snow cover effects, has an improved urban treatment,and considers surface emissivity properties.
5. Planetary Boundary Layer (PBL) – Theplanetary boundary layer (PBL) is responsible for vertical sub-grid-scalefluxes due to eddy transports in the whole atmospheric column, not just theboundary layer. Thus, when a PBL scheme is activated, explicit verticaldiffusion is de-activated with the assumption that the PBL scheme will handlethis process.
The most appropriate horizontal diffusion choices are made suchthat horizontal and vertical mixing are treated independently. The surfacefluxes are provided by the surface layer and land-surface schemes. The PBLschemes determine the flux profiles within the well-mixed boundary layer andthe 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 effectsin the vertical stability that determines the mixing. The schemes areone-dimensional, and assume that there is a clear scale separation betweensub-grid eddies and resolved eddies. This assumption will become less clear atgrid sizes below a few hundred meters, where boundary layer eddies may start tobe resolved, and in these situations the scheme should be replaced by a fullythree-dimensional local sub-grid turbulence scheme such as the TKE diffusionscheme.
ThePBL scheme implemented in this study is the Mellor-Yamada-Janjic (Eta) TKEscheme. In this implementation, an upper limit is imposed on the master lengthscale. This upper limit depends on the TKE as well as the buoyancy and shear ofthe driving flow. In the unstable range, the functional form of the upper limitis derived from the requirement that the TKE production be nonsingular in thecase of growing turbulence.
In the stable range, the upper limit is derivedfrom the requirement that the ratio of the variance of the vertical velocitydeviation and TKE cannot be smaller than that corresponding to the regime ofvanishing turbulence. The TKE production/dissipation differential equation issolved iteratively. 2.3 Model parametersand field specificsThecontrol runs and sensitivity runs were performed for two different dayscorresponding 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 studybecause of the noticeably aberrant weather conditions observed on both the daysin terms of net solar input and cloud cover. Both the runs were performed overa same set of domains on a 2-way nesting basis; centered over the College Parkarea (38.
9897 N, 76.9378 W).Theouter domain was of a grid size of 3.
6 km X 3.6 km. The inner domains are of areduced grid size, by a factor of 3 from the outer domain containing them. The 2mair temperature was estimated by accounting for the urban heat flux (with theground flux) in the net flux equation. The skin temperature was estimated fromthe heat budget at the surface.
