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GFS Unified UGWP Scheme


The "unified_ugwp" scheme was the first "unification" of the GSL Drag Suite Scheme with the Unified Gravity Wave Physics Scheme - Version 0 orographic and non-stationary gravity wave drag schemes. It was coded as a testing platform to determine the optimal combination of schemes to use for future physics prototypes. For the large-scale orographic gravity wave drag (OGW) and blocking scheme, the user may specify either those of the Unified Gravity Wave Physics Scheme - Version 0 or those of the GSL Drag Suite Scheme. The GSL Drag Suite includes two new "small-scale" orographic drag schemes, which may be used in combination with either "large-scale" orographic schemes. Finally, the non-stationary gravity wave drag (NGW) scheme is that of the Unified Gravity Wave Physics Scheme - Version 0 scheme.

Description of the GSL Drag Suite:

The GSL Drag Suite Scheme, developed by NOAA's Global Systems Laboratory, is a set of subgrid-scale orographic drag parameterizations that calculate momentum tendencies due to the effects of unresolved topography. The drag forces they represent are those due to: 1) large-scale gravity (mountain) waves that propagate vertically and break in the free atmosphere of the troposphere, stratosphere and above; 2) low-level flow blocking; 3) small-scale gravity wave drag (GWD) due to mountain waves generated in stable planetary boundary layer (PBL) conditions, typically at nighttime, which break at or near the PBL top; and 4) turbulent orographic form drag (TOFD), which is generated by turbulent pressure perturbations that are correlated with the terrain slope. The distinction between the large-scale and small-scale gravity waves is that the former are generated by topography with horizontal scales on the order of 5 km and greater, which can support vertical propagation through the typical static stabilities found in the free atmosphere, while the latter are generated by topography with smaller horizontal scales down to about 1 km, which can support vertical propagation only in very stable conditions, typically found in nocturnal PBLs.

The large-scale GWD scheme is based on Kim and Doyle (2005)[106] and Choi and Hong (2015) [38] and the code originated from the NCAR Weather Research and Forecasting (WRF) model and NOAA RAP/HRRR. The low-level blocking scheme is adapted from Kim and Doyle (2005) [106], with the code also originating from the WRF and RAP/HRRR. The small-scale orographic GWD scheme is based on Steeneveld et al (2008) [179] and Tsiringakis et al. (2017) [184], and the TOFD scheme is adapted from Beljaars et al. (2004) [15].

All four orographic GWD schemes require static input data files that contain statistical information about the subgrid terrain within each model grid cell, such as the standard deviation of the subgrid topography, which comes from the high resolution USGS 30-second GMETED2010 dataset. These data files augment the usual "oro_data.tile*.nc" files, which contain orographic height data and GWD static data for the GFSv16 GWD parameterizations. The static data files for the large-scale GWD and blocking schemes are named "oro_data_ls.tile*.nc". The source topography for the datasets are calculated from a 2.5-minute lat-lon grid to filter out small-scale topographic variations. The static data files for the small-scale GWD and TOFD schemes are named "oro_data_ss.tile*.nc". The data is from the 30-second topographic dataset, but band-passed filtered from approximately 20 km down to approximately 2 km as per Beljaars et al. (2004)[15].

The large-scale GWD and blocking schemes are explicitly tapered off from horizontal grid resolutions starting at approximately 13 km down to approximately 3 km resolution, at and below which the scheme is not active.

Description of the non-stationary GWD scheme of the Unified Gravity Wave Physics Scheme - Version 0 scheme:

The NGW physics scheme parameterizes the effects of non-stationary waves unresolved by dynamical cores. These non-stationary oscillations with periods bounded by Coriolis and Brunt-Väisälä frequencies and typical horizontal scales from tens to several hundreds of kilometers, are forced by the imbalance of convective and frontal/jet dynamics in the troposphere and lower stratosphere (Fritts (1984) [64]; Alexander et al.(2010) [2]; Plougonven and Zhang 2014 [158]). The NGWs propagate upwards and the amplitudes exponentially grow with altitude until instability and breaking of waves occur. Convective and dynamical instability induced by GWs with large amplitudes can trigger production of small-scale turbulence and self-destruction of waves. The latter process in the theory of atmospheric GWs is frequently referred as the wave saturation (Lindzen(1981) [117]; Weinstock(1984) [187]; Fritts(1984) [64]). Herein, “saturation” or "breaking" refers to any processes that act to reduce wave amplitudes due to instabilities and/or interactions arising from large-amplitude perturbations limiting the exponential growth of GWs with height. Background dissipation processes such as molecular diffusion and radiative cooling, in contrast, act independently of GW amplitudes. In the middle atmosphere, impacts of NGW saturation (or breaking) and dissipation on the large-scale circulation, mixing, and transport have been acknowledged in the physics of global weather and climate models after pioneering studies by Lindzen(1981) [117] and Holton(1983) [89]. Comprehensive reviews on the physics of NGWs and OGWs in climate and weather models have been discussted in Alexander et al. 2010 [2], Geller et al. 2013 [71], and Garcia et al. 2017 [68]. They are formulated using different aspects of the nonlinear and linear propagation, instability, breaking and dissipation of waves along with different specifications of GW sources (Garcia et al. 2007 [67]; Richter et al 2010 [161]; Eckermann et al. 2009 [48]; Eckermann 2011 [49]; Lott et al. 2012 [123]).

The concept of UGWP was first proposed and implemented in the Unified Forecast System (UFS) with model top at different levels by scientists from the University of Colorado Cooperative Institute for Research in the Environmental Sciences (CIRES) at NOAA's Space Weather Prediction Center (SWPC) and from NOAA's Environmental Modeling Center (EMC) (Alpert et al. 2019 [4]; Yudin et al. 2016 [192]; Yudin et al. 2018 [193]). The UGWP considers identical GW propagation solvers for OGWs and NGWs with different approaches for specification of subgrid wave sources. The current set of the input and control parameters for UGWP version 0 (UGWP v0) enables options for GW effects, including momentum deposition (also called GW drag), heat deposition, and mixing by eddy viscosity, conductivity and diffusion; however, note that the eddy mixing effects induced by instability of GWs are not activated in this version.

Namelist parameters control the number of directional azimuths in which waves can propagate, number of waves in a single direction, and the level above the surface at which NGWs can be launched. Among the input parameters, the GW efficiency factors reflect intermittency of wave excitation. They should vary with horizontal resolution, reflecting the capability of the dynamical core to resolve mesoscale wave activity with the enhancement of model resolution.

Prescribed distributions for vertical momentum flux (VMF) of NGWs have been employed in global numerical weather prediction and reanalysis models to ease tuning of GW schemes to the climatology of the middle atmosphere dynamics in the absence of the global wind data above about 35 km (Eckermann et al. 2009 [48]; Molod et al. 2015 [137]). These distributions of VMF qualitatively describe the general features of the latitudinal and seasonal variations of the global GW activity in the lower stratosphere, observed from the ground and space (Ern et al. 2018 [53]). Subgrid GW sources can also be parameterized to respond to year-to-year variations of solar input and anthropogenic emissions (Richter et al 2010 [161]; 2014 [162]).

Note that in UGWP v0, the momentum and heat deposition due to GW breaking and dissipation have been tested in the multi-year simulations and medium-range forecasts using a configuration of the UFS weather model using 127 levels with model top at approximately 80 km.

Along with the GW heat and momentum depositions, GW eddy mixing is an important element of the Whole Atmosphere Model (WAM) physics, as shown in WAM simulations with the spectral dynamics (Yudin et al. 2018 [193]). The impact of eddy mixing effects in the middle and upper atmosphere, which is not included in this version, need to be tested, evaluated, and orchestrated with the representation of the subgrid turbulent diffusion and the numerical dissipation.

The representation of subgrid GWs is particularly important for WAMs that extend into the thermosphere (top lid at about 600 km). In the mesosphere and thermosphere, the background attenuation of subgrid waves due to molecular and turbulent diffusion, radiative damping and ion drag will be the additional mechanism of NGW and OGW dissipation along with convective and dynamical instability of waves described by the linear (Lindzen 1981 [117]) and nonlinear (Weinstock 1984 [187]; Hines 1997 [86]) saturation theories.

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General Algorithm