Description

The GSL drag suite, 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 are 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) [109] and Choi and Hong (2015)[42] 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)[109], with the code also originating from the WRF and RAP/HRRR. The small-scale orographic GWD scheme is based on Steeneveld et al.(2008)[184] and Tsiringakis et al. (2017) [191], 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 ~20km down to ~2km as per Beljaars et al.(2004) [15].

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

Intraphysics Communication

Argument Table

General Algorithm

GFS Orographic GWD Scheme General Algorithm