DOI: 10.26050/wdcc/uaicon_gw_na
title: UA-ICON Current-Climate Simulation: Northwest America Gravity Wave Forcing (NA)

description: Project: UA-ICON Current-Climate Simulations with Localized Gravity Wave Forcing Sensitivity Experiments - This project investigates the role of gravity waves (GWs) in shaping middle atmosphere dynamics under present-day climate conditions. Specifically, it explores how localized GW forcing in Northern Hemisphere hotspot regions, including the Himalayas, East Asia, and North America, affects the circulation in the middle atmosphere and the vertical coupling between atmospheric layers. This includes the Arctic stratospheric polar vortex and its downward influence on polar and midlatitude climate. The project employs the UA-ICON high-top general circulation model in a long-term ensemble simulation setup. These ensemble simulations provide a robust basis for assessing the climatic impacts of regionally intensified GW forcing, including its influence on the polar vortex state—such as sudden stratospheric warmings, exceptionally strong vortex events associated with the ozone hole, and vertical coupling between atmospheric layers. The project aims to improve understanding of how spatially localized, non-zonal dynamical forcings in the stratosphere can drive remote climate effects. This contributes to reducing uncertainties in climate predictions by revealing key mechanisms linking regional GW activity to large-scale atmospheric variability. The project is funded by the Deutsche Forschungsgemeinschaft (DFG) through grant No. KA 5835/3-1 (https://gepris.dfg.de/gepris/projekt/516378721) and through Project Number 268020496 – TRR 172, within the Transregional Collaborative Research Center “Arctic Amplification: Climate Relevant Atmospheric and Surface Processes, and Feedback Mechanisms (AC)³” (https://ac3-tr.com/). This work utilized resources of the Deutsches Klimarechenzentrum (DKRZ), granted by its Scientific Steering Committee (WLA) under project ID bb1438.
Summary: This experiment provides output from a six-member ensemble simulation using the UA-ICON (Upper Atmosphere ICON) general circulation model, version 2.6.6. The model setup uses a high-top configuration with 120 vertical levels extending to ~150 km altitude and a horizontal resolution of ICON R2B4 (~160 km), allowing detailed representation of processes in the troposphere, stratosphere, mesosphere, and lower thermosphere. The experiment uses a time step of 360 seconds and includes a comprehensive physics package (ICON numerical weather prediction, NWP, package), notably gravity wave (GW) parameterizations for both subgrid-scale orographic (SSO) and non-orographic (NO) sources. Radiative transfer is handled via the ecRad radiation scheme.

The experiment includes six ensemble members to account for internal atmospheric variability. Initial conditions are based on ERA5 climatology (1979–2022). Ensemble 1 uses the mean January 1 state from ERA5 data (1979–2022), while ensembles 2–6 each exclude one year (1984, 1992, 2000, 2008, or 2016) to introduce slight variations in the initial conditions. Each ensemble simulation spans 30 years, with the first year treated as spin-up and excluded from output, resulting in a data range from 1991-01-01 to 2019-12-31 (arbitrarily numbered dates). All simulations are conducted under seasonally repeating boundary conditions to represent a stationary present-day climate. Sea surface temperature and sea ice are based on ERA5 climatology (1979–2022), greenhouse gas concentrations follow CMIP6 historical means (1979–2020), and ozone climatology is derived from MACC and GEMS datasets.

The key experimental perturbation is an artificial tenfold enhancement of the stratospheric SSO drag component within the Northwest America region (NA: 30°–60°N, 100°–130°W). The scaling factor of 10 was determined experimentally, ensuring that the enhanced drag remains within the range of natural variability and preserves realistic dynamical forcing. It is the only applied external perturbation in this experiment. This approach is designed to isolate the influence of this known GW hotspot on atmospheric dynamics and circulation patterns. In this experiment, Unlike the other ensemble members, which span 30 years, ensemble member 5 terminated early in 2005 due to numerical instability. the output of the ensemble is archived from 1991 onward, excluding the spin-up year. The dataset is well-suited for studying Northwest America stratospheric GW forcing effects on stratospheric variability, polar vortex dynamics, and for quantifying signal-to-noise characteristics via ensemble analysis.

Daily-mean atmospheric fields are stored as monthly NetCDF files over the global domain on model levels. Variables include 3D fields (temperature, winds, pressure, vertical velocity), GW drag tendencies (SSO and NO), and surface values such as 2-meter temperature and surface pressure.