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  1. Studies of atmospheric angular momentum Authors: NOAA, Climate Diagnostics Center, Science Review Published: 25th/26th July, 2001 Chapter 4: Empirical and Process Studies Introduction to chapter 4, part 3: Atmospheric angular momentum (AAM) provides a convenient framework to study the role of mountains, surface wind stresses and various transport mechanisms in variability ranging from intraseasonal to interdecadal and beyond. Quantitative studies are feasible with current global assimilated datasets which show a good budget balance for global integrals, intraseasonal variations and during northern winter/spring. The budgets get much worse when gravity wave drag is included, if zonal integrals are considered or during summer/fall seasons. AAM is useful as an index of the large scale zonal flow since it is highly correlated with independent length-of-day measurements and with phenomena such as the QBO, ENSO, the MJO and possibly global warming. CDC scientists have examined several aspects of AAM variability, including: the link to MJO tropical convection, a linear model of global AAM and its torques, the global AAM budget imbalances due to gravity wave drag, the forcing for the semiannual seasonal component of AAM and the AAM response to global warming in an ensemble of coupled ocean-atmosphere model runs. CDC also monitors in real time the complete vertically integrated budget as part of its web-based maproom activities and distributes AAM and torque data to other researchers. Link to full paper: Link to Introduction to Chapter 4: Link to full Science Review:
  2. Gravity wave drag Authors: Météo-France Centre National de Recherches Météorologiques ARPEGE-Climate Version 5.2, Algorithmic Documentation Published: September, 2011 Abstract: None (part of climate model manual) Link to Chapter 13 (on Gravity Wave Drag): Link to full Manual:
  3. Improving weather forecasts via impacts of turbulent orographic form & small-scale orographic gravity wave drag on boundary layers Authors: Maarten Minkman (Master thesis) Published: November, 2017 Abstract: Numerical Weather Prediction (NWP) models have difficulties with representing stable boundary layer conditions. During these conditions NWP models need more drag than physically observed to increase the representation of cyclonics. Therefore, NWP models use a non-physical enhanced momentum drag (long-tail formulation), which negatively affects the skill for the near-surface wind speeds and boundary layer depth. To describe the extra needed drag for NWP models in a more physical way, this study investigates whether the already developed parameterizations of turbulent orographic form drag (TOFD) and small-scale orographic wave drag (GWDSBL) could describe the needed drag instead of the longtail formulation. On top of the short-tail formulation TOFD and GWDSBL were added individually and combined. The short-tail formulation is supported by observations and LES simulations. Polar WRF (version 3.7.1.) was used in a 10-day case study during the northern hemispheric winter over North America. Individual GWDSBL and TOFD both reduced the wind speeds with a bias reduction of 2% and 13% respectively compared to the short-tail formulation. As a result the 2-meter temperatures reduced locally and increased the general cold bias within WRF by 1-4◦C where the wind speeds reduced due to GWDSBL and TOFD. Besides the boundary layer the synoptic scale was affected to by GWDSBL and TOFD. The cyclonic core pressure was improved and TOFD and the combination of GWDSBL and TOFD even outperformed the long-tail formulation by reducing the biases with approx. 5%. The 500 hPa geopotential heigth was raised with more drag added to short-tail and decreased the bias. However, the Arctic and number of model levels showed a significant impact in the representation of the 500 hpa geopotential height. TOFD also reduced on average the jet stream by 1-4 m s−1 and resulted in a northward shift. In the end the combination of GWDSBL and TOFD is seen as the best option to describe the extra needed drag instead of the long-tail formulation. Link to full paper:
  4. Interannual variability in the gravity wave drag – vertical coupling and possible climate links Authors: Petr Šácha, Jiri Miksovsky and Petr Pisoft Published: 24th May, 2018 Abstract: Gravity wave drag (GWD) is an important driver of the middle atmospheric dynamics. However, there are almost no observational constraints on its strength and distribution (especially horizontal). In this study we analyze orographic GWD (OGWD) output from Canadian Middle Atmosphere Model simulation with specified dynamics (CMAM-sd) to illustrate the interannual variability in the OGWD distribution at particular pressure levels in the stratosphere and its relation to major climate oscillations. We have found significant changes in the OGWD distribution and strength depending on the phase of the North Atlantic Oscillation (NAO), quasi-biennial oscillation (QBO) and El Niño–Southern Oscillation. The OGWD variability is shown to be induced by lower-tropospheric wind variations to a large extent, and there is also significant variability detected in near-surface momentum fluxes. We argue that the orographic gravity waves (OGWs) and gravity waves (GWs) in general can be a quick mediator of the tropospheric variability into the stratosphere as the modifications of the OGWD distribution can result in different impacts on the stratospheric dynamics during different phases of the studied climate oscillations. Link to full paper: Link to the Supplement:
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