Gravity wave drag parameterization developments in the Whole Atmosphere Community Climate Model (WACCM)
Abstract
Parameterized gravity-wave (GW) drag plays an important role in transporting sub-grid scale momentum in atmospheric models; these waves force the large-scale circulations responsible for phenomena such as the cold summer polar mesosphere, warm winter polar stratosphere and the quasi biennial oscillation. The development and improvement of parameterization schemes has thus been of some importance in models that include the middle atmosphere. However, some difficulties remain including the long-standing "cold-pole problem" where the southern winter polar stratospheric temperature is too low. GW drag parameterization (GWDP) scheme designs compress the broad, multi-parameter wave spectrum in a variety of ways in order to simplify the parameterization task. These simplifications are influenced by aspects of model design such as grid-cell and time step size, as well as by observations. Associated tuning parameters have allowed these schemes to be successfully applied within the limitations described above. This relative success has led to stability in GWDP design. Much effort has been applied to comparing model parameterizations to observations. The relationship between GWDP design and our ability to measure momentum flux (the key GWDP parameter) over the needed GW parameter ranges has limited the success of these comparisons and the ability to draw conclusions from them. It is usual practise to separate the parameterization of orographic GW from those generated by other processes. It is also common to combine non-orographic GWs and use a simple factor (intermittency) to describe source activity. The Whole Atmosphere Community Climate Model (WACCM) developers have been working for some time to further separate non-orographic GWs into specific source processes. Recent work in this area has sought to identify an observation-driven improvement path for the WACCM GWDP schemes. This process has highlighted ongoing difficulties in comparing observations to current parameterizations. In this paper, the relationship between model design and recent observations, (including the presence of a log-normal spectrum of momentum fluxes,) and the role of aggregation of the GW spectrum in space and time, will be explored. Changes to elements of the WACCM GWDP scheme will be described and some preliminary results will be presented.
- Publication:
-
43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E.704M