Silvio Kalisch

Interaction of gravity waves with the QBO: A satellite perspective

One of the most important dynamical processes in the tropical stratosphere is the quasi-biennial oscillation (QBO) of the zonal wind. Still, the QBO is not well represented in weather and climate models. To improve the representation of the QBO in the models, a better understanding of the driving of the QBO by atmospheric waves is required. In particular, the contribution of gravity waves is highly uncertain because of the small horizontal scales involved, and there is still no direct estimation based on global observations. We derive gravity wave momentum fluxes from temperature observations of the satellite instruments HIRDLS and SABER. Momentum flux spectra observed show that particularly gravity waves with intrinsic phase speeds <30m/s (vertical wavelengths <10km) interact with the QBO. Gravity wave drag is estimated from vertical gradients of observed momentum fluxes and compared to the missing drag in the tropical momentum budget of ERA-Interim. We find reasonably good agreement between their variations with time and in their approximate magnitudes. Absolute values of observed and ERA-Interim missing drag are about equal during QBO eastward wind shear. During westward wind shear, however, observations are about 2 times lower than ERA-Interim missing drag. This could hint at uncertainties in the advection terms in ERA-Interim. The strong intermittency of gravity waves we find in the tropics might play an important role for the formation of the QBO and may have important implications for the parameterization of gravity waves in global models.

Observations and Ray Tracing of Gravity Waves: Implications for Global Modeling

Vertical coupling by atmospheric waves is essential for the wind and temperature structure of the middle atmosphere. In particular, momentum carried by atmospheric gravity waves (GWs) governs the global circulation in the mesosphere and is for instance the reason for the cold summer mesopause. However, the small horizontal scales of GWs (tens to thousands of km) are challenging both global modeling and observations from satellite. Further, due to the small scales involved, there is a severe lack of understanding about GWs themselves, as well as dynamical phenomena involving GWs. Until recently, global observations of GWs were sparse and little was known about the global distribution of GWs, as well as their seasonal variation. Therefore, several projects in the priority program Climate And Weather of the Sun-Earth System (CAWSES) of the Deutsche Forschungsgemeinschaft (DFG) have addressed a number of the most pressing problems. Global distributions of GW activity and momentum fluxes have been derived from observations with number of satellite instruments, resulting in the first multi-year global data sets of GW parameters, covering time scales from seasonal variations up to the duration of almost a full 11-year solar cycle. In addition, seasonal and tidal variations of sporadic E layers in the ionosphere were studied in Global Positioning System (GPS) radio occultation data. Satellite observations of GWs and sporadic E layers were complemented by ground-based observations (radar and low-frequency (LF) drift measurements). All these observations, as well as accompanying modeling activities provided important constraints for GW parameterizations. Further activities addressed important aspects of GW propagation usually neglected in global modeling: GW ray tracing studies revealed the importance of non-vertical propagation of GWs and first steps were undertaken to develop an improved GW parameterization based on GW ray tracing techniques.