Convection waves: Observations of gravity wave systems over convectively active boundary layers

Widespread gravity wave systems have been found to exist over convectively active boundary layers (CBLs) in the presence of vertical wind shear. In contrast to mountain waves, these ‘convection waves’ occur over flat terrain and are ubiquitous over fields of shallow fair weather cumuli or clear air thermals. They extend vertically to at least 9km a.m.s.l., probably to the tropopause. First discovered by glider pilots as ‘thermal waves’, they have now been systematically investigated by research aircraft during the NCAR Convection Wave Project, which also incorporates an effort in numerical simulation. the flight results are reported in this paper, while the numerical simulations are described in a companion paper. Typical wavelengths of convection waves were found to range from 5 to 15 km, (average 9 km) and typical vertical motion amplitudes from ± 1 to ± 3 m s −1 . In all cases vertical wind shear exceeded 3×10 −3 s −1 . Under these conditions cloud tops were measured to move with relative velocities of 8 to 10 m s −1 against their environment, hence they present an obstacle to the surrounding airflow. the analogy to mountainous obstacles launching gravity waves into the atmosphere is suggestive and borne out by the aforementioned numerical simulations, which describe this process step by step. The interaction between the CBL and the overlying stable layers is studied by spectral and cross‐spectral analysis of aircraft data including coherence and phase relations between wave motions and cloud population underneath. Consideration is given to other potential sources of the observed gravity waves. At this stage the conclusions of our pilot study may be considered tentative. Some implications for further research are also discussed.