Nonorographic inertia‐gravity waves over New Zealand's Southern Alps: A case study

Wind profiles from radiosondes launched over New Zealand on June 29, 2014 during the Deep Propagating Gravity Wave Experiment (DEEPWAVE; June–July 2014) showed an interesting feature of two sharp peaks in wind speed: one in the upper troposphere and the other in the lower stratosphere. Analysis showed that the lower peak at around 11.5 km was associated with the upper‐tropospheric jet. Inertia‐gravity waves (IGWs) were found over the South Island from the upper troposphere to the lower stratosphere. The IGWs perturbed the environmental winds, leading to strong and weak wind layers that were tilted in a westerly direction and extended from 10 to 16 km in the IGW zone over the South Island. As a result, the upper wind peak was observed at ~14.5 km and the weak winds immediately below at ~13 km in the IGW zone. These IGWs had vertical wavelengths of ~3 km, horizontal wavelengths of 300–400 km, periods of 9–10 h, and a phase speed of ~11 m∙s−1. Numerical experiments showed that airflow over New Zealand's Southern Alps was not the main source for these IGWs. Further analysis suggested that the source of these IGWs in the lower stratosphere was likely due to the spontaneous adjustment of airflow associated with the upper‐tropospheric jet streak. Inertia‐gravity waves (IGWs) with the horizontal and vertical wavelengths of ~400 and ~ 3 km, respectively, were found in the upper troposphere and the lower stratosphere over the South Island of New Zealand on June 29, 2014 during DEEPWAVE. The IGWs perturbed environmental winds, leading to strong and weak wind layers that were tilted in a westerly direction and extended from 10 to 16 km in the IGW zone over the South Island. Mountain waves are the dominant waves over the South Island of New Zealand. However, to our surprise, numerical experiments showed that airflow over New Zealand's Southern Alps was not the main source for these IGWs.