Numerical Modeling of the Excitation, Propagation, and Dissipation of Primary and Secondary Gravity Waves during Wintertime at McMurdo Station in the Antarctic

Abstract

Abstract We analyze the results of the gravity wave (GW)‐resolving, high‐resolution Kühlungsborn Mechanistic general Circulation Model in July at McMurdo Station (166.69°E and 77.84°S), where strong downslope eastward winds create strong mountain wave (MW) events. These MWs have horizontal wavelengths of λ H ≃230 km, propagate to z ∼ 40–60 km, and can have upward phases in time if the eastward wind accelerates in time. Additionally, inertia‐GWs (IGWs) with λ H ∼ 500–800 km and ground‐based periods of τ r ∼ 5–6 hr are generated in the troposphere from unbalanced, large‐scale flow. The density‐scaled GW amplitudes are ∼10 times smaller at z ∼ 80–100 km than at z < 50 km because of severe wave dissipation. “Fishbone” structures are seen at z ∼ 30–60 km with upward (downward) phases in time below (above) the “knee” at z knee . We horizontally filter the perturbations to isolate the GWs in a fishbone structure for a particular MW event. We find that these GWs have strikingly similar parameters below and above z knee =46 km, with ground‐based horizontal phase speeds of c H ∼ 40–60 m/s, τ r ∼ 9–10 hr, λ H ∼ 1,600–2,050 km, vertical wavelengths of λ z ∼ 18–25 km, and azimuths of Υ= 145° –151° east of north. We show that these are secondary GWs excited by a body force at z knee created by MW dissipation approximately 400 km northwest of McMurdo 2.5 hr earlier and that the secondary GW scales and propagation directions are consistent with this force. Importantly, we show that most of the GWs at z > 70 km are secondary GWs not primary GWs from the troposphere.