Precipitation efficiencies in a climatology of Southern Ocean extratropical cyclones

• Published in: Journal of Geophysical Research: Atmospheres
• Main Authors: Dacre, H. F., Martinez-Alvarado, O., Hodges, K. I.
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Geophysical Union 2023
• Subjects: Southern Ocean
• Online Access: https://centaur.reading.ac.uk/114224/; https://centaur.reading.ac.uk/114224/8/114224%20VoR.pdf; https://centaur.reading.ac.uk/114224/1/SH_cyclones_JGR_hfd.pdf

Precipitation efficiency refers to the amount of water that is lost from the atmosphere through precipitation compared to the available water vapour in the atmosphere. This metric plays a critical role in understanding precipitation patterns. However, calculating precipitation efficiency for extratropical cyclones can be challenging because cyclones are dynamic and move through the atmosphere as they evolve. To overcome this challenge, our study uses ERA5 reanalysis data to estimate precipitation efficiencies for 400 Southern Ocean cyclones, with a frame of reference that moves with the individual cyclones. Our findings indicate that at maximum intensity, average precipitation efficiencies reach a maximum of 60$\%$/6 hours near the warm front where ascent rates are the largest. Typically, within 24-36 hours after cyclogenesis, all of the initial water vapour available within 500 km of a cyclone center is lost due to precipitation. However, a cyclone's precipitating phase is prolonged due to local evaporation and moisture flux convergence, which replenish the moisture lost via precipitation. Close to the cyclone centre, moisture flux convergence provides additional moisture from the environment into which cyclones are travelling. On average, this extends a cyclone's precipitation phase to over 60 hours after cyclogenesis. Thus, while moisture from the genesis location is quickly removed from the cyclone via precipitation, cyclones are replenished by moisture along their track, which doubles the timescale for a cyclone's precipitating phase.