Precipitation Efficiencies in a Climatology of Southern Ocean Extratropical Cyclones

Precipitation efficiency refers to the amount of water that is lost from the atmosphere through precipitation compared to the available water vapor 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 hr near the warm front where ascent rates are the largest. Typically, within 24-36 hr after cyclogenesis, all of the initial water vapor 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 (MFC), which replenish the moisture lost via precipitation. Close to the cyclone center, MFC provides additional moisture from the environment into which cyclones are traveling. On average, this extends a cyclone's precipitation phase to over 60 hr 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. Precipitation efficiency is a measure of how much water in the atmosphere falls as rain compared to how much is available to fall. In this study, we estimate how efficient precipitation is for 400 cyclones in the Southern Ocean. We find that when the cyclones are strongest, about 60% of the available water vapor turns into rain every 6 hr near the warm front of the cyclone where air is rising rapidly. Normally, within a day or two of a cyclone forming, all of the available water vapor in the cyclone gets turned into rain. However, the rain can persist for longer because cyclones pick up more moisture as they move, which gets turned into more rain. This can make the rain from a cyclone last on average for 60 hr, even after the initial moisture is all gone. The initial moisture content of extratropical cyclones is removed via precipitation within 30 hr after cyclogenesisLocal evaporation and moisture flux convergence doubles the precipitating phase of extratropical cyclonesThe feeder airstream provides a continuous supply of moisture to cyclones in the developing stage of their evolution