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Column Relative Humidity and Primary Condensation Rate as Two Useful Supplements to Atmospheric River Analysis
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  • Ruping Mo,
  • Rita So,
  • Melinda M. Brugman,
  • Curtis Mooney,
  • Anthony Q. Liu,
  • Matthias Jakob,
  • Armel Castellan,
  • Roxanne Vingarzan
Ruping Mo
National Laboratory-West, Environment and Climate Change Canada, National Laboratory-West, Environment and Climate Change Canada

Corresponding Author:ruping.mo@canada.ca

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Rita So
National Laboratory-West, National Laboratory-West
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Melinda M. Brugman
National Laboratory-West, National Laboratory-West
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Curtis Mooney
National Laboratory-West, Environment and Climate Change Canada, National Laboratory-West, Environment and Climate Change Canada
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Anthony Q. Liu
National Laboratory-West, National Laboratory-West
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Matthias Jakob
BGC Engineering Inc., BGC Engineering Inc.
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Armel Castellan
Client Services, Client Services
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Roxanne Vingarzan
Applied Sciences, PSOW, Environment and Climate Change Canada, Applied Sciences, PSOW, Environment and Climate Change Canada
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Abstract

Landfalling atmospheric rivers (ARs) frequently trigger heavy and sometimes prolonged precipitation, especially in regions with favored orographic enhancement. The presence and strength of ARs are often described using the integrated water vapor (IWV) and the integrated vapor transport (IVT). However, the associated precipitation is not directly correlated with these two variables. Instead, the intensity of precipitation is mainly determined by the net convergence of moisture flux and the initial degree of saturation of the air column. In this study, a simple algorithm is proposed for estimating the heavy precipitation attributable to the IVT convergence. Bearing a strong resemblance to the Kuo-Anthes parameterization scheme for cumulus convection, the proposed algorithm calculates the large-scale primary condensation rate (PCR) as a proportion of the IVT convergence, with a reduction to account for the general moistening in the atmosphere. The amount of reduction is determined by the column relative humidity (CRH), which is defined as the ratio of IWV to its saturation counterpart. Our analysis indicates that the diagnosable PCR compares well to the forecast precipitation rate given by a numerical weather prediction model. It is also shown that the PCR in an air column with CRH < 0.50 is negligibly small. The usefulness of CRH and PCR as two complements to standard AR analysis is illustrated in three case studies. The potential application of PCR to storm classification is also explored.