Column Relative Humidity and Primary Condensation Rate as Two Useful Supplements to Atmospheric River Analysis

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.

Concurrent extreme events of atmospheric moisture transport and continentalprecipitation: the role of landfalling atmospheric rivers

“…The moisture transport by itself will only be linked to extreme precipitation if there is an instability mechanism that forces the water vapor to rise, which in any case would inevitably result in a high convergence of the integrated water vapor flux value. As most of the moisture is confined at low levels, a high IVT convergence value implies ascents, and therefore IVT convergence implies both moisture arrival at a place and ascent, that is why its relationship with precipitation (and extreme precipitation) must necessarily be stronger than that between precipitation and the moisture transport itself (Mo et al, 2021). Some of the results reached in this paper, such as the general tendency towards a decrease in the influence of landfalling ARs in recent (warmer) periods, may be better understood if the causal linkage between moisture transport and precipitation through moisture transport convergence is taken into account.…”

“…From the amount of water vapor in an air column at a given time, it is not possible to know how much water vapor is involved in precipitation, the water vapor in the column is changing, it is necessary to know how much water vapor transported from the surrounding areas converges in the column. In fact, for actual extreme precipitation events, the amount of precipitation is much greater than the highest amount of moisture measured in the air column at a given time, indicating that, depending on the time scale taken to define the water vapor content, the convergence of water vapor is the most determining factor in the amount of moisture that will result in precipitation (Benton and Estoque, 1954;Mo et al, 2021). Large moisture transports do not guarantee large moisture convergence.…”

An anomalous atmospheric river linked to the late June 2021 western North America heatwave

“…ARs are often defined based on the vertically integrated vapour transport (IVT) derived from specific humidity and wind profiles (Methods) [1][2][3][21][22][23] . Based on the ERA5 global reanalysis 24 , the IVT distributions over the North Pacific basin valid at 0000 UTC June 21-26, 2021 are shown in Fig.…”

“…Landfalling ARs are capable of producing heavy precipitation in mountainous areas, especially in the cold season with water vapour supplied by tropical moisture sources 8,11,23,[32][33][34][35][36][37] . ARs in the warm season should produce less orographic precipitation because the warmer atmosphere can contain more water vapour before condensation begins than can a cooler atmosphere.…”

Identifying Eastern US Atmospheric River Types and Evaluating Historical Trends

“…New methodologies are developing algorithms to track AR cores in an attempt to track the lifecycle of global midlatitude ARs and associated precipitation (Shearer et al., 2020). Further, relative methods for defining IVT thresholds based on background moisture have been employed (e.g., Guan & Waliser, 2015; Lavers et al., 2012; Lora et al., 2017; Mo et al., 2021).…”