Abstract
This study documents the frequency and intensity of precipitation at Montreal, Canada, from 1979 to 2018 as it relates to four quadrants of a 500-hPa wave, identified by the position of troughs, ridges, and inflection points. These quadrants provide a simplified conceptualization of the contributions from the temperature and vorticity advection forcing terms in the quasigeostrophic (QG) omega equation. Precipitation is found to be significantly more intense in every season except summer in the quadrant immediately upstream of the 500-hPa ridge, where differential cyclonic vorticity advection (DCVA) and a local maximum in horizontal warm-air advection (WAA) tend to promote unambiguous QG ascent. In summer, the average precipitation is still most intense in the DCVA-WAA quadrant, but not significantly more than in the quadrant immediately downstream of the 500-hPa trough, where DCVA and a local maximum in horizontal cold-air advection (CAA) are expected to compete, resulting in ambiguous QG vertical motion. Precipitation in the DCVA-CAA quadrant is more intense in every season than in the expected differential anticyclonic vorticity advection (DAVA) quadrants, with significantly higher intensities in spring and fall. Furthermore, the DCVA quadrants exhibit significantly stronger ascent compared to the DAVA quadrants and the DCVA-WAA quadrant features significantly warmer 850-hPa equivalent potential temperatures compared to the three other quadrants in every season. Odds ratios indicate a statistically significant association between heavy precipitation episodes and the DCVA-WAA quadrant. Heavy precipitation episodes in the DCVA-CAA quadrant are associated with a negatively tilted 500-hPa geopotential height pattern in winter and fall.
Significance Statement
Operational weather forecasters apply conceptual models that connect upper-atmospheric weather patterns to vertical motion and precipitation. However, few studies have quantified this connection over a longer, continuous period of time. In this study, we examine the relationship between historical subdaily precipitation at Montreal, Canada, and a simple large-scale conceptual model that relates vertical motion to the position of upper-level troughs and ridges. We find significant evidence for heavy precipitation to occur upstream of the upper-level ridge, and for very little, or very light, precipitation to occur upstream of the upper-level trough. These results provide quantitative support to some of the conceptual methods available to operational weather forecasters in preliminary analyses that support their precipitation forecasts.
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