Research

Ph.D. work: Extreme precipitation regimes in eastern North America (supervised by Prof. John Gyakum)

Climatological study of synoptic-scale weather patterns and physical mechanisms

Note: I wrote an article for the Canadian Meteorological and Oceanographic Society (CMOS) bulletin summarizing this work, stemming from an outstanding presentation nomination at the 2024 annual CMOS congress held virtually. A more detailed, technical analysis can be found at Low et al. 2022 and another paper in review.

While precipitation is a very common occurrence, prolonged periods of widespread and heavy precipitation (or extreme precipitation regimes or EPRs) are not common and lead to widespread flooding and other major impacts. What kinds of meteorological patterns and physical processes are favorable for such events in eastern North America during winter?

Source: NWS

A continental-scale wave train consisting of an upper-level ridge, trough, and ridge in the central North Pacific, western North America, and eastern North America respectively is conducive for transport of Gulf of Mexico and western Atlantic moisture northward into eastern North America and an active storm track from the central Plains to New England or eastern Canada that take up the moisture and deposit it as precipitation. The amplitude of the wave train is important in determining how much precipitation falls. However, the strengths of individual cyclones are not so important; rather, it is important for the continental-scale pattern to remain more or less stagnant, even if cyclones move quickly along the jet stream. Not surprisingly, these events are much more likely during Pacific ridge regimes, as defined in Lee et al. 2023, and Pacific jet retractions, as defined in Winters et al. 2019, than during other types of regimes.

a): Composite 500-hPa height (solid contours, 6 dam interval) and anomaly (shaded in dam according to color bar) for three days before EPR start (t_s-3); b): As in a) but for two days before EPR start (t_s-2); c): As in a) but for one day before EPR start (t_s-1); d): As in a) but for EPR start (t_s); e): As in a) but for EPR mid-time (t_s-m); f): As in a) but for EPR end (t_e). Stippling indicates field statistical significance of anomalies at the α_{false discovery rate}=0.1 or α_global=0.05 level. The yellow boundary encloses the eastern North American region, and the navy blue dot marks the composite EPR centroid. Taken from Fig. 4 of Low et al. 2022.

(a) Cyclone frequency anomaly during EPRs shaded according to color bar, with EPR IVT anomaly vectors (reference vector on top right). Frequency represents the fraction of EPR’s lifetimes in a high or low pressure footprint. (b) As in (a), but for anticyclone frequency anomalies. The magenta dot marks the composite EPR centroid. Stippling indicates field statistical significance of anomalies at the α_{false-discovery-rate} = 0.1 or α_global = 0.05 level. The black boundary encloses the eastern North American region. Taken from Fig. 6 of Low et al. 2022.

 

(a) EPR frequency of an AR, or the fraction of EPR time steps featuring an AR. The magenta dot marks the composite EPR centroid. (b) Climatological frequency of an AR. The black boundary encloses the eastern North American region. Taken from Fig. 7 of Low et al. 2022.

But the presence of this pattern does not mean that it will persist for a long time. What contributes to the longevity? The frequent cyclones characteristic of this pattern actually contribute to the pattern's longevity in a feedback loop. Cyclones tracking northeastward to the northwest of upper-level ridges over the central North Pacific and eastern North America transport abundant moisture northward in warm conveyor belts to the southeast of the cyclones, where the air is lifted, and the moisture condenses aloft. This condensation releases latent heat, and this diabatic heating raises upper-level heights on the northwestern side of the upper-level ridges, slowing or stopping their eastward movement. This contributes to the unusually slow Rossby wave speed across most of the U.S. observed during these events, as well as the longevity. The greater the persistence of the upper-level ridges, and the broader the upper-level troughs and ridges, the longer the event persists.

Composite EPR time-averaged Rossby wave phase speed (calculated via Fragkoulidis and Wirth 2020's algorithm) (contoured in m s^-1, 2 m s^-1 interval, thicker and darker contours indicating lower RWP phase speed) and RWP phase speed anomaly (shaded in m s^-1 according to color bar). Stippling indicates field statistical significance of anomalies at the α_{false discovery rate}=0.1 or α_global=0.05 level. The western and eastern sides of the pink rectangle indicate the western and eastern bounds of the Hovmöller in the figure below. The cyan boundary encloses the eastern North American region.

 

Hovmöller plot averaged from 30-55°N throughout the EPR's life cycle of 250-hPa height anomaly (shaded in dam according to color bar) and v-wind anomaly (contoured in m s^-1, 4 m s^-1 interval except no contour of 0 m s^-1, solid black contours for positive values and dashed black contours for negative values)

 

Case study: Extreme precipitation during February 2019

In this section, I go into a more in-depth analysis over a particular case: the extreme precipitation period of 4-25 February 2019 that is composed of 3 EPRs. February 2019 was an exceptionally wet month across eastern North America. This month had many of the features listed above: an extremely persistent central North Pacific upper-level ridge, lasting for several weeks, promoting a trough in western North America, a ridge in eastern North America, and repeated cyclones tracking northeastward through eastern North America bringing heavy precipitation. Record rainfall occurred in the Ohio and Mississippi Valleys, with record snowfall in the Upper Midwest U.S and parts of Ontario and Quebec. The wet period finally ended at the end of February when the central North Pacific upper-level ridge migrated to Alaska, promoting a cold air outbreak with broad upper-level troughing over southern Canada and the northern U.S.

The 4-25 February 2019 extreme precipitation period was not well predicted beyond 10 days in advance. What led to the lack of predictability? ECMWF and GEFS ensemble simulations generally did not simulate even the large-scale weather pattern, let alone the more complex precipitation patterns, beyond 10 days in advance. Covariance tests of the ensemble members indicate that the development of the central North Pacific ridge was highly sensitive to the details of rapid cyclogenesis (e.g. latent heat release) in the northwestern Pacific and the interaction between a Kona low and the mid-latitude Pacific jet stream in the one week before the wet period. With in-situ data over the ocean lacking, and rapid cyclogenesis being difficult to simulate accurately, the errors quickly grow beyond a 5-day lead time, making long-range predictions of even large-scale patterns very difficult.

Source: CPC

More to come later!