Gyakum Group Contributions: Northeast Tropical Conference

Rensselaerville, NY, May 28-31^st

1. Oral Presentation Abstract:

The Extreme Precipitation Index (EPI) and its applications to Extratropical Transition

Shawn M. Milrad¹, John R. Gyakum², Eyad H. Atallah²

¹Applied Aviation Sciences Department, Embry-Riddle Aeronautical University, Daytona Beach, FL

² Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, QC

Recent high-impact extratropical transition (ET) events such as Irene (2011) and Sandy (2012) have highlighted the major threat to life and property that these storms pose. One often overlooked aspect of ET events is inland flash flooding due to heavy precipitation; during Irene (2011), such flash flooding resulted in the worst natural disaster in the history of Vermont. The aim of this study is to apply recent work aimed at dynamically diagnosing extreme precipitation events to precipitation distributions associated with ET events.

First, a novel, yet relatively simple and intuitive composite index, the Extreme Precipitation Index (EPI), is introduced. The EPI, which is designed to identify and predict extreme mid-latitude precipitation events, is based on an equation found in the original lecture notes of Professor Fred Sanders:

This equation relates the precipitation rate (P) to the integrated ascent ω (assumed for the purpose of this work to be quasi-geostrophic [QG]) and to the incipient air mass (represented by dr_s/dp, where r_s is the saturation mixing ratio). Since there is more QG ascent for a given QG forcing in the presence of weaker static stability, the equation suggests that the most value for the dollar is achieved with more ascent AND warmer, less stable air masses, represented by dr_s/dp.

Second, previous work has indicated that intensifying ETs feature a left-of-center (LOC) precipitation distribution, while weakening ETs exhibit a right-of-center (ROC) precipitation distribution. Using the NCEP Climate Forecast System Reanalysis (CFSR), 53 ET cases (1979-2012) are examined to explore the usefulness of the EPI in diagnosing storm dynamics, evolution, and precipitation distribution during ET. Composite and case diagnostics contrasting LOC and ROC events will be presented, and the potential for the use of EPI as an operational forecast tool will be discussed.

2. Poster Presentation Abstract:

Precipitation modulation by the Saint Lawrence River Valley in association with transitioning tropical cyclones

Shawn M. Milrad¹, John R. Gyakum², Eyad H. Atallah²

¹Applied Aviation Sciences Department, Embry-Riddle Aeronautical University, Daytona Beach, FL

² Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, QC

The St. Lawrence River Valley (SLRV) is an important orographic feature in eastern Canada that can affect surface wind patterns and contribute to locally higher amounts of precipitation. The impact of the SLRV on precipitation distributions associated with transitioning, or transitioned, tropical cyclones that approached the region is assessed. Such cases can result in heavy precipitation during the warm season, as during the transition of Hurricane Ike (2008), in which as much as 70 mm of precipitation was observed in the SLRV.

Thirty-eight tropical cyclones tracked within 500 km of the SLRV from 1979-2011. Utilizing the National Centers for Environmental Prediction (NCEP) North American Regional Reanalysis (NARR), 19 of the 38 cases (Group A) had large values of ageostrophic frontogenesis within and parallel to the SLRV, in a region of northeasterly surface winds associated with pressure-driven wind channeling. Using composite and case analyses, results show that the heaviest precipitation is often located within the SLRV, regardless of the location of large-scale forcing for ascent, and is concomitant with ageostrophic frontogenesis.

The suggested physical pathway for precipitation modulation in the SLRV is as follows: Valley-induced near-surface ageostrophic frontogenesis is due to pressure-driven wind channeling as a result of the along-valley pressure gradient (typically exceeding 0.4 hPa (100 km)^-1) established by the approaching cyclone. Near-surface cold-air advection as a result of the northeasterly pressure-driven channeling results in a temperature inversion, similar to what is observed in cool-season wind channeling cases. The ageostrophic frontogenesis, acting as a mesoscale ascent-focusing mechanism, helps air parcels to rise above the temperature inversion into a conditionally unstable atmosphere, which results in enhanced precipitation focused along the SLRV.