The Formation and Impact of an Incipient Cold-Air Precipitation Feature on the 24-25 January 2000 East Coast Cyclone
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Date
2005-07-22
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Abstract
The 24–25 January 2000 East Coast cyclone was characterized by a major operational forecast failure. In an effort to understand why short-range operational numerical weather prediction (NWP) model forecasts were so poor, the impact of a cold-air incipient precipitation (IP) feature that developed prior to the rapid cyclogenesis on 24 January is investigated using potential vorticity (PV) analysis. The IP was poorly forecasted by the operational NWP models, and these models failed to produce heavy precipitation far enough inland over the Carolinas and Virginia later in the cyclone event. Here the formation of the IP is examined from an observational perspective, the impact of the IP is quantified using PV methodology, and the ability of a NWP model to simulate its formation is tested by varying model physics, initial conditions and grid spacing.
It was hypothesized that latent heating associated with the IP that formed over the Gulf Coast states early on 24 January generated a lower-tropospheric PV maximum that was important to the moisture transport into the Carolinas and Virginia and the track and intensity of the surface cyclone later in the cyclone event. Calculations from a PV budget and piecewise PV inversion found that the IP was associated with the genesis of a lower-tropospheric PV maximum and that the balanced flow associated with the PV maximum contributed significantly to moisture transport into the region of heavy snowfall. Operational NWP models that failed to forecast the IP did not generate the PV maximum or the heavy precipitation over the Carolinas and Virginia.
Observational analyses and radar imagery showed that the IP formed in a region of elevated convective symmetric instability (a mixture of gravitational conditional instability and conditional symmetric instability) where forcing for ascent was provided by an approaching upper-level trough/jet streak. Short-range forecasts from NWP models under-forecasted the strength of the forcing and instability, and were unable to generate the IP in the region where it was observed.
An 18-member mesoscale model ensemble with 20-km horizontal grid spacing varying initial condition analyses and model physics was unable to generate the IP feature. Variance associated with the cyclone?s sea-level pressure and precipitation distributions due to initial condition variation was larger than that due to variations in model physics, although significant variation was due to poor performance by ensemble members initialized from the Global Data Assimilation System analysis. A high-resolution model simulation with 4-km grid spacing showed that the IP initially formed within a layer of elevated CSI, consistent with analyses. Buckling of absolute geostrophic momentum surfaces indicated adjustment to slantwise convection at later times. Simulations with 12-km and 20-km grid spacing degraded the representation of these features, suggesting that models run with even coarser grid spacing would be unable to capture the initial formation of the IP. Other simulations initialized only three or six hours later showed a marked improvement in the representation of the IP, the cyclone track and intensity, and the final precipitation distribution, confirming the importance of properly representing the IP feature in successful simulations of this event.
The current configuration of operational models with CP schemes and grid spacing insufficient to properly resolve the effects of slantwise convection suggests that future cases may occur where NWP models fail to capture the impact of a cold-air precipitation feature (possibly associated with elevated gravitational and slantwise instability), resulting in poor forecasts of downstream moisture transport and cyclone track and intensity. Operational forecasters should be aware of this possibility and be able to anticipate the potential feedbacks from precipitation (in NWP models and in reality) onto atmospheric dynamics. Available observations and high-frequency model analyses can be used to evaluate NWP model forecasts of precipitation and the lower-tropospheric PV distribution, allowing forecasters to recognize instances when model guidance can be adjusted to improve forecasts of high-impact cyclone events.
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latent heating, extratropical cyclones, diabatic processes, numerical modeling, cyclone dynamics, potential vorticity inversion
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Degree
PhD
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Marine, Earth and Atmospheric Sciences
