1. Subtropical PV Streamer Variability and Impact in the Subtropical North Atlantic Basin
32nd Conference on Hurricanes and Tropical Meteorology
The Condado Hilton Plaza, San Juan, Puerto Rico
17-22 April 2016
Philippe P. Papin*, Lance F. Bosart, Ryan D. Torn
Department of Atmospheric and Environmental Sciences – University at Albany: SUNY, Albany NY
*Corresponding author
Fig. 1. Example of a PV streamer. (a) 350-K PV (shaded, PVU) and winds (barbs, kt), (b) is 850 – 200-hPa vertical wind shear magnitude (shaded, m s-1) and direction (barbs, kt), and (c) is precipitable water standardized anomaly (shaded, sigma) with the 40 mm contour overlaid.
1) Motivation
3) Climatological Results
0000 UTC 9 Jun 2013
Upper-tropospheric potential vorticity (PV) streamers are synoptic features resulting from Rossby Wave Breaking (RWB) in the subtropical Atlantic Basin (Fig. 1a)
These PV streamers modify the local environment by:
Enhancing vertical wind shear (VWS) corridors (Fig. 1b)
Enhancing moisture anomalies (Fig. 1c)
These environmental factors influence tropical cyclone (TC) activity in the Atlantic basin
This study aims to investigate:
Seasonal PV streamer activity combining both their size and intensity
How seasonal TC activity is modulated by seasonal PV streamer activity
Fig. 4 (a) Time series of seasonal PV streamer count (blue line) and activity (red line, sigma x 104). (b) 1 June – 30 November PV streamer frequency for (c) as in (b) except for 1995.
PV Streamer
High VWS
Fig. 3 1 June – 30 November PV streamer frequency (shaded, %) and time-mean 200-hPa streamlines for the 1979 – 2014 climatology
Highest PV streamer frequency occurs poleward of 20oN in the North Atlantic and overlaps the location of the time-mean mid-ocean trough
PV streamer activity integrated over area and intensity shows a trend towards decreasing intensity since the 1980s despite only small changes in seasonal PV streamer count
Large year-to-year variance in PV streamer activity (Fig. 4b,c)
Dry Midlatitude Air
2) PV Streamer Identification
Fig. 2. (a) 350-K PV (shaded, PVU) with 2-PVU overlaid in black contour. (b) 350-K 2-PVU (blue contour) and points where a meridional gradient reversal in PV is observed (red contour). (c) as in (b) except including area of PV streamer candidate (black shading) with distances provided for the width, perimeter, and area of the PV streamer. (d) as in (c) except plotting standardized PV anomaly (shaded, sigma) with the average PV anomaly of the PV streamer provided.
[σ]
Points along contour where meridional PV gradient reversal is observed
Similar to Postal and Hitchman (1999)
4) Impact on TC Activity
Fig. 5. Scatter plot of PV streamer intensity metric (sigma 104, x-axis) and ACE (kt2 104, y-axis), and their linear regression (black line). Correlation coefficient provided in the top right.
Strong negative correlation (r = -0.62) between seasonal accumulated cyclone energy (ACE) and PV streamer activity (Fig. 5)
Composite 8 highest and lowest ACE years reveals decreases and increases in PV streamer frequency basin wide in the subtropical Atlantic (Fig. 6 a,b)
Example: 0000 UTC 9 Jun 2013
w= 2556 km
p =12,823 km
Area = 8,569,380 km2
Similar but more inclusive than Wernli and Sprenger (2007)
Average PV Anomaly sigma
p must be three times w
p must be > 3000 km
Fig. 6. Change in PV streamer frequency (shaded, %) in (a) 8 highest ACE seasons and (b) 8 lowest ACE seasons.
[σ]
Identification technique combines previous methodologies in order to link PV streamer areas to RWB using the 0.5o Climate Forecast System Reanalysis (CFSR)
Process:
Identify 2-PVU contour on 350-K surface from 0-70oN and 120oW – 20oE (Fig. 2a)
Identify points along contour where a meridional PV gradient reversal is observed (Fig. 2b)
Calculate line orthogonal to first several points of PV reversal (Fig. 2c)
Check PV streamer is large & elongated using perimeter (p) and width (w)
Calculate the intensity of the PV streamer as a standardized quantity (Fig. 2d)
5) Conclusions
References
PV streamers govern intensity and position of time-mean mid ocean trough
PV streamer activity inversely correlated with TC activity
Future work will composite PV streamers of different intensity and investigate their antecedent conditions
Postel, G. A., and M. H. Hitchman, 1999: A Climatology of Rossby Wave Breaking
along the Subtropical Tropopause. J. Atmos. Sci., 56, 359–373.
PVstd_anom = (PV – PVmean_climo) / PVstandard_deviation_climo
Wernli, H., and M. Sprenger, 2007: Identification and ERA-15 Climatology of Potential Vorticity Streamers and Cutoffs near the Extratropical Tropopause. J. Atmos. Sci., 64, 1569–1586.