Remote Impact of North Atlantic Hurricanes on Mediterranean Intense Rainfall
Study examines how North Atlantic hurricanes influence episodes of intense rainfall over the Mediterranean. It explores the upstream and downstream effects, showing how hurricanes interact with the midlatitude flow, triggering Rossby wave trains that lead to intense rainfall events. The research investigates the remote impact of hurricanes during specific observation periods and highlights case studies of hurricanes Leslie, Rafael, and Nadine on intense rainfall and predictability. Meso-NH sensitivity experiments are conducted to understand the impact of hurricanes Leslie and Rafael further.
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Impact of North Atlantic hurricanes on episodes of intense rainfall over the Mediterranean Florian Pantillon1,2Jean-Pierre Chaboureau1and Evelyne Richard1 1 Laboratoire d A rologie (LA), Toulouse 2 Karlsruher Institut f r Technologie (KIT), Karlsruhe AMA 2015 Toulouse
Upstream: remote impact of a hurricane Hurricanes can modify the midlatitude flow when they leave the tropics the hurricane outflow accelerates an upper-level jet and builds a ridge the downstream trough elongates and triggers surface cyclogenesis the impact quickly propagates downstream as a Rossby wave train Archambault et al. MWR 2013 Hurricanes interact with the midlatitude flow in autumn mostly
Downstream: Mediterranean intense rainfall Rossby wave trains can be precursors of intense rainfall the breaking of a Rossby wave train elongates an upper-level trough the trough induces strong low-level flow over the warm sea the strong flow transports moisture towards the topography Nuissier et al. QJRMS 2011 500-hPa height and 2-pvu wind speed 925-hPa wind Elongated upper-level troughs trigger intense rainfall in autumn mostly
Remote impact of hurricanes on intense rainfall During HyMeX SOP1 in autumn 2012 Selected Intense Observation Periods (rainfall above 100 mm in 24 h) a) triggered by a Rossby wave train b) located downstream of a hurricane Two case studies: Leslie remote impact on IOP4 Rafael remote impact on IOP14 - - ECMWF analysis
Remote impact of hurricanes on intense rainfall During HyMeX SOP1 in autumn 2012 Selected Intense Observation Periods (rainfall above 100 mm in 24 h) a) triggered by a Rossby wave train b) located downstream of a hurricane Two case studies Leslie remote impact on IOP4 Rafael remote impact on IOP14 - - Additional case study Nadine: impact on predictability IOP6 - ECMWF ensemble forecast
PART I Remote impact of Leslie and Rafael on intense rainfall Meso-NH sensitivity experiments
Synoptic evolution Leslie and Rafael H = Leslie IOP4 +48h +48h hurricane approaches Rossby wave train hurricane interacts with upstream trough downstream trough elongates cut-off low forms over the Mediterranean H = Rafael +48h +48h IOP14 330-K wind (vectors) and MSLP (blue contours) Did Leslie and Rafael impact the downstream wave breaking and the IOPs?
Numerical experiments with Meso-NH Domain North Atlantic and Europe, dx=16 km, integration 120 h Two runs for each hurricane control run: initialized from ECMWF analysis filtered run: hurricane filtered out from initial analysis impact of hurricanes = difference between control and filtered runs Leslie: Rafael: tiny hurricane deep hurricane Filtered anomalies at the initialization of Meso-NH runs 850-hPa wind (vectors) and MSLP (magenta contour)
Local impact and downstream propagation Evolution of the difference between control and filtered runs H = Leslie +24h +24h upstream trough downstream trough elongated trough cut-off low upstream trough delayed downstream trough deeper cut-off low deeper H = Rafael upstream +24h +24h trough elongated trough downstream trough cut-off low 330-K potential vorticity in control run (green contour at 2 pvu) and wind difference (vectors) The hurricanes amplify the Rossby wave breaking downstream
Remote impact on the Mediterranean Observation and analysis Control runs Filtered runs <100 mm >300 mm Leslie IOP4 >300 mm cut-off cut-off cut-off deeper with hurricanes more intense without hurricanes Rafael IOP14 300 mm 150 mm >200 mm 330-K potential vorticity (magenta) and 850-hPa moisture flux (vectors) Rossby wave breaking and intense rainfall also occur without the hurricanes
PART II Impact of Nadine on the downstream predictability ECMWF ensemble forecast
Synoptic evolution Nadine Unusual loop over the eastern North Atlantic 20-22 September: Nadine moves slowly while a cut-off low approaches from the north 23-25 September: Nadine is steered westward while the cut-off low is steered eastward ? Track of Nadine ECMWF ensemble forecast cutoff cutoff cutoff cutoff cutoff IOP6? MSLP (black contours) Did Nadine impact the downstream predictability during IOP6?
Clustering the ECMWF ensemble forecast 50 members from 00 UTC 20 September clustered in 2 steps 1) Principal Component Analysis at 00 UTC 24 September (t+96) 2) Ascending hierarchical classification with 4 clusters (arbitrary number) Cluster 4 Cluster 1 PC2 Cluster 3 analysis control deterministic outliers Cluster 2 PC1 Deviation from ensemble mean
4 scenarios of interaction Nadine-cutoff Nadine moves westward 1. weakest interaction Nadine moves slower cutoff moves eastward 2. weak interaction cutoff moves slower cutoff cutoff outliers Nadine control Nadine deterministic analysis Nadine merges with cutoff 4. strongest interaction cutoff moves southward Nadine makes landfall 3. strong interaction cutoff cutoff Nadine Nadine The interaction strongly impacts both Nadine and the cutoff
Impact on the Mediterranean during IOP6 1. weakest interaction: cutoff over Europe Observation and analysis 2. weak interaction: delayed cutoff over Europe Precipitation (too early) Precipitation (good timing) 3. strong interaction: no cutoff 4. strongest interaction: no cutoff Surface cyclone ~ cutoff over Europe Convergence line along cold front Moisture flow from the warm sea Orographic lifting over C vennes No precipitation No precipitation Precipitation > 70 mm in 6 h locally The interaction between Nadine and the cutoff impacts the Mediterranean
Summary Impact of hurricanes on the Mediterranean: 2 types of impact during HyMeX 1) Leslie and Rafael: remote impact on Rossby wave breaking downstream Meso-NH simulations initialized with and without hurricanes Wave breaking amplified with hurricanes compared to without hurricanes Wave breaking and intense precipitation also occur without hurricanes The interaction of a hurricane with the midlatitude flow is mostly a perturbation to Rossby wave breaking downstream Pantillon et al. QJRMS, early online release 2) Nadine: impact on downstream predictability ECMWF ensemble forecast clustered into 4 scenarios Track Nadine very sensitive to interaction with cutoff Precipitation downstream very sensitive to interaction Nadine with cutoff The interaction of a hurricane with the midlatitude flow can strongly impact the predictability downstream Pantillon et al. QJRMS, in prep.
Thank you! AMA 2015 Toulouse
The origin of uncertainty Ensemble sensitivity tracked back in time(Torn and Hakim 2008, Chang et al. 2013) ensemble sensitivity defined at each point as correlation among 50 members between 500-hPa geopotential height and Principal Component 1 shows a coherent pattern around the cut-off low = source of uncertainty JM= PC1 of member M xiM= Geopotential Height at point i of member M
What could have happened... Landfall for 8 members (6 from strong interaction cluster + 2 outlyers) Intensity: Nadine weakens prior to landfall but reaches 981 hPa in two cases Precipitation: moderate to intense due to the tropical moist air mainly 981 hPa 985 hPa 993 hPa 991 hPa 992 hPa 981 hPa 992 hPa 982 hPa
