exceptional bora outbreak in winter 2012: validation and analysis … · high-resolution...

06 – 10 October 2014 14th EMS Annual Meeting

Exceptional Bora outbreak in winter 2012: validation and analysis of

high-resolution atmospheric model simulations in the northern Adriatic area

Silvio Davolio, Paolo Stocchi – CNR ISAC (Bologna) Sandro Carniel, Alvise Benetazzo – CNR ISMAR (Venice)

Emanuele Bohm – CNR ISAC (Roma) Mariangela Ravaioli, Francesco Riminucci – CNR ISMAR (Bologna)

XiaoMing Li – RADI CAS (Beijing)

OUTLINE Framework and motivation The Bora event of winter 2012 in the northern Adriatic Modelling tools and available observations Modelling results

- validation: meteorological variables of interest for oceanographic applications - analysis of the meteorological event (heavy snowfall)

Conclusions

This study is undertaken in the framework of the Italian Project “RITMARE” coordinated by the National Research Council (CNR), within WP4 “Coastal Oceanographic Modelling” whose aim is to implement meteorological, oceanic and wave numerical models in order to attain a better understanding and representation of the main processes in the coastal zone and of their interactions.

Deep Water Outflow Current

Target area: northern Adriatic Sea, a region often affected by Bora winds, producing relevant air-sea exchange which in turn impacts the Adriatic circulation, triggering dense water formation processes. Suitable “laboratory”

The exceptional Bora event of winter 2012 is very interesting from both: -  meteorological perspective -  oceanographic perspective -  air-sea exchange perspective and has been selected as case study for validation of NWP models. Aim: - Assess the capability of our NWP model chain of reproducing the Bora event and associated phenomena (snowfall, fluxes, etc.) - Assess the impact of using different initial/boundary conditions (from Global models) to drive the NWP chain - Evaluate the impact of different SST initialization

MOLOCH forecast +33 h

NWP models sometimes do not evolve SST during short-range forecasts, assuming it is a slowly varying variable. However it may be unrealistic in a small and shallow basin like the Adriatic Sea, during particular meteorological events such as Bora.

Bora event: 29 January – 13 February 2012 Transition from zonal circulation to more meridional flow in the European area, associated with a blocking pressure ridge meridionally elongated over the eastern Atlantic, and the westward movement of a retrogressive wave from Eurasia towards the Mediterranean basin à descent of a cold trough toward central Europe, which brought very cold air masses of continental origin in the Mediterranean àthe Mediterranean basin was characterized by a persistent cyclonic circulation associated with an exceptional cold anomaly ! exceptional persistence of Bora.

NCEP Reanalysis – T at 850 hPa NCEP Reanalysis, GPH at 500 hPa

ISAC WEB Page: www.isac.cnr.it/dinamica/projects/forecasts/

NWP MODELS developed at CNR ISAC

BOLAM: a limited area, hydrostatic, convection-parameterized model, typically applied to simulations on continent-scale areas. It is employed to bridge the gap between the "coarse" global model fields (0.5 deg. for the NOAA-GFS data and about 0.20 deg. for the ECMWF data) and the high resolution MOLOCH forecast. MOLOCH: non hydrostatic, convection-permitting model, applied to simulations/ forecasting at high resolution (1÷4 km) on regional/national domains. Nested in BOLAM. Physical schemes (radiation, PBL turbulence, soil and microphysics) are common in the two models. Dynamical core, vertical coordinate discretization differ. “Slab ocean model”: SST modulated by sfc fluxes and relaxed toward climatological value.

BOLAM 11 km MOLOCH 2.3 km

Ini$al and Boundary condi$ons

GFS NCEP Forecasts 0.5 deg

IFS ECMWF 0.25 deg

GFS NCEP SST form

Satellite data (MyOcean

0.0625x0.0625)

Three different sets of IC/BC

Initial and boundary conditions

FORECASTS SCHEME

BOLAM forecasts MOLOCH forecasts Model output considered for the analysis (+12h - +36h)

General picture of the Bora event – Comparison with ASAR (ENVISAT) retrieved wind field 02 Feb 2012, 21 UTC

ASAR

MOLOCH Different “jet” are observed and correctly predicted. Wind intensity over N-Adriatic is well captured (> 25 m/s). Hor. Res. allows to properly describe the critical topographical forcing responsible for the swing of high-speed low-level wind jets and wake areas. No deflection along the Italian coast.

28 Jan Onset

13 Feb Offset

WIND SPEED VIDA

ACQUA ALTA (Ve) S1 buoy

•  OBS MOLOCH GFS MOLOCH IFS MOLOCH GFS+SAT

27 Jan 2 Feb 6 Feb 11 Feb 16 Feb

27 Jan 2 Feb 6 Feb 11 Feb 16 Feb 27 Jan 2 Feb 6 Feb 11 Feb 16 Feb

Acqua Alta (Ve)

Vida Buoy

S1 Buoy Po outlet

MOLOCH IFS MOLOCH GFS MOLOCH GFS+SAT

2m TEMPERATURE

S1 buoy

VIDA

ACQUA ALTA (Ve)



•  OBS MOLCH GFS MOLOCH IFS MOLOCH GFS+SAT

SST

S1 buoy

VIDA



ACQUA ALTA (Ve)


Cold fresh water from the Po river

MOLOCH-IFS MOLOCH-GFS

MOLOCH GFS+ SAT

SST: MOLOCH initial time – 01 Feb 2012

SURFACE HEAT FLUXES VIDA

ACQUA ALTA



ΔT = T1st level - SST

S1 buoy

VIDA

ACQUA ALTA (Ve)




MOLOCH Surface Sensible Heat Fluxes 01 Feb 2012 at 15 UTC (+12h forecast)


MOLOCH GFS+ SAT


MOLOCH GFS+ SAT

MOLOCH Surface Latent Heat Fluxes 01 Feb 2012 at 15 UTC (+12h forecast)

Wind Speed - Acqua Alta Temperature - Acqua Alta

Wind Speed - Vida Temperature - Vida

u Obs u MOLOCH_IFS u MOLOCH_GFS u MOLOCH_GFS+SAT

Statistical Summary : Taylor Diagrams Standard DeviaRo

n Standard DeviaRo

n

Standard DeviaRo

n Standard DeviaRo

n

Good agreement between forecasts and observaRons and small differences among different simulaRons. Slightly worse results at S1 buoy

SST Acqua Alta SHF Acqua Alta LHF Acqua Alta

SST Vida SHF Vida LHF Vida

Standard DeviaRo

n

Standard DeviaRo

n

Standard DeviaRo

n

Standard DeviaRo

n

Standard DeviaRo

n

Standard DeviaRo

n

Statistical Summary : Taylor Diagrams u  Obs u  MOLOCH_IFS u  MOLOCH_GFS u  MOLOCH_GFS+SAT

Low correlation Similar correlation, but MOLOCH IFS in much better agreement

Emilia Romagna Snow Height - Period: 1-12 February 2012

Source: ARPA SIMC

Apennines and ….

Source: ARPA SIMC

city of Bologna

Source: ARPA SIMC

24-h Accumulated Precipitation

from 31 Jan 12 UTC to 01 Feb 12 UTC

MOLOCH-IFS

MOLOCH-GFS

Different IC/BC

Different SST initialization but also

different upper air fields!

MOLOCH-GFS

MOLOCH-GFS+SST



Different SST initialization

Very weak impact of satellite SST initialization

on rainfall (snowfall) forecasts

MOLOCH-IFS

MOLOCH-IFS SST + 2°C



Experiment to assess sensitivity to SST

initialization

Very weak impact of SST on snowfall over the

Apennines, increased precipitation over the central

Adriatic Sea




initialization

Very weak impact of SST on snowfall over the

Apennines, decreased precipitation over the central

Adriatic Sea

MOLOCH-IFS

MOLOCH-IFS SST - 2°C




initialization

Same results for GFS driven simulations: very weak

sensitivity

MOLOCH-GFS

MOLOCH-GFS SST + 2°C




initialization

Same results for GFS driven simulations: very weak

sensitivity

MOLOCH-GFS

MOLOCH-GFS SST - 2°C

CONCLUSIONS -  Different SST initializations shows:

-  Much better agreement of ECMWF analyses, derived from OSTIA system (UKMO)

-  Remarkable warm bias of GFS analysis (SST analysis based on IR satellite only)

-  Limited impact of SST OI-analysis from satellite (MyOcean) mainly due to persistent cloud cover during the period

-  Dynamical variables (e.g. wind) and Bora evolution are well forecast -  SST initialization is critical for correctly forecasting surface fluxes and

temperature à importance for oceanographic applications -  SST initialization has a weak impact on precipitation forecast for this severe

weather event à moisture is supplied by mesoscale circulation around the Mediterranean cyclone and not by the Adriatic Sea (short fetch, very cold air)

-  Indication for model development: -  the slab ocean model allows for a too fast cooling à revising heat capacity

(or thermocline depth) -  Some discrepancies in forecast wind direction à increasing model horizontal

resolution -  More event, also with different Bora characteristics need to be analysed

exceptional bora outbreak in winter 2012: validation and analysis … · high-resolution...

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