Ernesto Barrera (ebarrera@inm.es) Centro Meteorológico de Santa Cruz de Tenerife

1. NWP Applications 2009 Course – Helsinki – Dec 2009The 31M Storm in Tenerife island Ernesto Barrera (ebarrera@inm.es) Centro Meteorológico de Santa Cruz de Tenerife Delegación Territorial en Canarias

2. The 31M Storm in Tenerife Island – A WRF simulation • Outline • Introduction • Objectives • The storm scenario • Experiment setting • Results • Verification • Conceptual model • Conclusions 2009 NWP Applications Course

3. The 31M Storm in Tenerife Island – A WRF simulation • Introduction (I) • The most severe rainfall episode ever recorded in Tenerife (Canary islands, Spain) occurred in 31 March 2002 (hereafter, 31M). • Torrential rains left 8 deads and considerable material damage affecting much more intensely a reduced area surrounding the capital city in the NE of the island. 80 Km Figure 1: Total accumulated precipitation in 24h recorded on 31M 2009 NWP Applications Course

4. The 31M Storm in Tenerife Island – A WRF simulation • Introduction (II) • Data from Santa Cruz Observatory revealed the exceptional nature of this downpour: • -The total accumulated in 24 hours (P24 ~ 232,6 mm) exceeded the yearly average (214 mm). • -Maximum precipitation rate: 167 mm/h. • -Longest persistence of high rates: 100 mm/h during about an hour. • -88 % of precipitation falled in less than 3 hours, between 16:00 and 18:30 UTC. Data from Santa Cruz station Torrentiality limit 2009 NWP Applications Course

5. The 31M Storm in Tenerife Island – A WRF simulation • Objectives • The aim of this work is to evaluate the capability of the Weather Research and Forecasting model (WRF-ARW) in simulating the main features of the 31M storm in the meso-gamma / micro-alpha scales. • A study of sensitivity of the model to several moisture physics schemes is performed and the impact of increasing spatial resolution is also discussed. • Verification was done by comparing model simulated reflectivities with those of real radar. 2009 NWP Applications Course

6. The 31M Storm in Tenerife Island – A WRF simulation • The storm scenario (I) • Heavy rains are relatively rare in Canary islands. The daily accumulated precipitation in Santa Cruz only exceeded 100 mm in 4 occasions since 1943. • This extreme rainfall was associated with a multi-cellular convective structure highly efficient in producing precipitation. It remained in a quasi-stationary regime, affecting in a continuous way the same sector NE of the island. • Many factors favoured the evolution of these anchored developments being the most remarkable the low level convergence enhanced by the orography and the dynamic forcings associated to a jet-streak and a cold air pool at high levels. 2009 NWP Applications Course

7. The 31M Storm in Tenerife Island – A WRF simulation The storm scenario (II) Z and T at 500 hPa, ECMWF forecast for 12:00 UTC 31M overimposed onto Meteosat7 IR channel: Cold pool at high levels 2009 NWP Applications Course

8. The 31M Storm in Tenerife Island – A WRF simulation The storm scenario (III) Z and RH at 850 hPa, ECMWF forecast for 12:00 UTC 31M overimposed onto Meteosat7 VIS channel: moisture at low levels 2009 NWP Applications Course

9. The 31M Storm in Tenerife Island – A WRF simulation The storm scenario (IV) Wind at 300 hPa, ECMWF forecast for 12:00 UTC 31M overimposed onto Meteosat7 WV channel: Jet streak. 2009 NWP Applications Course

10. The 31M Storm in Tenerife Island – A WRF simulation • Experiment setting (I) • Numerical experiments were carried out using the WRF/ARW (version 2.2, December, 2006). • In order to focus high resolution over the area of interest we considered three configurations based on triply nested domains with 1:3 grid size ratios and two-way interaction. • All three settings had the same geographical coverture but increasing resolutions of 1, 2 and 3 km in the finer grid. • Vertical stratification was an slightly altered version of the default 31 levels set supplied by the model itself. 2009 NWP Applications Course

11. The 31M Storm in Tenerife Island – A WRF simulation • Experiment setting (I) • Input data were obtained from the ECMWF operational model at the time of the episode (horizontal resolution ~ 40 Km/h). • Initialization with data supplied by another model is known to cause internal imbalances which result in unrealistic increases of precipitation in the early forecasts (spin-up). • In order to minimize this effect, simulations were initialized with the analysis of 00:00 UTC 31 March 2002 (~12 h before the storm started developing) • The 3-hourly forecasts from H+03 to H+24 of the same run were used to generate the lateral boundary conditions for the coarser grid. • We set a basis configuration that was mantained fixed for all simulations: • Moninhou-Obukhov surface layer. • Planetary boundary layer scheme from Yonsei University. • RRTM / Dudhia schemes for long/short wave radiation. • Land surface scheme of the RUC model. 2009 NWP Applications Course

12. Nesting (Km) Convection scheme Microphysics 27 / 9 / 3 27 and 9 Km 3 km All domains • Kain-Fritsch • Betts-Miller • Grell-Devenyi Explicit • Lin • Ferrier • WSM 6-class • Thomson 18 / 6 / 2 18 and 6 Km 2 km All domains • Kain-Fritsch • Betts-Miller • Grell-Devenyi Explicit • Lin • Ferrier • WSM 6-class • Thomson 9 / 3 / 1 9 Km 3 and 1 km All domains • Kain-Fritsch • Betts-Miller • Grell-Devenyi Explicit • Lin • Ferrier • WSM 6-class • Thomson The 31M Storm in Tenerife Island – A WRF simulation • Experiment setting (II) • 12 combinations of cumulus parameterization and water microphysics schemes were used to evaluate the impact of the choice in the atmospheric simulated fields. • There is no unified criteria about at which resolutions a cumulus parameterization should be required. • Besides that, the interaction between resolved and parameterized convection on nested domains remains unclear. • We will assume the usual approach that convection can be explicitly resolved in scales finer than 5 km and will use cumulus schemes in larger grid sizes. • Only microphysics schemes dealing with mixed phase were used since graupel was observed in 31M storm. • To avoid issues related to mixing physics in two-way mode, the same scheme was kept fixed in domains of the same nesting. • A total of 36 simulations were performed according to settings shown in the table 2009 NWP Applications Course

13. The 31M Storm in Tenerife Island – A WRF simulation • Results (I) • Hourly precipitation fields suggest that the main features of the event (location, duration and rates) were well captured in all simulations. • A detailed intercomparison show that cumulus parameterization on parent domains actually affects the explicitly resolved convection on the inner nest. • Betts-Miller-Janjic (BMJ) and Kain-Fritsch (KF) schemes tended to underestimate the precipitation and to spread out the affected area. This is probably due to a limitation of these schemes to work properly on grid sizes < 10 km. • Grell-Devenyi (GD) seems to have better performances which is attributed to its efficient ensemble approach what makes it more robust to grid size issues. • Microphysics schemes also make some differences in the results. • Lin scheme tends to generate more structured patterns with multiple precipitation nucleii (P24 ~ 130-150 mm) embedded on a spreaded area. • Ferrier and Thomson simulations seem to be quite similar as much in pattern shapes as in forecasted amounts (100 mm< P24<150 mm). • WSM6 was the only scheme that forecasted P24>200mm which is in good agreement with observed data. The precipitation appears distributed over a compact, elongated nucleus that match in size and shape with affected area. 2009 NWP Applications Course

14. Lin Ferrier WSM 6 Thomson mm Grell-Deveny Betts-Miller Kain-Fritsch The 31M Storm in Tenerife Island – A WRF simulation Results (II) 24-hour precipitation for 2 km resolution simulations suggest the existence of quasi-stationary patterns which contributed efficiently to precipitation along the NE area of the island. This feature appear to be enhanced in the GD/WSM6 case. Similar results are obtained at 3 and 1Km simulations: 24h precip 2009 NWP Applications Course

15. The 31M Storm in Tenerife Island – A WRF simulation • Verification (I) • Radar station is located in Gran Canaria island at 1880m height and at 90 Km distance from storm area in Tenerife. • In normal propagation the beam is refracted making the echoes from storm come from about 3Km height. • For verification purposes, simulated reflectivities at 3Km height were calculated and compared with available radar imagery. 2009 NWP Applications Course

16. Real radar 3 km simulation 1 km simulation 3 km 1 km 3 km 1 km The 31M Storm in Tenerife Island – A WRF simulation • Verification (II) • The impact of spatial resolution is considerable in lower levels. • Wind, temperature and moisture related fields from surface up to 700 hPa show complex structures according the rough nature of the terrain. • Convection appears enhanced in 1km simulatons. 2009 NWP Applications Course

17. Conceptual model (I) • Some aspects of the storm dynamics can be reproduced by the model. • Maps of moisture convergence (defined as ) were constructed to identificate the areas with stronger forcing at low levels. • Fig 1: Moisture convergence (shaded) and wind at 850 hPa. • Fig 2: Vertical cross section along line AA’ of maximum convergence at 16:00 UTC (showed in top-right map). It is represented the circulation on the plane of the cross (vectors), the simulated radar reflectivity (shaded colors) , the graupel mixing ratio value of 0.5 g/Kg (red dashed line) and 0ºC isotherm (black dashed line). • Fig 3: Same as Fig 2 but along the transect BB’ transversal to convergence line. Fig 1 Fig 2 Fig 3 The 31M Storm in Tenerife Island – A WRF simulation 2009 NWP Applications Course

18. The 31M Storm in Tenerife Island – A WRF simulation • Conceptual model (II) • The humid flow from SW splitted into two branches when arriving the island surrounding the steep orography and collide again on the lee side. • As a result of this convergence, intense ascents occurs along a narrow fringe oriented SW-NE. • Circulations on these maximum convergence zones indicate that southern branch of flow was initially stronger so vertical development appears slightly tilted towards the N. • Convergence stops above 800 hPa but convective ascent still continues up to 500 hPa. • Graupel was formed above the 0ºC level, about 700 hPa during the initial phases of the storm what agree with the observed small size hail. 2009 NWP Applications Course

19. The 31M Storm in Tenerife Island – A WRF simulation • Conclusions • A total of 36 WRF simulations of the 31M storm in Tenerife were done. • All experiments captured fairly well the precipitation related features of the storm although better results were obtained with particular combination of schemes. • Grell-Devenyi ensemble scheme (convection) gave better results operating with WSM 6 class microphysics. • Interaction between resolved/non-resolved convection in nested grids was found although feedback mechanism is not straightforward. • Increasing resolution enhances detailed structures, specially in low levels fields. Despite this, the improvement introduced by 1Km simulations not always justify the computational cost when compared with the ones at 2 or 3Km. • Dynamical aspects of storm can be described from simulations. Interaction of the flow with the topography combined with thermal instability is clearly suggested as the responsible mechanism for the intense convective activity. • Further research should complete the analysis of the data and investigate the interaction between schemes on nested domains which still remains unclear. A second phase of this work will include planetary boundary layer schemes in the sensitivity studies. 2009 NWP Applications Course

20. The 31M Storm in Tenerife Island – A WRF simulation Thanks for your attention 2009 NWP Applications Course