Rossella Ferretti
Department of Physics, University of L'Aquila-Coppito-L'Aquila
Center for the Forecast of Severe Weather, CETEMPS
Tiziana Paolucci
Scientific and Technology Park of Abruzzo
Center for the Forecast of Severe Weather, CETEMPS
Livio Bernardini
Scientific and Technology Park of Abruzzo
Center for the Forecast of Severe Weather, CETEMPS
Guido Visconti
Department of Physics, University of L'Aquila-Coppito-L'Aquila
Center for the Forecast of Severe Weather, CETEMPS
Abstract. Recent and repeated episodes of severe weather in Italy have stressed the need to have a sufficiently accurate forecast in order to give an adequate warning to the involved areas. The impact of the precipitation however is also a function of the characteristics of the hydrological basin. From this point of view a rather startling example is the disaster which hit the Campania region May 5th, 1998 in which a moderate precipitation (about 100 mm in 24 hours) produced a huge landslide which killed or injured several tens of people and produced serious damage to the area. Such localized events do require among other things the use of a high resolution weather forecast. In this paper a forecast of the Campania event using a limited area model at 3 km grid resolution is presented. The forecast rainfall at several grid resolution is used to initialize a simple hydrological model to estimate the runoff. The numerical experiments suggest that high resolution may be a key factor in predicting the runoff.
1. INTRODUCTION
The precipitation forecast over complex topography is still a challenging problem. The Mediterranean area is often interested by catastrophic events related to heavy precipitation. In the last 10 years more than one flood hit Italy producing severe damages.
During May 2-5 1998, cyclogenegesis over Tyrrhenian sea produced rainfall over Campania, a region of central Italy (Fig. 1); the rainfall caused a severe landslide destroying several villages and killing many people.
To prevent these events, both a good forecast of the precipitation and the knowledge of the characteristics of the terrain are necessary. Most of the Italian weather forecasts are performed using hydrostatic models reaching resolutions lower than 9 km. A few studies using this resolution for the model simulation showed good skill for the precipitation forecast but at time missing the location of maximum rainfall (Paccagnella et al., 1992, 1995). The hydrostatic version of the MM5, from Penn State/National center for Atmospheric Research (PSU/NCAR), also showed good skill for the precipitation forecast, but the location of the maxima of the rainfall were not correct (Ferretti et al., 2000b), whereas the same event analyzed using the non-hydrostatic version of the MM5 showed better results (Paolucci et al., 1997). A study of the Piedmont flood using the non-hydrostatic version of the MM5 showed good skill for both the precipitation amount and the extent of the rainfall (Ferretti et al., 2000a).On the other hand, a comparison between the MM5 score and a few other models (Richard, 1998) revealed a poor skill by the MM5 , but the resolution used for that study was 10km.
Recently, in the framework of a cooperation between University of L'Aquila and Scientific and
Technology Park of Abruzzo the MM5 has been made operational, being the only high resolution non-hydrostatic model used for weather forecast in Italy. The probability of occurrence of post events, (landslides and avalanches), like the one that will be analyzed in this study, is known to be correlated with complex statistical moments of the meteorological variables (in this case precipitation) besides, of course, the structural parameter, like ground conditions. Therefore, a high resolution forecast of the meteorological variables may help to forecast these events.
An attempt to forecast the hydrological impact of this event is presented: the runoff, related to the heavy precipitation of this case, using an off line scheme for the soil is computed.
The aim of this paper is to show how a high resolution weather forecast may help to correctly forecast a few hydrological parameter. To this purpose sensitivity test to the grid resolution are carried out using a non-hydrostatic model. The model results are used to initialize a simple hydrological scheme, the Biosphere-Atmosphere Transfer Scheme (BATS).
The meteorological characteristics of this event and the precipitation recorded by an high resolution rain gauge are presented in section 2; the experimental set up for both MM5 and BATS is discussed in section 3. The results are presented in section 4 and the conclusion are given in section 5.
2. METEOROLOGICAL CHARACTERISTICS
This event was characterized by cyclogenesis over the Mediterranean sea a deep low level depression entering this region from the south intensified over the Tyrrhenian Sea on May 3rd, and reached a minimum of 990 hPa by May 4th at 0000 UTC (Fig. 2a). The cyclone was associated with a cut-off low (Fig. 2b) which swept over the Mediterranean area. During its passage the axis rotated clockwise producing a westerly flow both at upper and lower level, by the late afternoon (Fig. 3a,b). Advection of warm and humid air persisted for the rest of the day over most of southern Italy. The day before the event, the local rain gauge recorded light precipitation over the disaster area, reaching about 75 mm in 24h (not shown). During May 5th the rainfall intensified both over this area and south of it (Fig. 6, the rain gauge data are superimposed to the model results). However, the amount of rain was less than half of that recorded during the Piedmont flood of 1994, which was a major severe weather event. Therefore, it makes sense to suppose that the damage produced by precipitation is strongly related to the nature and morphology of the terrain for this event, besides the correlation with the meteorological variables as already pointed out.
3. NUMERICAL EXPERIMENT SET UP
3.1 Mesoscale model description
The MM5 (PSU/NCAR) is a non-hydrostatic limited area model fully compressible at the primitive equations (Dudhia, 1993, Grell et al., 1994). To enhance to forecast over the area of the slide, three domains two-way nested and 24 vertical levels (sigma coordinate) unequally spaced are used. The MRF (Troen and Marth, 1986, and Hong and Pan, 1996) Planetary Boundary Layer parameterization and the Grell (Grell, 1993) cumulus convection parameterization associated with an explicit computation of rain water for domain1 and 2, whereas only the last one for domain 3, are used. The forecast is performed by using three domains, the first domain (1) has a resolution of 27 km and contains 50 (latitude) x 66 (longitude) grid points; the intermediate domain (2) has 9 km resolution and 58x67 points and the high resolution domain (3) has 61x58 grid points with a 3 km grid size. To enhance the local forcing by the soil a 1 km resolution land use data (USGS) specifies the vegetation and the soil characteristics.
A few model simulations are performed to the aim to verify the sensitivity of the run-off to the grid resolution of the MM5. A simulation using only one domain (D11) at 27 km is performed, and the results are compare with both the observations (OBS) and the results of domain 1 (D21) obtained by using two domains two way nested at 27km and 9km respectively for domain1 and 2. A last simulation performed using three domains (D3) respectively at 27, 9, 3km for domain1 , 2 and 3 is performed. The D3 results are compared with both the observations and the previous simulations: domain 2 of two domains only (D22) is compared with domain2 of three domains (D32), and finally D33 (domain3 ) is compared with the OBS only (Table 1).
All the MM5 forecasts are initialized by using the European Center for Medium-Range Weather Forecast (ECMWF) data analysis and the Boundary Conditions are upgraded every 6 hours by using ECMWF forecast.
3.2 Hydrological model description
The MM5 output is coupled off line with the Biosphere Atmosphere Transfer Scheme (BATS) to evaluate the runoff expected from the precipitation. BATS can predict a number of soil variable based on the specification of soil characteristic and vegetation cover.
The BATS (Dickinson et al., 1986) estimates the transfer of momentum, heat and moisture between the surface and the atmosphere when the boundary meteorological conditions are specified, it also computes soil moisture content and runoff. In the past, this scheme has been coupled to the MM4, the hydrostatic version of the MM5, by Giorgi (1989) for climate studies. BATS is applied to the landslide area and it is initialized by using both the observations and the MM5 precipitation, temperature and wind forecast for May 5th . The 1 km land-use for the MM5 allow for estimating the vegetation type in the slide area: cropland and woodland mosaic with urban and built-up land in the nearby area is mostly the soil type of this region.
4. RESULTS
4.1 MM5 precipitation forecast
Two operational forecasts covering the whole period are performed: one starting at 1200UTC on May 3rd and lasting for 48h; the second one starting at 1200UTC on May 4th and ending at 1200UTC on May 6th. The results of both of them, for the experiments in table 1, are used for initializing the BATS. For each run the first 12h are discarded in order to take into account the spin up time, therefore, the two forecasts have an overlapping period from 0000UTC of the 5th to 1200UTC of the same day. The OBS are available from 0000UTC of the 4th to 0000UTC of the 6th . The comparison between the model rainfall and the observations is now presented.
To ensure a multi-scale analysis all the results of the model on domains 1, at the same resolution but using different nesting levels, are compared with the OBS. The same comparison is performed for the domains 2.
The comparison between the 24h accumulated precipitation for D11 (Fig. 4a) and the OBS (Fig. 6) shows a large underestimation of the rainfall, furthermore the location of the maximum is too much inland. If higher resolution associated with the two way nesting for domain 1 and 2 (D22) is used, a measurable improvement both in the total amount of rain and in the position of the maximum is found (Fig. 4b). Indeed, the most intense precipitation has now moved toward the coast. In this area is located Sarno, the town more affected by the flood. The differences between D11 and D21 are produced by the feedback process due to the two-way nesting technique; indeed, in this case a feedback from the fine mesh to the coarse resolution ensure the small-scale forcing to affect the large scale. The rainfall produced on domain 2 (D22, Fig. 5a), which feeds back to domain 1, is closer to the OBS (Fig. 6) than both D11 and D21 . This imply a large gain in the precipitation forecast by increasing the resolution only. If three domains two way nested are used , the rainfall on domain 2 (D32) is further improved (Fig. 5b). It can be easily noticed that there is a considerable improvement in the rain predicted in both cases D22 and D32 with respect to domains 1.
A direct comparison between the high resolution rainfall (D33) and the OBS is now presented. Figure 6, where the rain gauges data are superimposed to the model prediction, shows that the rainfall in the southern area agrees reasonably well with the D33 results, with peak values around 100 mm in 24h. On the other hand, the model seems to under predict the rain in the southernmost region while the northern patch of rain seem an almost complete artifact. The rain produced by the model is strongly influenced by the updraft forced by the orography and this may very well be the cause of the northern patch of rain.
The 24hr accumulated precipitation recorded (not shown) light precipitation starting from May 1st and decreasing by the end of May 3rd ; during May 4th a maximum of the rainfall reaching
84.6mm in the Campania region was recorded, whereas during the day of the slide 103mm are reached south of the slide area. In the disaster area only 60mm are reached. D33 shows a similar pattern to the OBS, both for the amount and the distribution of the precipitation, reaching values higher than 50mm. The day before the catastrophe light precipitation occurred reaching 24.4mm and doubling during the following 24h; during the first day, the precipitation forecast (not shown) produces 25mm, and it reached 50mm the day after (Fig. 6), showing a correct tendency. These results confirm the good skill by the model to forecast precipitation even at very high resolution. Indeed, the model forecast show a very good agreement with the high resolution precipitation data on the fine mesh; the precipitation are highly localized, and the maximum of the precipitation is close to the hill where the landslide occurred.
4.2 Run-off results
An attempt to estimate the run-off using BATS is now presented. The observed rainfall is used to estimate the run-off that will be used as reference (obs). The run-off produced by BATS using the rainfall from D11, D22 and D33 is compared with obs. To ensure a correct response by BATS to the precipitation event the simulation started a few day before the disaster to allow the soil to reach the right amount of soil moisture. In fact the day before the disaster precipitation occurred making the soil wet. The run-off produced by this precipitation is shown on figure 7 before May 3rd, but it is not real.
The comparison between the run-off produced by both the observed and the forecast rainfall shows (Fig. 7) that as the resolution increases the signal get closer to the real one. Indeed, the rainfall obtained by using one domain only produces a strong underestimation of the runoff (Fig. 7a), only a weak signal during May 5th is obtained. If the resolution is increased, that is the rainfall from D22 is used to estimate the runoff (Fig. 7b), a slightly improvement is obtained. Finally, if higher resolution is used the runoff (Fig. 7c) is in good agreement with the one obtained forcing BATS with the observations (Fig. 7obs). The maximum run-off is still underestimated but the bimodal structure is clearly reproduced by the high resolution rainfall. It is clear from this results that increasing the weather forecast resolution the right amount of run-off is obtained.
5. CONCLUSIONS
The MM5 forecasts and the run-off produced by BATS show that increasing the resolution a large improvement in both the rainfall and the run-off is obtained. At the low resolution the rainfall is largely underestimated and the maximum of the precipitation is displaced. Increasing the resolution (up to 3km) both a good precipitation forecast and an estimation of the run-off in very good agreement with the one produced by using the observed precipitation, are found. The high resolution rainfall forecast is in good agreement with the observations, but a model tendency to overestimate the rainfall, if strong uplifting is present as for the mountain regions, is found. However, D33 is the only one able to separate areas with different rainfall. These results suggest that the high resolution is required for correctly evaluate the hydrological impact. Indeed, even using a simple hydrological model the role of the grid resolution in the rainfall forecast is highlighted .
Acknowledgements. This work has been partly supported by Parco Scientifico and Tecnologico d'Abruzzo and by GNDC (Gruppo Nazionale Grandi Catastrofi). Servizio Idrologico of Campania Region is also acknowledged for the rain gauge data. NCAR is acknowledged for the MM5 model.
REFERENCES
Dickinson R.E., A. Henderson-Sellers, P.J.Kennedy and M.F. Wilson, Biosphere-Atmosphere Transfer Scheme (BATS) for the NCAR Community Climate model. NCAR Tech. Note NCAR/Tn-275+STR. Natl. Cent. for Atmos. Res., Boulder, CO. ,1986.
Dudhia J. A nonhydrostatic version of the Penn State-NCAR mesoscale model: Validation tests and simulation of an Atlantic cyclone and cold front. Mon. Wea. Rev., 121, 1493-1513,1993.
Ferretti R., S. Low-Nam, and R. Rotunno, Numerical simulations of the Piedmont flood of 4-6 Nov 1994. Tellus, 52, 162-180, 2000a.
Ferretti R., T. Paolucci, W. Zheng, G. Visconti and P. Bonelli, Analyses of the precipitation pattern on the Alpine region using different cumulus convection parameterizations. J. Appl. Meteorol., 39, 182-200, 2000b.
Giorgi, F. Two-Dimensional simulation of possible mesoscale effects of nuclear war fires. 1 Model description. J. Geophys. Res., 94, 1127-1144, 1989.
Grell G.A., Prognostic evaluation of assumption used by cumulus parameterization. Mon. Wea. Rev., 121, 764-787, 1993.
Grell G.A., J. Dudhia and D.R. Stauffer, A Description of the Fifth-Generation Penn State/NCAR Mesoscale Model (MM5). NCAR Tech. Note NCAR/Tn-398+STR. Natl. Cent. for Atmos. Res., Boulder, CO. , 1994.
Hong, S.-Y. and H.-L. Pan, Non local boundary layer vertical diffusion in a medium-range forecast model. Mon. Wea. Rev., 124, 2322-2339, 1996.
Paccagnella T., S. Tibaldi, R. Buizza and S. Scoccianti, High-resolution numerical modeling of Convective precipitation over Northern Italy. Meteorol. Atmos. Phys., 50, 143-163, 1992.
Paccagnella T., P. Patruno and C. Cacciamani, Operational quantitative precipitation forecast of the Piedmont flood event at the Regional Meteorological Service of Emilia-Romagna. Map Newsletter, 2, 7-11, 1995.
Paolucci T., R. Ferretti and G. Visconti, Numerical simulations of summer Convective precipitation cases in the Alpine region using different parameterizations: Preliminary results. Preprints, Seventh PSU/NCAR Mesoscale Model User's Workshop, 21-23 July 1997.
Richard, E., J. Charney, S. Cosma, R. Benoit, S. Chamberland, A. Buzzi, L. Foschini, R. Ferretti and J. Stein, Intercomparison of simulated precipitation fields for some MAP episodes. Map Newsletter, 9, 12-13,1998.
Troen, I. and L. Mahrt, A simple model of the atmosphere boundary layer: Sensitivity to surface evaporation. Bound. Layer Meteor., 37, 129-148, 1986.
FIGURE CAPTIONS
Figure 1. Model domains. Domain 1 has grid resolution of 27km, domain2 9km and domain 3 3km.
Figure 2. Ecmwf data analyses for May 4th at 0000UTC: a) sea level pressure (c.i. = 2hPa) and wind (one barb = 10 knots) at 925hPa; b) 500hPa-heght field (c.i. = 30m) and wind (one barb = 10 knots).
Figure 3. As Fig. 2 but for May 4th at 1800UTC.
Figure 4. 24h accumulated precipitation (mm) at ending at 0000UTC May 5th predicted on domain 1 by using : a) one domain at 27 km grid resolution.; b) 2 domains two-way nested at 27 and 9 km resolution respectively.
Figure 5. As for figure 4 but on domain 2 using a two way nesting: a) between domain 1 and 2 at the same resolution of fig. 4b; b) between domain 2 and 3 at 9 and 3 km resolution, respectively.
Figure 6. 24h accumulated precipitation (mm) ending at 0000UTC May 5th both predicted on domain 3 using three domains two way nested and observed at a few stations (colored circles). The circles are filled with the color corresponding to the scale shown on the right.
Figure 7. The run off in mm/days from 1 to 7 May, 1998, obtained when BATS is coupled to : the observations (obs); a) the rainfall produced by the model simulation performed using one domain only at 27km; b) the rainfall produced by the model simulation performed using 2 domains with the highest resolution being 9km; c) the rainfall produced by the model simulation performed using three domains with the highest resolution being 3km.
TABLE CAPTION
Table 1. The model simulation performed using different nested levels. D11 for model simulation using one domain only. D21,2 for model simulation using two domains. D31,2,3 for model simulation using three domains.
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MODEL |
DOMAINS |
GRID SIZE (km) |
ACRONIMUS |
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MM5 |
1 |
27 |
D11 |
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MM5 |
2 |
27 9 |
D21 D22 |
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MM5 |
3 |
27 9 3 |
D31 D32 D33 |
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