Sensitivity of precipitation forecasts to cumulus parameterizations in Catalonia (NE Spain)

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1 Sensitivity of precipitation forecasts to cumulus parameterizations in Catalonia (NE Spain) Jordi Mercader (1), Bernat Codina (1), Abdelmalik Sairouni (2), Jordi Cunillera (2) (1) Dept. of Astronomy and Meteorology, University of Barcelona, Barcelona, Spain (2) Meteorological Service of Catalonia, Barcelona, Spain 1. Introduction Precipitation forecasts in Catalonia (Northeastern Iberian Peninsula) have always been a challenge due to the complexity of its orography and its situation beside the Northwestern Mediterranean Sea (Figures 1a and 1b). Heavy rainfall events are frequent and 24-hour precipitation amounts exceeding 200 mm are reached almost every year. Despite being a small region (32,000 km 2 ), contrasts between precipitation amounts recorded in a particular event can be very large. In order to evaluate the sensitivity of precipitation forecasts to the cumulus parameterizations and some of the cloud microphysical schemes provided by the WRF model, as well as the ability of this model to simulate the most important features of rain events in Catalonia, several simulations for selected episodes are being performed. This work is included in a larger scope project carried out by the Meteorological Service of Catalonia (SMC) and the University of Barcelona. This project is aimed to set up a suitable operational configuration of WRF- ARW to be used as a forecasting tool in SMC. In the poster, preliminary results obtained for two convective precipitation events have been selected to get an appreciation of the performance of WRF model: August 25 and September 13, during later summer of a) b) Figure 1. a) Situation of Catalonia (orange square), in NE Iberian Peninsula. b) Enlarged area displaying the most important mountain ranges. In both maps, the green square indicates the situation of Barcelona. 2. Selected events The first event considered as a case took place on August 25, 2006 and was characterized by cold and moist air in midlevels of the troposphere. According to the Barcelona sounding, at 12Z the temperature at 500 hpa was close to -13ºC and the CAPE 649 J. The main event of that day was an isolated storm that originated a flash-flood in a quite populated touristic area of the coast (Figures 3a,c,e,g). More than 35 cars parked in a usually dry stream bed were swept to the sea. The economical losses were noticeable. Also, the effects of a tornado were observed. The synoptic situation on September 13, 2006 was characterized by a through with cold and moist air that crossed the Iberian Peninsula from NW to SE. Precipitations affected all Catalonian areas, many stations

2 recorded 24-hour precipitation amounts exceeding 100 mm, with a maximum record reaching 238 mm, while other nearby observatories recorded total daily amounts between 10 and 20 mm. Most of the rainfalls were concentrated in two rain bands (Figures 4 a-k) that crossed the region following a SW- NE direction. The second one became stationary for almost 3 hours when reaching the NE of Catalonia and caused the largest amounts of precipitation of that event (Figure 4k). 3. Model simulation configuration In this work, the WRF-ARW model version 2.2 has been used, with the configuration described below Model domains Simulations were performed using a set of 3-nested domains with horizontal grid-point resolutions of km. The outer domain is a grid of 94x102 points which covers the SW of Europe; the second domain has a grid size of 70x70 points, and the finest mesh covers Catalonia and its surrounding area (Figure 3) with a grid of 88x88 points. Vertical levels used in these simulations are the default 31 levels defined in WRF (Wang, W., et al., 2007) Initial and boundary conditions To generate both initial and boundary conditions, GFS forecasts are used, with a horizontal resolution of 1º and a 6-hour time interval between two consecutive forecast outputs. Specifically, to initialize a WRF simulation at 00 or 12Z, GFS forecasts initialized 12 hours earlier have been used. Consequently, the first guess is actually generated from a GFS 12-hour forecast and subsequent 6-hour forecasts are used to provide boundary conditions for the coarsest domain. For simulations in nested domains, both initial and boundary conditions are supplied from their parent domain s run. Both 12 and 4- km simulations are initialized at the same time as the coarsest domain run and boundary conditions are updated every 3 hours Observational data assimilation Conventional surface and upper-air data are assimilated, using WRF-3DVar applications (Barker, D.M, et al., 2004), in order to improve the first guess and the boundary conditions in the coarsest domain. This configuration described above has been made in order to compare WRF with MM5 forecasts generated in SMC for operational purposes (RAM-SMC, 2005) Parameterization settings Figure 2. The 3-nested domains with horizontal resolutions of km. These simulations have been carried out using the NOAH (4 subsoil layers) land surface model, the Yonsei University (YSU) planetary boundary layer scheme, the Monin-Obukhov scheme for surface layer physics, along with the Dudhia and RRTM parameterizations for short-wave and long-wave radiation processes. (Skamarock, W.C., et al., 2005). These parameterization settings have been kept without change for all simulations. On the other hand, in order to evaluate the sensitivity of precipitation forecasts to each of the cumulus parameterizations supplied by the WRF model and some of the available clouds microphysical schemes, several

3 simulations have been performed for each selected event trying different combinations of these physical parameterizations (see table 1). Domain/ resolution Cumulus schemes Clouds microphysical 36 km - Kain -Fritsch - Betts Miller Janjic - Grell-Devenyi - WSM3 - WSM5 - Thompson - Kain-Fritsch - WSM3 12 km - Betts-Miller-Janjic - WSM5 - Grell-Devenyi - WSM6 - NO convection - Thompson - Kain-Fritsch 4 km - Betts-Miller-Janjic - WSM6 - Grell-Devenyi - Thompson - NO convection Table 1. For each domain, cumulus and cloud microphysical schemes that had been used to evaluate the sensitivity of precipitation forecasts to them. 4. Results 4.1. August 25, 2006 event All runs were initialized at 12Z. The 12- km runs with the cumulus parameterizations of Betts Miller-Janjic and Grell-Devenyi forecasted the convective rainfalls at the mountainous area of Pyrenees, but they were not able to reproduce the isolated storm that developed in the Pre-litoral mountain range and later reached the coastline and produced a flash-flood. In fact, both simulations forecasted little precipitation on the sea when the storm had already passed over the coastline, where that storm actually finally went (results not shown). In contrast, the results of the 12-km run with the Kain-Fritsch convection scheme (not shown) displayed a rainfall between 18Z and 22Z that can be associated to the radar signature of the storm. Although cumulus parameterizations should not been used in simulations with horizontal grid-point resolutions of 4-km, one of these schemes is needed to forecast this storm (Figures 3b,d,f,h). Otherwise, this event is missed. The results provided with the runs that had used different clouds microphysical schemes showed more differences in rainfall intensity than in any other feature of the precipitation field, such as timing or location (not shown). Specifically, the Thomson scheme gives, in general, higher intensities than the WSM5 or WSM6 schemes. In addition, the sensitivity of the precipitation field of a nested run to the cumulus parameterization used in its parent domain s run has been also evaluated. In fact, many simulations in this event had shown that sensitivity to this factor can be greater than the sensitivity to the clouds microphysics schemes (not shown). a) RADAR: 18Z b) WRF: 18Z c) RADAR: 19Z d) WRF: 19Z e) RADAR: 20Z f) WRF: 20Z g) RADAR: 21Z h) WRF: 21Z Figures 3a,c,e,g. 1-hour accumulated precipitation from the radar imagery of the August 25, 2006 storm. From 18Z to 21Z it followed a NW-SE trajectory, crossed the coastline about 20Z and produced a flash-flood. Figures 3b,d,f,h. 1- hour accumulated precipitation output for the WRF 4-km run with the Kain-Fritsch cumulus parameterization (and the Betts-Miller-Janjic cumulus parameterization on its parent domain s simulation). Despite the storm was forecasted in this work, precipitation intensity was underestimated.

4 4.2. September 13, 2006 event Both the 12-km and 4-km runs, which were initialized at 00Z of September 13, were not able to capture the occurrence of the first rain band (Figures 4a,4c), probably because of the spin-up of the model and the wind direction simulated in low-levels, which was different from the actual wind direction in environment. However, the second rain band was quite well forecasted in terms of the shape and movement, but only when no convective schemes had been used, both in the 4-km runs and, surprisingly, in the 12-km runs. In the 12-km run with explicitly-resolved convection, the second rain band was actually advanced in time, because it can be seen in the model output (Figure 4b) such early as 02Z, while the first signals of this rain band did not appear in the radar signature until 07Z (Figure 4e). After its formation, this rain band remained stationary for two hours and finally it began to move towards NE (Figure 4g). This behavior was forecasted by simulations too, despite a little advance in time (Figures 4f,4h). Finally, the rain band became smaller, changed its orientation and remained stationary near the coastline in the NE of Catalonia (Figure 4k), producing the largest amounts of precipitation of that event. As it can be seen in figures 4j and 4l, this movement and the final placement of the rain band were also well simulated with a little displacement. The best agreements with the 4-km runs were achieved with explicitly-resolved convection too (not shown). In comparison with the simulations provided by the 12-km runs, they showed the second rain band moving faster. On the other hand, the experiments designed to evaluate the sensitivity of the forecasted precipitation field to the cloud microphysics schemes revealed that simulations with the Thompson scheme produced higher intensities of precipitation than the simulations with other cloud microphysics parameterizations, such as the WSM5 or WSM6 (results not shown), as it has been seen for August 25 event yet. a) RADAR: 02 Z b) WRF: 02 Z c) RADAR: 05 Z d) WRF: 05Z e) RADAR: 08 Z f) WRF: 08Z g) RADAR: 11 Z h) WRF: 11 Z i) RADAR: 14 Z j) WRF: 14 Z k) RADAR: 16 Z l) WRF: 16 Z Figures 4a-k. In the left-hand side, 1-hour accumulated precipitation from the radar imagery of September 13, In the right hand side, 1-hour accumulated precipitation output for the WRF 12-km run, with explicitly-resolved convection (and the Kain-Fritsch cumulus parameterization on its parent domain s simulation).

5 In addition, the influence of the cumulus scheme used in the parent domain s run was evaluated (results not shown). Such influence increases with time and it implies differences both in the position of the rain bands and the intensity of the rainfall. Consequently, the maximum 24-hours amounts of the forecasted rain fields have shown to depend on the cumulus parameterization scheme used in the parent domain s run. For instance, the nested 4-km run from a 12-km run with explicitly resolved convection showed a maximum amount of 24-hour forecasted precipitation up to 228 mm (good agreement with observed records) while the simulation whose parent was a 12-km run with the Betts-Miller-Janjic parameterization showed a maximum amount of 137 mm. Finally, the rain field from the operational output of the MM5 12-km run (with the Grell convection parameterization and the Schultz microphysics scheme) has been compared with the rainfall forecasts from the WRF simulations. At initial times, MM5 showed precipitation over the Northern areas of Catalonia, as it was observed by the radar imagery, while the WRF 12-km run (with explicitly-resolved convection) concentrated the rainfall in SW Catalonia (Figures 5a-c). However, after the first 6 hours, the WRF output displayed a rain band that began to move towards NE, while MM5 showed precipitations moving with this direction but reproducing some rainfall cores rather than a clearly band-shaped rainfall. 5. Conclusions Two convective rainfall events in Catalonia have been presented to get an appreciation of the sensitivity of precipitation forecasts to both cumulus parameterizations and cloud microphysics schemes. In the August 25 event, a cumulus parameterization in both 12-km and 4-km runs is needed to reproduce the storm that caused a flash-flood in a coastal area of Catalonia. However, for the September 13 event, neither of the simulations using cumulus schemes in the 12-km and 4-km domains were able to reproduce the observed pattern of precipitation, while the simulations with explicitly-resolved convection showed the best agreement with observations after the first 6 hours. Figures 5a-f. On the left hand side, at the top, 1-hour accumulated precipitation from the radar imagery of September 13, 2006 at 04Z; in the middle, 1-hour accumulated precipitation output for the MM5 12-km run at the same time, (with the Grell convection scheme) and in the bottom, 1-hour accumulated precipitation output for the WRF 12-km run (explicitly-resolved convection). On the right hand side, the same but at 10Z. Such results suggest the need for a new cumulus scheme at intermediate scales (between 3-4 and 10 km) because the actual parameterizations were designed for horizontal grid-point resolutions coarser than 10 km. It has been noted also that the most influence of the clouds microphysics schemes on precipitation forecasts is done in the precipitation intensity. In these events, the Thompson scheme produced higher intensities than the WSM5 or WSM6 parameterizations. Moreover, the sensitivity of nested runs to the cumulus parameterization used in their parent domain s run has been shown. This sensitivity increases with time and exerts influence on the location, timing and intensity of the rain bands.

6 Finally, a comparison between the operational MM5 12-km run and the WRF 12- km simulations has been shown. The MM5 forecasts agreed observations better than WRF during the first hours, but the WRF 12-km run with explicitly-resolved convection performed better after these first 6 hours, specially concerning the rain band structure. More comparisons between the performance of the WRF model and the other models used operationally at SMC, specifically MM5 and MASS, have to be done. Acknowledgements The radar imagery has been supplied by Remote Sensing team of SMC. The information about domain configuration of MM5 in SMC was given by Jordi Moré. We would thank the help and advice given by J.R. Miró, J. Toda, M. Aran and M. Bravo, from the Applied Research and Modeling Area of SMC. References Applied Research and Modeling Area, Meteorological Service of Catalonia (RAM- SMC), Verificació dels models operatius de mesoescala al SMC (Verification of mesoscale operational models in SMC) (Internal Technical Note) Barker, D. M., W. Huang, Y. R. Guo, and Q. N. Xiao., A Three-Dimensional (3DVAR) Data Assimilation System For Use With MM5: Implementation and Initial Results. Mon. Wea. Rev., 132, Skamarock, W. C., Klemp, J. B., Dudhia, J., Gill, D. O., Barker, D. M., Wang, W., and Powers, J. G., 2005 (revised 2007): A description of the Advanced Research WRF Version 2. NCAR Tech Notes-468+STR. Wang, W., Barker, D., Bray, J., Bruyère, C., Duda, M., Dudhia, J., Gill, D., Michalakes, J., User s Guide for Advanced Research WRF (ARW) Modeling System Version 2.2. Mesoscale and Microscale Meteorology Division - National Center for Atmospheric Research (MMM-NCAR).