Simulating tropical cyclone activities using regional climate model Tuan-Long TRINH, Hoang-Hai BUI, Van-Tan PHAN, Quang-Trung NGUYEN Department of Meteorology Hanoi University of Science, VNU ABSTRACT The Regional Climate Model (RegCM3) from ICTP (International Center for Theoretical Physics) with roughly 36-km horizontal grid spacing is used to simulate the inter-annual variability of tropical cyclones activities in the Northwest Pacific Ocean. The simulation domain is from 0N to 33N and from 100E to 160E. Numerical experiment was carried out in 1995-1997, remarkable years when the ENSO event shifted from cold phase to warm phase. Output fields of RegCM3 were interpolated to 4 levels (850hPa, 700hPa, 500hPa, 300hPa) and the criterion for a possible candidate of TC was selected with a standard of vortices (0.5x10 -5 ). If the center of vortex and sea level pressure (SLP) center are found with bi-rational interpolation, other criteria will be checked to confirm “TC is detected”, including the anomaly of temperature at each level, the outer core wind strength (>5m/s on 850 mb) and the decrement of pressure (> 1hPa). Results show that the number, position and motion of detected TCs agree well with the observation. The number of TCs is in general seasonally well represented with the highest one in August. However, “detected TCs” occur also in the end of winter, which is usually not the case of the observed TCs. Keywords : tropical cyclone, hurricane, regional climate model, Northwest Pacific Ocean, tropical cyclone detection scheme. 1. Introduction Today, there are many evidences showing that climate change is related to abnormal changes of weather and climate in many regions on the earth. The cause of large scale circulation changes is mainly from global warming and there is an increasing trend for extreme events, including tropical cyclones (TCs). The detection scheme which uses outputs of regional climate models has been tested in this study. Outputs of RegCM3 with the roughly 36-km horizontal grid spacing in North Western Pacific domain (0-33N, 100-160E) to simulation operation of TCs. Then the results of detection process are assessed detection ability to develop scenario of operation TCs in the future. The formation conditions of TCs have been concluded in details by previous studies, these conditions include: high enough surface temperature, weak enough vertical wind shear strong enough, large scale vortices in the troposphere… Usually, The resolution of regional climate models using lower than scale of tropical cyclone, if Its exist on climate model, Its express quite weak. For global climate models have low resolution very low, tropical cyclone area is just expressed as an anomaly point of thermal, pressure and vortex field on a grid cell. For regional climate models which have higher resolution, tropical cyclone like vortices is expressed more strongly comparing to lower resolution but its amplitude is still weaker than the real therefore, to detect TC from outputs of regional climate models. It is necessary to develop an ensemble of criteria which is weaker than the condition of real TCs. The TC detection is called tropical cyclone like vortices (TCLV) or simulated TCs. In the research of Bengtsson at al (1995), simulated tropical cyclones have different atmospheric general circulation with other regions: vortex at 850hPa, wind at 10m, temperature field at 850hPa, 700hPa 500hPa, 300hPa and sea level pressure field. According to Carmargo and Zebiak (2002), the TC detection method not only miss a lot of TCs but also catch many TCs which just are local minimum pressure. Base on their experiments, they concluded that TC detective scheme should set up the criteria that depend on individual region and Models. Walsh and Watterson (1997) used a criteria that include vortices at 850hPa greater than a value (2x10 -5 s -1 or 5x10 -6 s -1 ) outer core wind strength at 10m greater than a value (10 ms -1 or 6 ms -1 ) and anomalies temperature on middle troposphere to determine vortex storm. They found that regional climate models with resolution of 120km are able to got good simulation but the results is better if the resolution is higher. Nguyen and Walsh (2001) detected TC in DARLAM model with resolution of 125km, and used the criteria of 6 condition: (1) vortices greater than 0.5.10 -5 s -1 , (2) scheme is able to detect almost minimum pressure local in radius 250km from condition (1). The central lowest pressure are considered as tropical cyclone like vortices. (3) The upper troposphere temperature profile satisfy hot tower structure : the sum of the temperature deviations at 750, 500 and 300 hPa exceeds 0.5K (4wind speed average in radius 300x300km surrounding the core at 850 hPa is greater than at 300 hPa. (5) Anomalies temperature in central vortices at 300 hPa is greater than at 850 hPa. (6) Outer core wind strength must be greater than 5ms -1 . Otherwise, if a TC like vortices already satisfy all of there conditions at least 24 hours, TC detective scheme can skip conditions (3),(4),(5) and just keep condition (6) until it no longer satisfies, the scheme will finish. Although the RegCM3 with resolution of 36km is still lower than actual tropical cyclone scale, but the results have proposed some climatology characteristics of TCs. In the past, it is few of studying about TC detection which uses output of regional climate models for South China Sea. Carmargo and Zebiak (2002), despite their TC detective scheme was using an improved with more criteria depend on individual different regions, TCs in South China Sea cannot represent the operation of tropical cyclone, it is caused by low resolution of global models. Recently, the regional climate models are improved quit well. The higher resolution of regional climate models offer a good ability of simulating, prediction and estimating the action of TCs. Therefore construction of tropical cyclone detective scheme important issue. This paper would like to present about a tropical cyclone detection method using output of RegCM3, and the initial results ability to simulate the operation of TCs over South China Sea and North West Pacific. Configuration and experiment methods to tropical cyclone detection with output of RegCM3 is presented in section 2. Some experiment results and comment in section 3 and section 4 are a few conclusions and recommendations. 2. Methodology 2.1 Model, reanalysis and best-track data With processing of research and development, the important of feature regional climate models was improved and upgraded. In this research, RegCM3 running with resolution 36km-mesh and 18 layers vertical scheme. The center of domain is 17 0 N, 130 0 E with 192x104 grid box as domain include 100-160 0 E and 0-33 0 N. That domain enough large to catch more tropical cyclones formation in South China Sea but also tropical cyclones formation on North West Pacific area and move to South China Sea. Schemes are used in RegCM3 include Community Climate Model Version 3 (CCM3), Biosphere Atmosphere Transfer Scheme (BATS) and four different tool with convection scheme: Kuo scheme, MIT-Enmanuel scheme, Grell scheme with close hypothesis Arakawa-Schubert (Grell-AS 1974) and close hypothesis Fritch-Chappel (Grell-FC). In our experiment, RegCM3 running with Grell-FC convection scheme. That field data atmosphere used data reanalysis ERA40 for input condition and boundary condition with mesh resolution about 2.5 x 2.5. Time integral in that Model 00UTC 12/01/1994 to 00UTC 12/31/1997. In that, the December of 1994 is spin-up time. Bottom layer boundary on ocean area used Optimum Interpolation Sea Surface Temperature Analysis (OISST). Observation number tropical cyclone data reproduce from website weather.unisys.com/hurricanedata. 2.2 Tropical cyclone detection method Tropical cyclone detection method was used in this experiment will be descript in this section. Criteria and algorithms to detect tropical cyclone from output of ReCM3. The first, meteorological factors need convert from model isobaric layer to four standard isobaric layer as 850hPa, 700hPa, 500hPa and 300hPa. Detection process is done with all time step of output model in follow five process. At each time step, Detection scheme check each grid point and to find the local maximum vortex greater than a target criteria (5x10 -5 s -1 ). Local maximum vortex is determined when vortices in a grid point greater vortices of four beside grid point. If a grid point satisfy this criterion, it is considered a candidate of eye tropical cyclone. If the grid point of the candidate tropical cyclone center is defined as where minimum surface pressure is at least 1 hPa within a radius of 250 km from that candidate by downhill method combined with spline interpolation of two-dimension rational. Center of sea level pressure not necessarily coincide with the grid point by interpolation method. If center of sea level pressure is found, more criteria will be use for calculation to determine this is a center tropical cyclone. The criteria include : Sea level pressure anomaly must lower than a criteria value (-1 hPa) The total temperature anomaly in the isobar greater than 0.5 0 C Outer core wind strength greater than a criteria value (5ms -1 ) When all criteria are satisfied, the center of sea level pressure is considered a center tropical cyclone. Because the detection is done at each step time (6 hour) so detection process should be identified as shown the center of storm formed a new storm or a storm have existed since before the step time. This process is done by checking the existence of a storm after step time radius 300km cycle whose center is at the storm. According to research by Weatherford and Gray (1988), Outer core wind strength is tangential wind speeds average scarf ring domain between the two circles radius 10 to 2.5 0 longitude. In that experiment, outer core wind is calculated by an average of 36 points on four cycle (Fig 2a). This process is done by interpolation of wind field to grid point by spline parallel rationale method. The anomaly of field values is calculated similar. The field of anomaly sea level pressure and anomaly temperature is done by subtraction of center vortex and value of 8 points the circle radius from the center 2.5 0 longitude (Fig 2b). Finally, to result remote high frequency noise, the vortices do not exist under 48 hour will be considered that only the vortex exists at least 2 days (8 step time) is considered swirling storm. Figure 1 Domain of RegCM3 simulation. Figure 2 Calculation outer core wind speed scheme and anomaly of value field scheme 3. Result and Discussion 3.1 Result of tropical cyclone detection In the period 1995-1997 are generated, ENSO (El-Nino Souther Oscilation) phenomenon conversion from La Nina phase (cold phase) to El Nino phase (warm phase). In that period time study (three years), the result of observations on website: weather.unisys.com/hurricanedata (figure 3) the North West Pacific basin about 111 TCs (about 37 TCs per year) more 23% than average of thirty year ago. The number of tropical cyclone impact to South China Sea so much (about 9 tropical cyclone). Figure 3 Tropical cyclones Observation in 1996 (weather.unisys.com/hurricanedata) The trajectory motion of tropical cyclone observation (figure 4) and the orbital of tropical cyclone detection (figure 5). Doman detection enough larger for tropical cyclone activity. When compared (figure 4) with (figure 5) show differences between tropical cyclone formation regional and activity regional. Result show that tropical cyclone of detection is located above 30 degrees north latitude and western of 150 west longitude. Tropical cyclones of detection overview move from West to East and parabolic form from Southeast to North West. The number of tropical cyclone detection (about one hundred) similar with tropical cyclone of observation on North West Pacific (111 TCs). Although trajectory tropical cyclone detection quite difference as tropical cyclone observation mainly in South China Sea. In this domain, TCs detection have trajectory anomalies and little movement difference with observation. Figure 4 Observed Tracks (1995-1997) Figure 5 Detection scheme Tracks (1995-1997) 3.2 Tropical cyclones Detection Analysis That report is a examine the ability to detect tropical cyclone of TCs detection scheme from output of RegCM3 model and simulation cavalier the operation of TCs in North West Pacific domain. RegCM3 are integral on domain enough large cover all activity of tropical cyclone in period 1995-1997. Figure 6 show that, monthly frequency of tropical cyclone detection quite same as observation. However experimental detection in winter of northern Hemisphere still have any tropical cyclone like vortices but un exist on reality. The cause of that problem is ability simulation of RegCM3, some factor of meteorological incomplete math with reality as observation. Also trajectory of any tropical cyclone detection moving strange. That problem may be criteria detection inappropriate. Condition of tropical cyclone detection weaker than observation condition. Example outer core wind strength (OCS), pressure anomaly and lifetime of tropical cyclone are quite short because simulation of model usual weak. Figure 6 Observed and model simulated seasonal cycle (number of hurricanes per month, 1 = January, 12 = December) 4 Summary In this study, the tropical cyclone detective scheme using the output of RegCM3 has got a good simulation for the some characteristic of TCs track in North West Pacific in 1995-1997. The number of simulated TCs in this region is quite similar to the observation. But this scheme still not well describe the monthly frequency of TCs specially in winter time of Northern Hemisphere. The normal trajectory simulation tropical cyclone form relatively same as general rules of movement of tropical cyclone over North West Pacific basin. Although that region still exist a difference between the trajectory tropical cyclone in simulation and trajectory monitoring, especially with tropical cyclone forming over the South China Sea. The experimental results show possibility of simulating tropical cyclone with output of RegCM3 as the primary rationale of tropical cyclone detection scheme. The difference between output of modeling simulation and observation may be related to the horizontal resolution of the RegCM3 is not enough smooth and the criteria used to detect non exactly to be able to get result more reasonable. That need to survey more other case necessary. 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The higher resolution of regional climate models offer. Simulating tropical cyclone activities using regional climate model Tuan-Long TRINH, Hoang-Hai BUI, Van-Tan PHAN, Quang-Trung