Height of Warm core in Very Severe Cyclonic Storm Phailin: insat-3d perspective
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- Bu səhifədəki naviqasiya:
- Introduction
- VSCS Phailin
- INSAT-3D Sounder
- Prior studies on warm core structure
- Results and Discussions
- Conclusions Acknowledgments References
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Height of Warm core in Very Severe Cyclonic Storm Phailin: INSAT-3D Perspective S. Indira Rani, V. S. Prasad, E. N. Rajagopal and Swati Basu National Centre for Medium Range Weather Forecasting (NCMRWF) Ministry of Earth Sciences (MoES), Earth System Science Organization (ESSO), Governemnt of India A-50, Sector-62, Noida, Uttar Pradesh – 201 309, INDIA. email: indira@ncmrwf.gov.in Abstract The warm core structure of Very Severe Cyclonic Storm (VSCS) Phailin formed over North Indian Ocean during 8 -14 October 2013 was analyzed using INSAT-3D sounder Brightness Temperature (BT). Unlike the conventional belief suggested warm core existence at 250 mb, present study explicitly showed multiple maxima, with strong primary maximum in the middle level (600 -650 mb) and a weak secondary maximum in the upper level (300 – 250 mb). Due to the high resolution of (10 km) INSAT-3D sounder channels, the warm core structure below 10 km of the atmosphere is well resolved. All the 18 infrared channels of the INSAT-3D sounder showed positive perturbation throughout the core with varying intensity maximizing in the middle level.
Satellite observations play an important role in monitoring and tracking of tropical cyclones. The first satellite observations of tropical cyclones were made by low earth orbiting weather satellites and since the low earth orbiting satellites became operational in the mid 1960s, no tropical cyclone has gone unobserved. Using imagery from polar orbiting satellites, Dvorak (1973, 1975) developed a technique, which undergone several refinements, but is still in use today, to estimate the tropical cyclone intensity. Later, the focus of tropical cyclone observation shifted from low earth orbiting satellites (polar satellites) to geostationary satellites, because of the frequent imaging capability of the latter (Purdom and Menzel 1996; DeMaria 1996). In due course, the Dvorak technique was expanded to accommodate characteristics revealed in infrared imagery from geostationary satellites (Dvorak 1984) and Velden et al. (1998) automated this technique. Warm core is the characteristic that distinguishes tropical cyclones from its extra tropical counter parts, where the centre of the cyclone is warmer than its environment. The existence of the tropical cyclone warm core has long been recognized, but the structure is little known. Two of the most common variables used to characterize the warm core are its strength and height. The strength is given by the magnitude of maximum perturbation temperature and the height is the level where the maximum perturbation temperature occurs. Combination of diabatic heating in the eye-wall updraft and dry adiabatic descent within the eye contributes to the warming of the core of tropical cyclones. The descent in the eye is generally believed to be a forced response to the eye-wall heating (Schubert and Hack, 1982; Nolan et al. 2007; Vigh and Schubert 2009; Pendergrass and Willoughby 2009). Thus the structure of the warm core may be sensitive to the distribution of diabatic heating (Stern and Nolan 2011). The strength of the warm core increases with the intensity of the cyclone. The factors which determine the values of warm core strength and height are poorly understood. According to Stern and Nolan 2011; there are a number of misconceptions appeared in the literature regarding the physical interpretation of changes in the strength and height of warm core. Lack of quality observations of the structure of warm core limits the knowledge. The conventional wisdom says that the typical warm core height is around 250 mb (~ 10 km). This conventional wisdom is mainly based on three case studies from 1960s and 1970s, using flight-level observations at three to five altitudes (La Seur and Hawkins, 1963; Hawkins and Rubsam, 1968; and Hawkins and Imbembo, 1976). Hawkins and Rubsam found the strength and height of warm core in Hurricane Hilda (1964) to be +16° C and 250 mb respectively. Hawkins and Imbembo found two maxima in the warm core for Hurricane Inez (1966) at 650 and 300 mb, both same magnitude of +9º C. In their study, this double maxima as low as 650 mb, was believed to be unusual and a warm core height of 250 mb was thought to be more typical. Individual simulation studies by Liu et al. 1999; and Kimbal and Dougherty, 2006 have found opposite behavior (decreasing warm core height with increasing cyclone intensity) in contrast to the conventional view. Study of Hurricane Erin (2001) by Halverson et al. (2006) based on dropsondes, reported the warm core to be near 500 mb. As Inez (1966) was stronger than Erin (2001), Halverson et al.(2006) associated the lower warm core with the weaker storm. However, Dolling and Barnes (2010) showed from dropsondes that the height of the warm core in Hurricane Humberto (2001) was found between 2 to 3 km on three consecutive days, with the intensity varying from a tropical storm to category 2 hurricanes over this time frame. Stern and Nolan (2011) also reported the warm core maxima in the middle level through various simulation experiments, which is contrast to the conventional wisdom. In this study, the warm core of Very Severe Cyclonic Storm (VSCS) Phailin was studied using INSAT-3D sounder Brightness Temperature. This is the first of this kind over the Tropics, to determine the warm core through geostationary sounder observations. Section 2 gives a brief description of the history of VSCS Phailin, followed by Section 3 gives overview of INSAT-3D sounder channels. An attempt to revisit the available prior studies on the warm core structure is compiled in Section 4. The results and discussions of the present study are described in Section 5, and followed the conclusions in Section 6.
An average of 4 to 6 storms forms in the two main seas in the North Indian Ocean, the Arabian Sea (ARB) and the Bay of Bengal (BOB), between May and December with the peak formation in October and November every year. Phailin was the second-strongest tropical cyclone which made landfall in India, after the 1999 Odisha cyclone. The system was first noted as a tropical depression within the Gulf of Thailand on 4th October, 2013. Further the system moved westward within an area of low to moderate vertical wind shear, and moved out of the Western Basin on 6th October. The system was named Phailin on 9th October; afterwards it had developed into a cyclonic storm and passed over the Andaman and Nicobar Islands into the Bay of Bengal. Phailin intensified rapidly and became a very severe cyclonic storm on 10th October. The system started to weaken on 12th October as it approached the Indian state of Odisha. It made landfall later in 12th October, near Gopalpur in Odisha coast around 2130 IST (1600 UTC). Phailin subsequently weakened over land as a result of frictional forces, before degenerating into a well marked area of low pressure. Figure 1 shows the track of Phailin (Ref.) 
Figure 1: Phailin Track
INSAT-3D is the second of geostationary sounder after Geostationary Orbiting Environmental Satellites (GOES), and the first geostationary sounder over India and surrounding oceanic regions. INSAT-3D is a multipurpose geosynchronous spacecraft with main meteorological payloads (imager and sounder). The mission goals are to provide an operational, environmental and storm warning system to protect life and property. The satellite has three payloads viz., Meteorological (MET), Data Relay Transponder (DRT), and Satellite Aided Search and Rescue (SAS&R). INSAT-3D imager provides imaging capability of the earth disc in six different channels, viz one in visible (VIS) and five in infrared (IR). The VIS channel operates in 0.52 – 0.72 µ. The other five infrared channels are in short wave infrared (SWIR) (1.55 – 1.70 µ), Mid wave infrared (MIR) (3.80 – 4.00 µ), water vapor (WV) (6.50 – 7.00 µ), and in two thermal infrared (TIR) channels. The split TIR channels are 10.2 – 11.2 µ (TIR-1) and 11.5 – 12.5 µ (TIR-2). The ground resolution at the sub-satellite point is 1km for both visible and SWIR channels, the ground resolution for MIR and TIRs is 4km each and 8 km for WV channel.  Distributed over long wave and short wave bands, INSAT-3D sounder has 18 IR channels, and an additional visible channel which provides synoptic view of the clouds and the earth, and hence the three dimensional map of temperature and humidity structure of the atmosphere. Out of the 18 IR channels, 6 bands are in the SWIR, five are in the MIR, and 7 are in the long wave infrared (LWIR). The ground resolution of all the channels is 10 km × 10 km. Table 1 describes the details of INSAT-3D sounder channels. Figure 2 shows the weighting function of INSAT-3D sounder channels over Indian region.
Figure 2: Weighting Function of INSAT-3D sounder channels over Indian Region. Table 1: INSAT-3D Sounder IR Channel Description
Observational, remote sensing and simulation studies related to the warm-core structure are amble in literature. As mentioned in the introduction, the observational studies assumes the conventional wisdom, the warm core is situated around 250 mb. But there are observational evidences of two prominent maxima, one in the middle atmosphere (~ 600 mb) other than the one at 250 mb (Hawkins and Imbembo, 1976), and with maxima only in the middle level (Halverson et al. 2006; and Dolling and Barnes 2010). Knaff et al. (2004) examined the structure of warm core from remote sensing observations Advanced Microwave Sounding Unit (AMSU) on-board polar satellites. Their study showed that the mean warm core is maximized near 12 km which is even higher than that found in the observational studies from the 1960s and 1970s. Another simulation study by Stern and Nolan (2011) showed the primary warm core in the middle level and a secondary in the higher level, in contrast to the 12 km warm core reported by Knaff et al. (2004). Almost all the temperature gradient below 10 km is found inside 30 km radius in Stern and Nolan simulation; however the 50 km resolution of AMSU could not see these gradients distributed with in 30 km radius below 10 km. Another simulation study by Rotunno and Emauel (1987) showed two maxima in perturbation temperature at 6 km and 15 Km. In a simulation study by Braun (2002) showed a cross section of perturbation virtual potential temperature maximized at 5 km. Liu et al. (1997) showed the warm core to be near 500 mb.
The warm core studies reported widely used the atmospheric temperature to determine the strength and height. There are several findings related to the warm core structure using micro wave remote sensing data, both brightness temperature and retrieved atmospheric temperature (Kidder et al. 2000 and the references therein). In this study we used the INSAT-3D sounder brightness temperature to investigate the strength and height of the warm core of VSCS Phailin, since temperature retrievals are available only at clear sky conditions. The resolution of each sounder channel is 10 km, and this resolution is even fine to see the inner core of cyclone, as compared to the coarse resolution of AMSU (Knaff et al. 2004). Figure 3 shows the zonal cross section of perturbation brightness temperature from INSAT-3D IR sounder channels along 85.65º E valid for 0023 UTC of 12th October 2013. The y-axis is the INSAT-3D sounder channel numbers and the x-axis is the latitude from 16.3º N to 17.3º N. It is noticed from figure 2 that all the channels showed positive perturbation temperature in the core of the cyclone and the maximum perturbation was observed in channel 14, which peaks around 600 -650 mb. Other than the surface channels, channels 10 and 11 in the middle levels also showed maximum perturbation. Channel 15, which peaks in 300 -250 hPa showed a perturbation of ~ 40 K, which can be treated as the secondary maxima. From Figure 3, it clear that the warm core extends from surface to the top of the atmosphere (~ 200 hPa) with varied intensity. F Calculation of perturbation temperature depends on the choice of reference profile to define the environment. Various definitions of the environment have been widely used in different studies, irrespective of observational, remote sensing or simulation of warm core. In the flight-level observational studies in 1960s and 1970s used annual mean Caribbean sounding of Jordan from 1958. Halverson et al. (2006) used a combination of two dropsondes at 340 km and 610 km from the centre. Knaff et al. (2004) used the reference state as the mean in the 500 – 600 km annulus, while Rotunno and Emanuel (1987) used the temperature at the outer boundary in the initial condition as the reference state. Since the temperature of the storm evolves with time, it is often meaningless to use the initial condition as the reference state. Persing and Montgomery (2003) used rapidly varying initial condition as the reference state. Liu et al (1997) used domain average temperature as the reference state in their simulation study. Domain average includes the warm core itself, and it can mislead to wrong interpretation of warm core. In this study we used a domain averaged reference profiles several hundred kilometers away from the storm, so that the warm core is excluded from the reference profile. Since the primary maxima is observed especially in the middle level, and secondary maxima in the high level, we restrict our detailed discussion to channels 10,11 and 14 in the middle levels and channels 12 and 15 in the upper levels. Figure 4 shows the brightness temperature perturbations in channels 10, 11, 12, 14 and 15 valid for 0023 UTC of 12th October 2013. Maximum perturbation is observed in channel 14, which peaks around 600 -650 mb, followed by channel 10 which peaks around 600 mb. The perturbation temperature is less for Channel 11, which peaks around 400 mb and channel 12 which peaks around 250 mb. The secondary maximum is showed in channel 15, which peaks around 300 -250 mb. F Unlike the conventional wisdom warm core structure which peaks around 250 mb, the present study agrees with the most of the simulation studies, which showed two maxima, with primary maxima peaking at the middle level (~600 hPa) and secondary maxima in the upper level (300 – 250 hPa). If the warm core height increases with the strength of the cyclone, the Phailin warm core has to be above 250 hPa, since it was declared a VSCS, the second strongest ever formed over the Indian Ocean. Present result is not agreeing with Knaff et al (2004), which made use of microwave remote sensing data of AMSU to study the warm core structure. Resolution of INSAT-3D sounder is 10 km, and hence it could resolve the perturbation features below 10 km unlike the 50 km resolution of AMSU, which missed the primary maxima of the warm core below 10km (Knaff et al. 2004). Present study matches better with the simulation study of Stern and Nolan (2011), where they reported through various simulations the existence of multiple maxima, the primary in the middle level and the secondary in the high level. From figure 4d, it can be noticed that the maximum perturbation in the brightness temperature in the 600 -650 mb level is > 40 K, whereas in the higher level (300 -250 hPa) it is between 30 and 40 K (Figure 4e). It is also noticed from Figure 3d that the environmental perturbation is more negative for channel 14, compared to other channels. This is also a good indication of the selection of the environment profile several hundred kilometers away from the centre, instead of taking the domain average.
Acknowledgments References Schubert and Hack, 1982; Nolan et al. 2007; Vigh and Schubert 2009; Pendergrass and Willoughby 2009 Stern and Solan 2011 La Seur and Hawkins, 1963; Hawkins and Rubsam, 1968; Hawkins and Imbembo, 1976 Liu et al. 1999; Kimbal and Dougherty, 2006 Halverson et al. (2006) Dolling and Barnes (2010) Halverson et al. 2006; Dolling and Barnes 2010. Knaff et al. (2004) Rotunno and Emauel (1987) Braun (2002) Persing and Montgomery (2003) Kidder et al. (2000) Kidder S. Q., Goldberg M. D., Zehr R. M., DeMaria M., Purdom, F. F. W., Velden C. S., Gordy N. C., and Kusselson S. J. 2000: Satellite Analysis of Tropical Cyclones Using the Advnaced Microwave Sounding Unit (AMSU), Bull. Amer. Met. Soc., Vol 81, No 6, pages: 1241-1259. Purdom and Menzel 1996; DeMaria 1996 Dvorak (1973, 1975) (Dvorak 1984) Velden et al. (1998) The North Indian Ocean Cyclone (NIOC) period is an event in the annual cycle of tropical cyclone formation, which usually occurs between May and December, with the peak in October and November. An average of 4 to 6 storms forms in the two main seas in the North Indian Ocean (NIO), the Arabian Sea (ARB) and the Bay of Bengal (BOB), every season. Warm core is the characteristic that distinguishes tropical cyclones from its extra tropical counter parts, where the centre of the cyclone is warmer than its environment. The existence of the tropical cyclone warm core has long been recognized, but the structure is little known. Two of the most common variables used to characterize the warm core are its strength and height. The strength is given by the magnitude of maximum perturbation temperature and the height is the level where the maximum perturbation temperature occurs. INSAT-3D, India's advanced weather satellite, is the first geostationary sounder over India and the surrounding Oceanic regions, and the second of its kind after the Geostationary Operational Environmental Satellites (GOES). INSAT-3D has 18 channel sounder with a resolution of 10 km to profile the atmospheric temperature and humidity. Brightness Temperature from INSAT-3D sounder channels are used to analyze the warm core structure of Tropical cyclones over NIO. Analysis shows multiple maxima, with strong primary maximum in the middle level (650 -600 mb) and the secondary maximum in the upper level (300 -250 mb), unlike the conventional belief suggested warm core existence at 250 mb. Due to the high resolution of (10 km) INSAT-3D sounder channels, compared to the Micro wave channels (AMSU-A of 50 km resolution), the warm core structure below 10 km of the atmosphere is well resolved. NCMRWF is working on to derive a technique to estimate the strength of NIOC using the potential of INSAT-3D sounder. It is always desirable to compare two or more observations of storms. The warm core of a tropical cyclone consists of two parts: a broad-scale, upper-level component, representing the overall magnitude of the tropical cyclone, and a small-scale, low-level warm core that is contained within the eye. Only occasionally is the eye large enough and the satellite pass close enough to the center of the storm so that the lower-level temperature anomaly can be observed. The upper-level warming can always be observed, but the lower-level warming is often obscured by the surrounding rain or is simply smaller than the 48-km resolution of the AMSU-A instrument. It is necessary, therefore, to have an independent estimate of the size of the eye of a storm, which can be provided by GOES or AVHRR imagery. Yüklə 57,82 Kb. Dostları ilə paylaş:
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