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Guimond Tropical Meteorology II 1 Midlatitude Trough Interaction with the Extratropical Transition of Hurricane Michael Stephen R. Guimond Center for Ocean-Atmospheric Prediction Studies, The Florida State University, Tallahassee, Florida 11 April 2005 ABSTRACT The interaction of Hurricane Michael (2000) with a Midlatitude Upper Tropospheric Trough (MUTT) as the storm was undergoing Extratropical Transition (ET) is examined. The utility of a potential vorticity (PV) framework for diagnosing the dynamic response of a Tropical Cyclone (TC) to an influx of high PV air in a baroclinic environment will be assessed. Output from the Aviation model (AVN) with a grid resolution of 1 was manipulated to calculate the full PV on horizontal isentropic surfaces and vertical cross sections to capture both the MUTT interaction and ET process. In addition, rain rates from the Special Sensor Microwave/Imager (SSM/I) were analyzed to provide a quantitative evaluation of the structural changes observed during an ET event that could assist the operational community. Results indicate that a trough interaction can be inferred from SSM/I data as the mean area average rain rate centered on the storm increased with distance from the TC center, a typical signature of the asymmetric structural change observed during trough interaction cases. PV was shown to be a rather versatile parameter with the ability to provide a dynamically sound depiction of the kinematics involved with a MUTT interaction and ET event that will continue to improve upon research into the complex prediction of TC intensity change. 1. Introduction The impacts of Midlatitude Upper Tropospheric Troughs (MUTTs) on Tropical Cyclones (TCs) are numerous. From a movement perspective, MUTTs can rapidly accelerate the translational speed of TCs as well as the direction of storm motion with the majority recurving to the north or northeast in the northern hemisphere (Jones et. al 2003; WMO 1995). From a structural standpoint, prolonged interaction with a MUTT can transform a TC from a warm-core symmetric vortex in a barotropic environment into a cold-core asymmetric vortex within a baroclinic environment, a process referred to as Extratropical Transition (ET) (Jones et. al 2003). The movement of a TC into an environment associated with a large temperature gradient allows for complex interactions Guimond Tropical Meteorology II 2 to take place between the transitioning storm and intense synoptic scale frontal systems. A key component of these interactions are the frontogenetic/frontolytic processes that occur as the front approaches and merges with the TC, a topic the author will address for his Master’s Thesis. The greatest challenges to operational forecasters during an ET event are the potentially massive amounts of precipitation, continuation of high winds with an expansion of the overall wind field and the development of large ocean swells (Abraham et. al 2004; Jones et. al 2003). The structural changes that occur with ET can have dramatic impacts for the Canadian Maritimes and marine communities as well as coastal locations in Western Europe (Abraham et. al 2004). Providing accurate and timely warnings for these impacts can be difficult especially when so little is known about the complete ET process. Finally, from an intensity/strength point of view, MUTT interactions with TCs can cause significant pressure drops on the order of several tens of millibars due to a secondary circulation feedback that results in a firing of eyewall convection and amplified low-level wind speeds (Hart 2005, personal communication). In this study, the MUTT interaction as well as the ET of Hurricane Michael (17-19 October 2000) will be examined and shown to undergo a similar motion, structure and intensity change to that of the generalization of such storms mentioned above (Stewart 2000). In order to describe the MUTT and ET evolution of Hurricane Michael, both synoptic and dynamic perspectives will be considered that follow the general flow of recent research on these subjects. a. Distinguishing MUTT interaction from ET It is important for the operational and research communities to bin storms into certain categories in order to discern a typical pattern of evolution that will assist in improving Guimond Tropical Meteorology II 3 the track, intensity and structural changes lacking in current forecasts. The main difference between a trough interaction and the extratropical transitioning process of a TC is the role of the trough in the structural change process. In trough interactions, a MUTT usually involves a rather short merging period with the TC on the order of 12-24 hours with the interaction usually resembling a smaller scale PV anomaly from the MUTT intensifying the TC (Molinari et. al 1998; Hanley et. al 2001). However, during the majority of ET events, the MUTT drives the evolution of the storm and therefore produces longer merging time periods lasting several days with the interaction depicting a deeper, wider PV anomaly from the MUTT advecting over and wrapping around the TC (Hart 2005, personal communication; Jones et. al 2003). b. Framework of paper A large portion of the dynamic component of this study will focus on the use of potential vorticity (PV) reasoning to explain how the approach of a MUTT can enhance the secondary circulation of a TC. This secondary circulation is necessary to maintain the primary circulation from the dissipative effects of angular momentum loss to the sea surface (friction) and thermal energy loss due to radiative (evaporation) and adiabatic (ascent) cooling (WMO 1995). It has been argued by Molinari et. al 1998, among others, that PV provides for a more concise, beneficial dynamical skeleton of TC-trough interactions than simply using basic variables such as vorticity, height and wind. Strong motions through a heat source (latent heat releasing convective clouds) follow lines of constant angular momentum and motions through a momentum source (MUTT or upper- level jet) follow lines of constant potential temperature (WMO 1995). Thus, because of the quasi-conservative properties of PV along surfaces of constant potential or equivalent Guimond Tropical Meteorology II 4 temperature, one can follow the motion of air parcels along isentropic surface at higher levels in the atmosphere where the trough and TC interact. In addition, PV links the dynamic and thermodynamic properties of the atmosphere together into one formulation while the traditional vorticity equation relies on implied heating from the divergence and twisting terms allowing for a more direct interpretation of the physical processes occurring in TC-trough interactions (Molinari et. al 1998). 2. Physical Mechanisms of PV Intensification PV anomalies (relative to the surrounding environmental PV) can have enhancing or diminishing effects on the intensity of a TC that can become complex to diagnose. An important question to consider is how the strength and duration of negative influences from a MUTT will augment and overtake the positive influences especially at close distances to a TC. A fine line exists between a “good” trough and a “bad” trough because in many instances a catch exists that may render the initial intensity change useless or reverse the process. The following sub-sections will explain these processes in detail. a. Positive Influences When an approaching synoptic scale trough moves within a certain distance (see Hanley et. al 2001) of a TC, the PV anomaly associated with the trough begins to become somewhat spatially and temporally coincident with the TC center, a procedure referred to as the “superposition principle”. When this happens, intensification of the TC can occur out of a combination of two main processes. First, “constructive interference” occurs where large eddy PV fluxes moving inward from the trough to the TC allow for enhanced cyclonic spin-up of the initial vortex and the excitation of a conditional instability of the Guimond Tropical Meteorology II 5 second kind (CISK) type response. Second, enhanced upward motion and an associated evaporative-wind feedback (“WISHE”) results from several different processes that may combine to produce the intensification. These include: (i) increasing cyclonic vorticity advection with height associated with the approaching trough in a Q-G omega framework, (ii) location of the TC core within the left front or right rear jet streak quadrants and (iii) enhanced evacuation of air near the tropopause due to a jet streak increasing the outflow anti-cyclone aloft (Hart 2005, personal communication). When (i) is coupled with low static stability well outside the core of the TC, a surface induced circulation can form seen through relatively lower values of vorticity. If this rotation is within a certain distance from the TC center, the long “arms” of the TC’s vorticity will tend to draw in sources of angular momentum and thus produce an enhancement of the cyclonic rotation (Hart 2005, personal communication). b. Negative Influences The effects of vertical wind shear on a TC are well known (WMO 1995; Molinari et. al 1995; Molinari et. al 1998; Hanley et. al 2001). The general consensus on the effects of vertical wind shear are to displace the heating and convection away from the center of the storm, which does not allow for an area of focused heating to produce the pressure falls necessary to maintain and/or strengthen a TC (Hart 2005, personal communication). The displacement of the heating aloft acts to ventilate the upper troposphere warm anomaly while decoupling the lower atmospheric frictional convergence and latent heat release. This process can be seen from satellite imagery when the low-level rotational center is displaced from the cirrus canopy and the TC begins to take on a more asymmetric appearance (Beven seminar, 2005). Dry air intrusions act to destabilize the Guimond Tropical Meteorology II 6 sounding away from the TC core producing unfocused convective heating ultimately resulting in a decrease of warm-core maintenance and the typical destruction of a TC. c. The Catch Generally speaking, larger troughs enable greater constructive interference effects of upper PV anomalies from the trough and lower PV anomalies from the TC that can lead to intensification. However, larger troughs produce substantial, long-lasting values of vertical wind shear that will destroy convection surrounding the TC and thus, the catch. A key aspect of the intensification from positive influences is a thinning of the approaching positive PV anomaly that attempts to match the scale of the TC’s positive PV anomaly. Deep convection surrounding the TC core along with the placement of an upper-level jet to the north of the storm, which will increase the evacuation of air at high levels, will act to lift the tropopause and build the ridge downstream. The result of this process will enhance the outflow anticyclone and thus decrease the horizontal scale of the trough producing the necessary thinning (Hart 2005, personal communication). The above process allows smaller-scale PV anomalies that can have substantial magnitudes to approach the TC core without experiencing the detrimental effects of vertical wind shear. 3. Data and analysis method a. Data Twelve-hourly 1 gridded analyses are taken from the Aviation model (AVN) from 00 UTC 15 October 2000 through 00 UTC 22 October 2000 which spans the period that Michael was a hurricane (17-19 October 2000) (Stewart 2000). In addition, the National Hurricane Center’s (NHC) “best track” dataset from 20 November 2000 was used for the Guimond Tropical Meteorology II 7 precise timing, positioning and intensity (based on central pressure and wind speed) of Hurricane Michael as the storm traversed the Atlantic Ocean (Stewart 2000). Rain rates from the Special Sensor Microwave Imager (SSM/I) on 0.25 resolution swaths taken from Remote Sensing Systems were used to diagnose the structural changes in precipitating convection of Michael (www.remss.com). Three different satellites (F13, F14, F15) were used to provide the densest network of precipitation measurements around the time of trough interaction with the storm. b. Diagnostics To make use of the SSM/I data, the time each satellite passed over the TC was found and linear interpolation between successive “best track” positions was used to find the approximate center of the TC at the time of satellite overpass. This process produces a TC center that is within a few tenths of a degree of the actual center due to the resolution of the satellite. Typical TC eye diameters range from ~15-30 km and are thus on scales smaller than the resolution of the satellites (WMO 1995). As a result, the rain rates retrieved from the satellites at the TC “center” are most likely picking up on the intense precipitation occurring just outside of the eye within the eyewall and beyond. To facilitate an analysis of how the structure of precipitating convection changes upon MUTT interaction and during the early stages of ET, 1x1, 2x2 and 3x3 area averages positioned about the TC center were calculated and reported in Table 1. Many of the dynamical techniques used by previous authors to explain both MUTT and ET processes will be incorporated in this study including vertical wind shear and the full PV. Guimond Tropical Meteorology II 8 The vertical wind shear was calculated as a simple vector difference between the 850 and 200 mb layers given by VS V200 V850 , (1) where V200 is the vector wind at 200 mb, V850 is the vector wind at 850 mb and VS is the shear vector within the layer. In order to plot vertical cross sections of PV in pressure coordinate form, the full PV equation can be expressed as ˆ V PV g p f k p , (2) p p where p is the vertical component of relative vorticity in pressure coordinates, p is the horizontal gradient operator evaluated on a pressure surface and V is the horizontal wind (Hanley et. al 2001). The full PV, which incorporates the twisting of horizontal potential temperature gradients seen in the second term of (2), will be expressed in the almost universal potential vorticity units [PVU, where 1 PVU = 1106 m 2Ks1kg1 ; Hoskins et al. (1985)]. Plots of PV in the horizontal plane on a representative isentropic surface, which allows one to view the interaction of the MUTT and TC as air flows along constant potential temperature, will be shown at the same times as the vertical cross sections to enable a thorough kinematic description of the trough interaction and ET process. To produce isentropic surfaces, a desired potential temperature value is interpolated between pressure levels to produce a field of coefficients, which is then used as a weighting function to find the appropriate value of PV (calculations done in FERRET). This process pulls out high PV air from both the MUTT and TC to view their interaction at various levels in the atmosphere. Guimond Tropical Meteorology II 9 4. Results a. Remote Sensing Component Table 1 contains the rain rates area averaged about the TC center of Hurricane Michael from the 18 October 2000 through the 19 October 2000. As mentioned previously, the rain rates at the TC’s “center” are shown along with 1x1, 2x2 and 3x3 box averages to show the structural changes occurring during a MUTT interaction and ET process. On the 18th of October, the rain rates at the center of Michael are on average quite large (~13 mm/hr) with fluctuations (max = 21.20 mm/hr, min = 3.00 mm/hr) in the intensity due to approximating the TC center and normal structural variability in the eyewall. On the 19th of October, rain rates are shown to increase to rather impressive levels (max = 25.00 mm/hr) early on in the day. SSM/I has an upper threshold level in rain rate returns of 25.00 mm/hr, therefore it is likely that Michael was producing more intense rainfall as this time than the satellite can detect. The 18th and 19th of October were chosen to exemplify the shift from a symmetric TC to a more asymmetric structure upon interaction with a MUTT. At 1324 and 1400 UTC October 19th the rain rates changed dramatically near the TC center to much lower intensities than previously observed. In addition, every other time period besides the two above displayed a decrease in the rain rate averaged over each box about the TC center, going from highest in the 1x1 box to lowest in the 3x3 box. At 1324 UTC on the 19th, the rain rate went up from 2.04 mm/hr in the 1x1 box to 4.87 mm/hr in the 3x3 box. Similar results were found at 1400 UTC on the 19th with a rain rate of 2.36 mm/hr in the 1x1 box to 4.79 mm/hr in the 3x3 box. As a supplement to these data, the NHC’s best track reports a 14 mb pressure drop in the six Guimond Tropical Meteorology II 10 hour period between 1200 UTC on the 19th to 1800 UTC on the 19th, a pressure change of more than 2.25 mb/hour (Stewart 2000). The structural and intensity changes observed during this time period are attributed to the interaction of a MUTT with Michael as can be seen in the Geostationary Operational Environmental Satellite (GOES) 8 water vapory imagery at 1200 UTC 19th October in Figure 1. The MUTT displaced the core of intense precipitation away from Michael’s center and as the TC began to rotate, the majority of heavy rainfall was located off to the north and northwest of the eye. This analysis indicates that structural changes in box averaged rain rates about the TC center can be a useful tool for diagnosing a trough interaction and the early stages of ET, which can be extended to include the complete ET process if so desired. b. Synoptic and Dynamic Component A step-by-step analysis of each twelve hour period from the GFS beginning 0000 UTC 19th October 2000 and ending 1200 UTC 20th October 2000 will be examined with heavy weight on PV-thinking to describe both the MUTT interaction and ET of Hurricane Michael. Figure 2a displays the horizontal plot of PV on the 335 K isentropic surface for 0000 UTC on the 19th overlaid with horizontal wind vectors and the hurricane symbol indicating the model’s location of the TC center at this time. From the plot, the center of Michael can be seen by the patch of ~1 PVU air out in the Atlantic Ocean with the strongest gradient of PV at 1.5 PVU (nominal tropopause level) and greater at ~434 km from the TC center representing the trough. The distance inferred from Figure 2a is on the lower end of the favorable distant interaction composite (PV maxima between 400 and 1000 km from TC center) as described by Hanley et al. 2001. The corresponding Guimond Tropical Meteorology II 11 time of vertical cross section (sliced at 34N) in Figure 2b shows the maximum PV anomaly associated with the TC concentrated in the lower portions of the atmosphere between 800 and 900 mb. Contours of potential temperature clearly show the warm core structure of the TC at this time with the tropopause (thick black line) at a fairly high, constant height. Figure 2c shows the vertical wind shear in the 200 – 850 mb layer to be quite low in the vicinity of the TC but with higher values associated with the trough to the northwest and jet maximum to the north. The relative humidity at 700 mb is between 80-90 % in the region of the TC which will help to sustain the convective core, but drops off rather quickly near the trough location depicted in Figure 2d. At 1200 UTC 19th October, the MLUTT has moved within ~325 km of the TC center and is now defined as a favorable superposition composite (PV maxima within 400 km of TC center) for TC intensification based on the criteria established by Hanley et al. 2001. Figure 3a shows the horizontal plot of PV on the 330 K isentropic surface at the time above with values of PV above 4 PVU at the TC center and a sharp gradient in PV extending above 1.5 PVU close to the TC’s PV anomaly. The horizontal winds overlaid on this plot clearly show the movement of high PV air from the base of the trough advecting into the core of the TC producing a constructive interference effect detailed in section 2. The vertical cross section plot (sliced at 40N) shows an arm like extension of PV associated with the trough containing values at higher levels above 5 PVU moving towards the center and a longer protrusion with values between 1 and 1.5 PVU located close to the storm at around 600 mb. The TC is shown to be more intense than the previous time period with large values of PV (between 4 and 5 PVU) at higher levels in the atmosphere yet the entire core of the PV anomaly remains well intact and matches the Guimond Tropical Meteorology II 12 scale of the trough. Contours of potential temperature reveal a distinct warm core structure throughout the center of the vortex with isentropes moving apart in the lower portions of the atmosphere within the TC’s PV anomaly. This observation indicates the TC was experiencing an increase in relative vorticity from conservation of momentum arguments that allowed, among other processes, for the rapid deepening (14 mb pressure drop in six hours) of Michael during and just after the time period of analysis. Figure 3c shows that the center of Michael is within a zone of low wind shear at this time, but is on the cusp of an extremely large gradient in shear associated with the approach of the trough. In addition, both the horizontal winds overlaid on the PV plot and the vertical wind shear display stronger winds to the north and northeast of the TC indicating that the outflow anticyclone is becoming enhanced producing trough thinning. This is a perfect example of scale matching that enabled the trough to provide the TC with the necessary benefits without letting the detrimental effects of wind shear overwhelm the storm. Figure 3d explains that the TC is still within a good source region of moisture in the mid levels to sustain convection and avoid the negative impacts of dry air intrusions. At 0000 UTC 20 October, the MUTT has begun to “drive” the evolution of Michael, which leads into the progression of the ET process. Figure 4a displays the horizontal plot of PV on the 330 K isentropic surface overlaid with the horizontal winds. The distance of the trough’s PV anomaly from the TC center is less than 100 km, but the interaction is difficult to discern as some of the high PV air from the trough has begun to mesh with the PV from the TC. The maximum PV of the TC has gone down from the previous time to roughly 3 PVU, but the anomaly has a larger diameter due to the wind field expansion in cases of ET (Hart 2005, personal communication). A large gradient in the PV field of the Guimond Tropical Meteorology II 13 trough exists to the southwest of the storm where high PV air has begun to wrap around the storm a further indication of ET with maximum values at around 8 PVU (Jones et al. 2003). Figure 4b displays the vertical cross section (sliced at 48N) of PV with a rather large region of air greater than 1.5 PVU (maximum of ~3 PVU) associated with the TC and a sharp lobe of PV greater than 7 PVU protruding into the middle part of the atmosphere. Potential temperature contours indicate that the vortex has begun to take on slight cold core structure reminiscent of extra-tropical systems although the densest cold air still remains within the narrow zone of the trough. Figure 4c shows that Michael is experiencing larger values of vertical shear at this time around 30 m/s while Figure 4d displays the movement of dry air associated with the trough. Strong wind magnitudes on the horizontal PV plot across the center and to the northeast of Michael indicates ascent along the 330 K surface and when coupled with the movement of the storm into a baroclinic zone with increased vertical wind shear, may imply warm frontogenesis, although further study is needed (Jones et al. 2003). Finally, at 1200 UTC 20 October the horizontal plot of PV along the 325 K isentropic surface in Figure 5a displays a more pronounced wrapping of the PV anomalies with an increased asymmetric structure. A large core of PV around 7 PVU is collocated with the model’s location of ET Michael at this time along with the generation of a cold front trailing down from the center of Michael indicated by the thin zone of PV around 6 PVU to the south and southwest of the storm. The horizontal winds surrounding the storm show an area of ascent along the 325 K surface to the northeast of Michael and descent to the southwest of the storm, which would correspond to areas of warm and cold frontogenesis, respectively if frontogenesis functions were analyzed in more detail. Guimond Tropical Meteorology II 14 However, GOES infrared imagery at this time period (Figure 6) clearly shows the development of cold and warm frontal features associated with the ET process of Michael. The vertical cross section (sliced at 51N) of PV shows a distinctly asymmetric cyclone with large values of PV above 5 PVU pushing down into the storm as the tropopause descends lower in the atmosphere. Although there is a slight indication of a warm core especially near the surface, the majority of the cyclone at this point is cold core in nature throughout the troposphere. Figure 5c shows that the wind shear has begun to subside near the center of the storm although there are higher values with a large gradient in shear off to the east. The long extension of dry air associated with the trough shows up well in Figure 5d as this air continues to wrap around the cyclone and hinder the growth of convection. 5. Conclusions The Midlatitude Upper Tropospheric Trough (MUTT) interaction with the Extra- Tropical Transition (ET) of Hurricane Michael (17-19 October 2000) from both a synoptic and dynamic perspective was shown to be a rather multifaceted event. The use of full PV in the diagnoses of a MUTT interaction and ET process proved beneficial for the analysis of storm structure in various phases. In particular, the storm composites identified by Hanley et al. (2001) seemed to fit rather well in this study, which further improves their utility for identification of trough interactions from both a research and forecasting perspective. PV thinking helped to elucidate the reasoning for Michael’s 14 mb pressure drop in only six hours as a MUTT approached the TC center. PV seemed particularly useful in explaining the ET process as high PV air wrapped around Michael Guimond Tropical Meteorology II 15 and began to interact with the baroclinic environment. Knowledge of this process will certainly help out the author in his work on a Master’s Thesis involving the structural change of mid-latitude frontal systems as they merge with transitioning TCs. Acknowledgements. I thank Dr. Robert Hart for his countless discussions and patience in a Tropical II class that proved to strengthen my interest and knowledge in a subject I certainly hope to be involved with in some way for my career. The author also wishes to acknowledge use of the Ferret program for analysis and graphics in this paper. Ferret is a product of NOAA's Pacific Marine Environmental Laboratory (Information is available at www.ferret.noaa.gov). REFERENCES Abraham, J., J. W. Strapp, C. Fogarty, and M. Wolde, 2004: Extratropical transition of hurricane Michael. Bull. Amer. Meteor. Soc., 95, 1323-1339. Hanley, D., J. Molinari, and D. Keyser, 2001: A composite study of the interaction between tropical cyclones and upper-tropospheric troughs. Mon. Wea. Rev., 129, 2570-2584. _____, 2002: The evolution of a hurricane-trough interaction from a satellite perspective. Wea. Forecasting, 17, 916-926. Jones, S.C., and co-authors, 2003: The extratropical transition of tropical cyclones: forecast challenges, current understanding, and future directions. Wea. Forecasting, 18, 1052-1092. Molinari, J, S. Skubis, and D. Vollaro, 1995: Exernal influences on hurricane intensity. part III: potential vorticity structure. J. Atmos. Sci., 52, 3593-3606. ____, ____, ____, F. Alsheimer, and H.E. Willoughby, 1998: Potential vorticity analysis of tropical cyclone intensification. J. Atmos. Sci., 55, 2632-2644. Stewart, S.R., 2000: Tropical cyclone report hurricane Michael 17-19 October 2000. NCEP Rep., 13 pp. [Available online at http://www.nhc.noaa.gov/2000michael.html.] WMO, 1995: Global perspectives on tropical cyclones. WMO Technical Document. 693 pp. Guimond Tropical Meteorology II 16 TABLES Table 1. Comparison of rain rates retrieved from SSM/I satellites during the times of overpass of Michael's center for the area averages indicated. Also shown is the distance of the TC center from the Potential Vorticity Anomaly (PVA = 1.5 PVU) for each overpass. TIME (UTC) TC CENTER(°N,°W)* Rain Rate (mm/hr) DISTANCE FROM PVA (km) 18-Oct-2000 Center|1°x1°|2°x2°|3°x3° 148 (30.50,71.00) 11.80|8.93|5.56|3.24 ~705 1100 (31.50,70.50) 21.20|15.16|11.44|7.34 ~598 1342 (31.75,70.25) 6.70|11.35|9.05|5.52 ~564 1418 (32.00,70.00) 20.50|14.15|9.38|5.33 ~557 2218 (33.75,68.50) 3.00|15.48|12.37|8.35 ~456 19-Oct-2000 106 (34.50,67.50) 10.50 |13.34|10.50|7.54 ~424 136 (34.75,67.25) 23.10|13.31|9.90|6.67 ~419 1042 (39.00,62.50) 25.00|11.83|8.62|7.12 ~337 1324 (40.75,60.75) 1.10|2.04|3.16|4.87 ~299 1400 (41.25,60.50) 1.30|2.36|3.45|4.79 ~288 *Closest data point to TC center from NHC since SSM/I data has 0.25° resolution. Guimond Tropical Meteorology II 17 FIGURES QuickTime™ and a GIF decompressor are needed to see this picture. Figure 1. GOES 8 water vapor imagery at 12:15 UTC 19th October 2000. Figure 2a. Horizontal plot of PV on the 335 K isentropic Figure 2b. Vertical cross section of PV in the x-z plane for surface for the time period indicated in PVU with 0.5 PVU the time period indicated in PVU with increment of 0.5 PVU increment. Horizontal wind vectors along the same surface are and 1.5 PVU (nominal tropopause) darkened. Contours of overlaid with reference vector in m/s. Model TC center potential temperature are overlaid in K with increment every indicated by the hurricane symbol. 5K. Latitude of slice indicated at top of plot. Guimond Tropical Meteorology II 18 QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. Figure 2c. Vertical wind shear in 200 - 850 mb layer Figure 2d. Relative Humidity at 700 mb contoured overlaid with streamlines at time period indicated. Values every 10 % for the time period indicated. are contoured every 5 m/s with streamline reference in m/s. Figure 3a. Horizontal plot of PV on the 330 K isentropic surface Figure 3b. Vertical cross section of PV in the x-z plane for the for the time period indicated in PVU with 0.5 PVU increment. time period indicated in PVU with increment of 0.5 PVU and Horizontal wind vectors along the same surface are overlaid 1.5 PVU (nominal tropopause) darkened. Contours of potential with reference vector in m/s. Model TC center indicated by the temperature are overlaid in K with increment every 5K. hurricane symbol. Latitude of slice indicated at top of plot. Guimond Tropical Meteorology II 19 Figure 3c. Vertical wind shear in 200 - 850 mb layer Figure 3d. Relative Humidity at 700 mb contoured every overlaid with streamlines at time period indicated. Values 10 % for the time period indicated. are plotted every 5 m/s with streamline reference in m/s. Figure 4a. Horizontal plot of PV on the 330 K isentropic surface Figure 4b. Vertical cross section of PV in the x-z plane for for the time period indicated in PVU with 0.5 PVU increment. the time period indicated in PVU with increment of 0.5 PVU Horizontal wind vectors along the same surface are overlaid and 1.5 PVU (nominal tropopause) darkened. Contours of with reference vector in m/s. Model TC center indicated by the potential temperature are overlaid in K with increment every hurricane symbol. 5K. Latitude of slice indicated at top of plot. Guimond Tropical Meteorology II 20 Figure 4c. Vertical wind shear in 200 - 850 mb layer Figure 4d. Relative Humidity at 700 mb contoured overlaid with streamlines at time period indicated. Values every 10 % for the time period indicated. are plotted every 5 m/s with streamline reference in m/s. Figure 5a. Horizontal plot of PV on the 325 K isentropic Figure 5b. Vertical cross section of PV in the x-z plane for the surface for the time period indicated in PVU with 0.5 PVU time period indicated in PVU with increment of 0.5 PVU and increment. Horizontal wind vectors along the same surface are 1.5 PVU (nominal tropopause) darkened. Contours of overlaid with reference vector in m/s. Model TC center potential temperature are overlaid in K with increment every indicated by the hurricane symbol. 5K. Latitude of slice indicated at top of plot. Guimond Tropical Meteorology II 21 Figure 5c. Vertical wind shear in 200 - 850 mb layer Figure 5d. Relative Humidity at 700 mb contoured overlaid with streamlines at time period indicated. Values every 10 % for the time period indicated. are plotted every 5 m/s with streamline reference in m/s. QuickTime™ and a GIF decompressor are needed to see this picture. Figure 6. GOES 8 infrared imagery at 12:15 UTC 20th October 2000.
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