Weather observations by aircraft reconnaissance inside Severe Typhoon Utor

Transcription

1 Weather observations by aircraft reconnaissance inside Severe Typhoon Utor P. W. Chan, W. K. Wong and K. K. Hon Hong Kong Observatory, China Introduction Since 2009, the Hong Kong Observatory (HKO) has been collaborating with the Government Flying Service (GFS) of the Hong Kong Special Administration Region Government to equip a fixed-wing aircraft, namely a BAe Jetstream 4100, with a dedicated meteorological measuring system. This system consists of a probe set up under the wing, two global positioning system (GPS) antennae and other components stored inside the cabin (including the navigation unit, signal processor and data storage). The system is capable of providing accurate meteorological measurements, including the three components of the wind, temperature, relative humidity (RH) and pressure at a frequency as high as 20Hz. Details of the system have been reported by Chan et al. (2011). The meteorological measuring system is used primarily for collecting wind-shear and turbulence data in the vicinity of Hong Kong International Airport. Starting from 2011, the Jetstream 4100 has also been used to fly into tropical cyclones over the northern part of the South China Sea in order to collect valuable data in support of the typhoon warning service in Hong Kong. In particular, the aircraft data may be used to locate the centre as well as the high-wind radii of the typhoon, such as the storm-force wind radius (the radius from the typhoon s eye in which storm-force winds occur) and the gale-force wind radius (the radius from the typhoon s eye in which gale-force winds occur), in order to assess the chance of the occurrence of high winds over the South China coastal areas. In the past 3 years, a total of 11 missions have been flown, and the meteorological data collected have been found to have a positive impact on the analysis and forecasting of tropical cyclones in the region; an example is the assimilation of flight data into a numerical weather prediction (NWP) model (see Wong et al., 2013, for details). On 13 August 2013, Severe Typhoon Utor moved into the northern part of the South China Sea and was expected to bring about gale-force or stronger winds to various parts of Hong Kong, according to its forecast structure and track. In order to assess the potential impact of Severe Typhoon Utor on Hong Kong, a tropical cyclone reconnaissance flight was conducted in the afternoon (local time) of the day. Compared with similar flights in the past, some special conditions applied to this particular flight, including: (i) a flight into a severe typhoon, which was among the strongest experienced by the aircraft reconnaissance flights conducted up to then over the northern part of the South China Sea (no such flight into a severe typhoon had been conducted since collaboration began with the GFS in 2009); (ii) a relatively long flight route at lower levels within the atmospheric boundary layer (FL020, i.e. flight level at a height of 2000ft) in order to determine the galeforce wind radius of the typhoon, which is important for assessing the chance of occurrence of gale-force winds in Hong Kong. An initial study of the meteorological data collected in this flight is presented in this paper. Moreover, a HKO colleague photographed the clouds and the sea conditions at different locations in this severe typhoon during the flight, and these valuable weather photographs are also presented in the paper. Flight route Before the flight was conducted, the flight route was determined considering the air traffic (restrictions on the flight path due to commercial jets) and the usefulness of the meteorological data (which was mainly considered with respect to the available information about the centre location and the high wind radii of the typhoon). As in previous flights, data collection was at two levels, in view of the need to collect data within the atmospheric boundary layer of the typhoon as well as at a suitable level related to the steering of the typhoon within the largescale atmospheric flow. Consideration was also given to the time limit of the flight because of the fuel capacity of the aircraft (giving a flying time of up to about 5h). The flight route is shown in Figure 1. After considering the various flying constraints, flying in the mid-tropospheric steering flow region was conducted at FL080, which included flying through the centre of the severe typhoon. On the return leg, a longer flight route was taken within the atmospheric boundary layer at FL020 in order to observe the wind distribution upstream of Hong Kong, which Figure 1. The upper panel shows the flight route of the aircraft with the wind fleches overlaid on a false-colour satellite image at 1030 UTC. The lower panel shows the heights of the aircraft between points A and C. 199

2 Aircraft reconnaissance inside Severe Typhoon Utor 200 (a) (d) (b) (c) Figure 2. Time series (UTC) of the data collected by the aircraft. (a) Red curve is the wind direction and black curve is the wind speed at the flight level. (b) Red curve is the wind direction at the flight level and green curve is the wind speed reduced to a height of 10m above mean sea level. (c) Purple curve is the calculated mean sea-level pressure at the location of the aircraft (as calculated from the station-level pressure, height and station-level temperature of the aircraft). (d) Blue curve is the vertical wind velocity: positive means upwards and negative means downwards. also included flying through the centre of the typhoon: in fact, once the centre of the typhoon was located, the aircraft flew around the centre twice in order to collect the valuable meteorological data in this region. Time series of meteorological measurements The time series of wind direction and wind speed at the flight level and reduced to a height of 10m above mean sea level are shown in Figure 2(a) and (b). The correction of the wind speed to 10m is made following the method described in Chan et al. (2011), namely, assuming a power law of the wind speed as a function of height over the lower troposphere and using a representative power exponent for open oceans. There are two points worth noting: (i) in flying around the centre of the severe typhoon between 1020 and 1030 UTC of 13 August 2013, there were significant changes in wind direction from northwesterly to, eventually, southeasterly as measured onboard (the location of the aircraft at times 1015 and 1030 UTC is indicated in Figure 1); (ii) there is a number of spikes in the wind speed, for instance, between 1015 and 1030 UTC, between 1100 and 1115 UTC and between 1130 and 1145 UTC (locations at these times shown in Figure 1); during these intervals, the aircraft was crossing areas of convection, as shown on the satellite image in Figure 1. The 10m winds were estimated to be in the order of 35 40ms 1, which is lower than the estimated strength of the typhoon of 48.9ms 1 (95kn) around that time based on estimation using the Dvorak technique (Velden et al., 2006). The Dvorak technique estimates the intensity of the tropical cyclone by inspecting the pattern of clouds in the satellite images and making comparison with patterns from historical cyclones. The approximate mean sea-level pressure (MSLP) as determined from the station-level pressure and the altitude of the aircraft is presented in Figure 2(c). The MSLP is estimated based on the formula in the Handbook of Meteorological Instruments (Met Office, 1980), that is, the addition of the station-level pressure and a correction of pressure (which is related to the station-level pressure, height of the station above mean sea level as well as the station-level temperature). There are two local minima in the MSLP time series, namely, between 1000 and 1015 UTC and between 1030 and 1045 UTC. The estimated MSLP has the lowest value of about 973.7hPa, which is higher than the estimated minimum pressure of 941hPa based on satellite analysis. The location of minimum MSLP is roughly consistent with the centre of the severe typhoon as determined from the satellite image (Figure 3). The tim e series of vertical velocity is provided in Figure 2(d). The correlation of the measured vertical velocity and the structure of the severe typhoon requires further study, but preliminary investigation shows that the vertical velocity exceeds 10ms 1 in magnitude at times. The maximum upward velocity is about 12ms 1 at about 1030 UTC, the largest downward velocity occurring at about 1108 UTC, reaching slightly more than 15ms 1 Determination of the position of the typhoon centre The flight route together with the wind data symbols are overlaid on the satellite imagery in Figure 1. The eye of Severe Typhoon Utor is readily observed in the figure, with winds of opposing directions surrounding the eye (the occurrence of easterly winds first followed by westerly winds over a short distance, as indicated at 1030 UTC in Figure 1).

3 Figure 3. Flight route for levels below 2000m with wind fleches. The estimated gale-force wind radius of Severe Typhoon Utor as obtained from the flight data is shown. The figure also shows the locations of the minimum estimated MSLP ( x ) and the maximum estimated 10m wind speed (circle). The satellite-determined location of the typhoon centre is shown as +. Figure 4. The wind distribution of Severe Typhoon Utor as estimated from multiplatform satellites: (a) 0600 UTC, 13 August 2013; (b) 1200 UTC, 13 August The colours refer to wind speeds in knots: strong wind in green, gale-force wind in yellow, storm-force wind in red. The contours refer to wind speeds in excess of 20, 35, 50, 65 and 80kn. The gale-force wind radius is given by the contour with the label 35. Its value was estimated from the latitude/longitude grids (each degree is about 111km). However, another location with opposing wind directions is also observed about 50km southwest of the eye of Severe Typhoon Utor. The wind data have been double-checked by the manufacturer of the onboard meteorological measuring system based on the raw measurements from the system and they are found to be in order. Thus, this secondary circulation centre is considered to be genuine, and it coincides with the edge of an outer rainband of Severe Typhoon Utor where overshooting cloud tops are observed from the satellite imagery (Figure 1). The existence of this secondary centre will be studied in detail and reported in a future publication, for example, through numerical simulation of Severe Typhoon Utor with the assimilation of the aircraft reconnaissance data. Secondary circulation is not a common feature and, to the knowledge of the authors, there is no similar report in the existing literature. Determination of gale-force wind radius The gale-force wind radius of the severe typhoon is estimated based on the estimated 10m winds from the aircraft measurements. It refers to the distance (radius) from the centre of the tropical cyclone where winds of gale force or above occur. An indication wind strength along the flight route is given in Figure 3. The galeforce wind radius is estimated to be about 290km from the available measurements. The present analysis shows that the relatively longer flight span at FL020 was useful for analysing the high-wind radii of the severe typhoon. In order to check the accuracy of the estimated 10m wind speed from the aircraft data, comparison was made with the available wind-speed measurements from oil rigs and weather buoys in the region. For instance, the aircraft-estimated wind speed at 10m above mean sea level is found to be about 40kn from the east at about 20 N E, which is consistent with the wind speed measured at an oil rig (35kn from the east) located at about 50km away. As a further check of the accuracy of the wind radii, the multiplatform satellite wind analysis (available at cira.colostate.edu/products/tc_realtime/) was used for comparison. From this analysis (Figure 4), the gale-force wind radius is estimated to be 290km at 0600 UTC and 350km at 1200 UTC, which are, in general, consistent with the estimation based on the aircraft data. Aircraft reconnaissance inside Severe Typhoon Utor Figure 5. Time (UTC) series of the derived cube root of eddy dissipation rate based on the flight data. The yellow line is the limit of moderate turbulence and the red line is the limit of severe turbulence. Turbulence intensity measurements Following the International Civil Aviation Organization (2010), turbulence intensity 201

4 Aircraft reconnaissance inside Severe Typhoon Utor Figure 6. Photograph taken at the outer rainband of Severe Typhoon Utor during the flight at the location indicated by the arrow. Figure 7. Similar to Figure 6 but at a location close to the eye of Severe Typhoon Utor. 202 is expressed in terms of the cube root of the eddy dissipation rate (EDR) of turbulent kinetic energy. This is calculated from the power spectrum of the vertical velocity as measured by the aircraft, by searching for the inertial range (where the power spectrum follows the slope of 5/3). The time series of the cube root of the EDR is shown in Figure 5. It is calculated from vertical velocity following the method of Haverdings and Chan (2010). In general, the air was rather turbulent inside the severe typhoon, but moderate turbulence (EDR 1/3 of 0.3m 2/3 s 1 ) and severe turbulence (EDR 1/3 of 0.5m 2/3 s 1 ) were observed at a number of locations along the flight route. In particular, the severe turbulence occurs in the region of an outer rainband associated with the typhoon (between 1100 and 1115 UTC). Photographs taken inside the typhoon Photographs were taken throughout the flight and some of those of interest are presentated in this paper. Figure 6 shows the cloud and sea condition near the outer rainband of the severe typhoon, with isolated patches of cumulus and white wave heads over the sea being observed. In general, the seas were rougher outside the centre of the typhoon, especially in those regions with stronger winds. Figure 7 shows conditions near the centre of the severe typhoon, where the sea was rather calm. Compared with Figure 6, there is a broad area of clouds that might be associated with the eye wall of the typhoon. Similar observations were made at lower altitudes of the flight (not shown). Conclusion Investigation flights have been conducted for tropical cyclones over the northern part of the South China Sea since This paper reports the first observations, in 2013, of a severe typhoon within the flight campaign for this region, especially information from near the eye of this typhoon,

5 using the data collected by the onboard meteorological measuring system and by human observations (supported by a photographic record). Two centres are indicated for the severe typhoon at the time of the observation. The estimated maximum 10m wind speed is generally consistent with the satellite estimation, although the estimated minimum MSLP appears to be higher. A special flight route was devised to provide an estimate of the gale-force wind radius of the severe typhoon, and this estimate is consistent with satellite estimations. Photographs were taken at the location of an outer rainband and near the typhoon centre. The meteorological observations from this investigation are valuable and will be used for further study of the structure of the severe typhoon, for example through numerical simulation by assimilating the flight data, with the results being reported in a future publication. References Chan PW, Hon KK, Foster S Wind data collected by a fixed-wing aircraft in the vicinity of a tropical cyclone over the south China coastal waters. Meteorol. Z. 20: Haverdings H, Chan PW Quick Access Recorder (QAR) data analysis software for windshear and turbulence studies. J. Aircr. 47: International Civil Aviatio n Organization Meteorological Service for International Air Navigation, Annex 3 to the Convention on International Civil Aviation, 206 pp. Met Office Handbook of Meteorological Instruments, Volume 1 Measurement of Atmospheric Pressure, Second Edition. HMSO: London. Velden C, Harper B, Wells F et al The Dvorak tropical cyclone intensity estimation technique, a satellite-based method that has endured for over 30 years. Bull. Am. Meteorol. Soc. 87: Wong WK, Tse SM, Chan PW Impacts of reconnaissance flight data on numerical simulation of tropical cyclones over South China Sea. Meteorol. Appl. doi: /met.1412 Correspondence to: P. W. Chan pwchan@hko.gov.hk 2014 Hong Kong Observatory. Weather published by John Wiley & Sons Ltd on behalf of Royal Meteorological Society. This is an open access article under the terms of the Creative Commons Attribution- NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. doi: /wea.2315 Aircraft reconnaissance inside Severe Typhoon Utor UK Citizen Rainfall Network: a pilot study Samuel Michael Illingworth, 1 Catherine Louise Muller, 2 Rosemarie Graves 3 and Lee Chapman 2 1 Centre for Atmospheric Science, School of Earth, Atmospheric and Environmental Sciences, University of Manchester 2 School of Geography, Earth and Environmental Sciences, University of Birmingham 3 Earth Observation Science, Physics and Astronomy, University of Leicester Introduction Crowd sourcing as a potential method to collect large amounts of data in a relatively inexpensive manner while also educating the general public, has received much press recently in both the scientific and public domain. It was first defined by Jeff Howe (Brabham, 2008, and references therein) as the act of a company or institution taking a function once performed by employees and outsourcing it to an undefined (and generally large) network of people in the form of an open call and, although this method has been used to great effect in a number of profitable cases by a variety of industries, its vast potential for scientific research has also recently begun to be exploited. From a scientific perspective, crowd-sourcing projects are often referred to as citizen science, which is defined by Wiggins and Crowston (2011) as a form of research collaboration involving members of the public in scientific research projects to address realworld problems. With its history of public involvement, weather monitoring lends itself readily to citizen-science-style activities, with recent examples including: real-time temperature monitoring using smartphone battery temperatures (Overeem et al., 2013), the use of social media broadcasts to obtain accurate snowfall data (e.g. Muller, 2013), smartphone weather apps (e.g. MetWit: Weather Signal: and the uploading of amateur weather station data (e.g. Met Office WOW: gov.uk/). Such activities also build on the number of urban meteorological networks, with a range of research or operational objectives, that now exist within the UK (Muller et al., 2013a). Currently, there are over 200 automatic weather stations across the UK, as well as 4000 registered open rain gauges, which can provide hourly, daily and monthly measurements, thus making the British rainfall network one of the densest in the world. However, even with this extensive network there are still many areas across the UK that do not provide measurements of rainfall. There is also a historical tendency for the rainfall measurements to be somewhat intermittent: rainfall-monitoring stations were reported at some point between 1961 and 2000, with only around 40% of these stations found to be active at any one time (Hollis and Perry, 2004). Citizen science provides a feasible and low-cost solution to increasing the number of British rainfall-monitoring stations, with the potential coverage of these measurements extended to cover all areas of settlement across the British Isles. Such a voluntary rainfall network already exists in countries outside the UK, with the Community Collaborative Rain, Hail and Snow Network (CoCoRaHS) in the USA being a particularly effective example (Cifelli et al., 2005; The CoCoRaHS is a unique, non-profit, community-based network of volunteers who work together to measure and map precipitation, using low-cost measurement tools and utilising an interactive website to 203