Comparative analysis of TEC disturbances over tropical cyclone zones in the North–West Pacific Ocean
Introduction
Tropical cyclones (TCs) are one of the most dramatical and destructive geophysical phenomenons. TCs are powerful vortex structures which originate in the equatorial troposphere over oceans. Having formed, TCs typically move in the direction of general circulation. The general circulation is a system of large-scale air flows in the Earth’s atmosphere. TC development stages are normally classified according to the maximum sustained wind speed in a cyclone, which is defined as the speed of the wind at an altitude of 10 m, averaged over 10 min, and is recorded in an open and flat space in eyewall (Emanuel, 2003). Eyewall is the ring of strongest winds and heaviest precipitation immediately bordering the eye of typhoon (Terry, 2007). The cyclone with the maximum sustained wind under 17 m/s is called tropical depression; about 18–32 m/s, tropical storm; over 33 m/s, hurricane. In Southeast Asia, TCs of hurricane intensity are termed typhoons (Emanuel, 2003).
TCs may be one of the sources of internal atmospheric waves (IAW). Investigations into gravity waves carried out based on modeling have shown that over TC zones in the upper atmosphere there are IAW with horizontal wavelengths of 15–2000 km and periods from 20 min to 11 h (Chane-Ming et al., 2002, Kim et al., 2005, Kuester et al., 2008). Originating in the lower atmosphere, IAW under favorable conditions can reach the ionosphere and generate electron density disturbances manifesting themselves as traveling ionospheric disturbances (TID) (Kazimirovsky, 2002).
The TC-generated ionospheric disturbances were registered using various methods: measurements of Doppler frequency shift and Faraday rotation of sounding signal, vertical and oblique sounding, rocket sounding, and GPS.
Xiao et al. (2007), relying on Doppler frequency shift measurements carried out during 24 typhoons revealed the presence of medium-scale TID with a period of about 20 min in 22 of them (Xiao et al., 2007). The disturbances were more intense when a typhoon reached the coastline or was over the continent. Besides, ionospheric disturbances with periods 13–14 min (Huang et al., 1985) and 90 min (Bishop et al., 2006) were detected using incoherent scatter radar data when strong TC passed near the point of observation.
Measurements of the F2-layer critical frequency made by ionosondes of vertical sounding and tomography sounding data revealed changes in f0F2 values amount to as much as 10–20% when typhoons were near the coastline (Liu et al., 2006, Rice et al., 2012, Vanina-Dart et al., 2011). Vanina-Dart et al. (2011) note that the increase (Rice et al., 2012) or decrease (Liu et al., 2006) in f0F2 may be associated with the time lag between the beginning of measurements and the cyclone onset.
The analysis of short-period variations in maximum observed frequencies of oblique sounding signals along Magadan–Irkutsk, Norilsk–Irkutsk, Khabarovsk–Irkutsk paths carried out from 2005 to 2009 revealed time intervals of signal spectrum energy amplification in certain frequencies which were not connected with heliogeomagnetic activity or with the passage of atmospheric fronts and solar terminator near the midpoint of oblique sounding ray path (Chernigovskaya et al., 2010a, Chernigovskaya et al., 2010b). The authors think that the spectrum energy amplification may be caused by traveling ionospheric disturbances associated with propagation of 1–5-h internal atmospheric waves in the ionosphere. Sources of such waves may have been TC occurring in the North–West Pacific Ocean that time.
Based on rocket measurements, Vanina-Dart et al. (2008) registered a significant decrease in electron density and a rise in the lower boundary of the D-layer during the TC active phase. It was suggested that the mentioned effects were caused by TC-generated gravity waves which affected the ionosphere.
Investigations into the TC effects on the ionosphere encounter difficulties because of the low amplitude of the ionospheric response to tropospheric disturbances (Perevalova and Ishin, 2011). So in Afraimovich et al. (2008) an attempt to detect disturbances generated by typhoon SAOMAI with the aid of GPS signals failed because the TC coincided with a magnetic storm. The authors did not manage to separate ionospheric disturbances uniquely associated with the typhoon. Thus, we can conclude that the detection of TC-generated weak ionospheric disturbances should be carried out under quiet geomagnetic conditions only. Furthermore, to exclude the influence of equatorial ionization anomaly the investigation of total electron content (TEC) behavior should be studied only in evening/night local hours (Perevalova and Ishin, 2011, Polyakova and Perevalova, 2011).
For example, Mao et al. (2010) using GPS data from more than 50 stations during typhoon MATSA under quiet geomagnetic conditions revealed that TEC increased by about 5 TECU relative to monthly averages. The increase was registered when the typhoon reached the coastline. Moreover, based on the investigation into more than 10 cyclones Bishop et al., 2006, Bishop and Straus., 2006 registered an increase in frequency of disruptions in the GPS signal in an area of about 1200 km from the cyclone center. Zakharov and Kunitsin (2012) based on analysis GPS data revealed that acoustic gravity waves (AGW) activity increases by almost 20–30% during a hurricane sharply grows/decay stage compared to the next days. The authors with the help of GPS interferometry showed that appearance of AGW disturbances connected with TCs action geographically associated with orography perturbed places. The excitation of the wave structures is most effective during the stages of rapid increase/decrease typhoon intensity.
We have developed a method for comparative analysis of GPS and NCEP/NCAR Reanalysis archive data, which enables us not only to register travelling disturbances in the ionosphere but also definitely ascertain their connection with cyclones. Analyzing hurricanes occurring near the US Atlantic Coast by our method, we established that during TC at its peak intensity the amplitude of TEC variations with periods of 02–20 and 20–60 min in the ionosphere increased; the amplitude of long-term variations increased more evident than did the amplitude of short-term variations (Polyakova and Perevalova, 2011). TEC variations were most intense when the wind speed in the cyclone was maximal.
In this paper, using the method for comparative analysis of GPS and NCEP/NCAR Reanalysis data, we examine variations in ionospheric plasma parameters observed during several tropical cyclones of different intensity in the North–West Pacific Ocean.
Section snippets
Data and analysis methods
We have analyzed an ionospheric response to TC for six cyclones of different intensity occurring in the North–West Pacific Ocean in September–November 2005: DAMREY (September 20–27, 2005), SAOLA (September 20–26, 2005), LONGWANG (September 25–October 2, 2005), KIROGI (October 10–19, 2005), TEMBIN (November 7–11, 2005), BOLAVEN (November 13–20, 2005). The trajectories of the mentioned cyclones are indicated by dots in Fig. 1 (the TCs data are taken from http://wunderground.com/hurricane/); the
TCs SAOLA, DAMREY, and LONGWANG
From September 20 to October 3, 2005, in the North–West Pacific Ocean there were three powerful cyclones simultaneously. TC DAMREY occurred from September 20 to September 27; on September 24 it reached the typhoon stage (the wind speed in the TC exceeded 33 m/s). On September 25, the cyclone had the highest wind speed (about 45 m/s) and the lowest pressure (948 hPa). TC SAOLA formed on September 20; through September 22–25 it had typhoon intensity. The maximum wind speed in the cyclone (over 50
On tropospheric delay
In addition to the ionosphere, GPS signal propagation is affected by the troposphere introducing errors to measurements of pseudorange. The radio signal delay in the troposphere is largely caused by refraction effects associated with irregularities in dielectric constant. The delay factor depends on meteorological parameters (pressure, temperature, humidity) and is proportional to the length of the signal path in the troposphere. Thus, the tropospheric delay is maximal at low elevation angle.
On multipath effect
The accuracy of measurements, made by a GPS receiver, may be affected by the multipath effect. The multipath effect implies that a receiver receives not only a direct signal from satellite but also signals reflected from various surfaces nearby the receiver. Combination of direct and reflected signals in the receiver leads to variations in amplitude and phase of the resulting signal as well as to a decrease in a signal-to-noise ratio. This causes errors in measurements of radio-signal delay and
Conclusion
We have examined the ionospheric response to six TCs of different intensity through a comparative analysis of TEC variations and dynamics of variations in meteorological parameters during the cyclones. The amplitude of TEC variations of different periods was shown to increase in the ionosphere over the cyclones at their peak intensity. An intensification of TEC disturbances is registered only when the wind speed in a cyclone exceeds a threshold value, i.e. the cyclone reaches the typhoon stage.
Acknowledgments
We are grateful to the Scripps Orbit and Permanent Array Center (SOPAC) for GPS data and to National Centers for Environmental Prediction (NCEP) for meteorological data from the NCEP/NCAR Reanalysis archive.
The study was supported by the Ministry of Education and Science of the Russian Federation (under projects Nos. 8699, 8388 and contract No. 14.518.11.7065) as well as by RFBR research projects Nos. 12-05-33032_a and 12-05-00865_a.
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