Measurement and Analysis of Weather Phenomena with K

2016 URSI Asia-Pacific Radio Science Conference August 21-25, 2016 / Seoul, Korea

HemisphericMeasurement and and annual Analysis asymmtry of Weather of NmF2 Phenomena observed by FORMOSAT-3/COSMICwith K-Band Rain RadioRadar occultation observations Jun-Hyeong Park Ki-Bok Kong Seong-Ook Park

Dept. of Electrical Engineering Development1* team 1 Dept. of Electrical Engineering KAIST V. Sai GowtamKukdong ,Telecom S. Tulasi Ram KAIST DaeJeon, Republic of Korea 1IndianNonsan, institute Republic of geomagnetism, of Korea DaeJeon, Republic of Korea [email protected] [email protected] Panvel, [email protected] Navi Mumbai, India.

Abstract——To the overcome average NmF2 blind valuesspots inof Decemberan ordinary solstice weather are 0.9wall83 exists AU for between December the andtransmitter 1.017 AU (Tx) for June).and receiver But varying (Rx) significantlyradar which higherscans thanhorizontally those at atJune a highsolstice altitude, at all longitudes.a weather Sunantennas – Earth to improve distance isolation can explain between only them.25% ofWith the these total Thisradar is which known operates as F2 - layervertically, annual so asymmetry.called an atmospheric This phenomenon profiler, observedmethods, leakage asymmetry. power between Lei et Tx al. and(2013) Rx could found be reduced. similar wasis needed. observed In thisand paper,reported a K-bandseveral decadesradar for ago observing but the possiblerainfall asymmetryFig. 1 shows inmanufactured thermospheric antennas neutral and the density separation and wall. they mechanismsvertically is introduced,are not clearly and understood. measurement In thisresults present of rainfall study, are by attributed this to the varying Sun – Earth distance. But, using the rich data set of Constellation Observing System for shown and discussed. For better performance of the atmospheric ionosphericB. Design of behavio Tranceiverurr is different from the thermosphere Meteprofiler,orology, the radar Ionosphere which and has Climate high resolution (COSMIC) even GPS with – Radio low because of its complex electrodynamics and transport Fig. 2 shows a block diagram of the K-band rain radar. occultationtransmitted power observations, is designed. we With investigated this radar, thea melting hemispheric layer is processes involved. Zeng et al. [2008] found significant asymmetry of equatorial ionization anomaly and its local time Reference signals for all PLLs in the system and clock signals detected and some results that show characteristics of the meting longitudinal variations in the asymmetry values. Their case and seasonal variations from 5 years data during ascending for every digital chip in baseband are generated by four layer are measured well. controlled simulation indicate that the solstice difference of phase of the solar cycle 24. Important findings from our study frequency synthesizers. In the Tx baseband module, a field Sun-Earth distance, offset between geomagnetic and are,Keywords—K-band; (i) during solstices, theFMCW; EIA crest rain in theradar; winter low hemisphere transmitted is programmable gate array (FPGA) controls a direct digital geographic center and the tilt of geomagnetic pole will play strongerpower; high than resolution; that in therainfall; summer melting hemisphere layer from morning to synthesizer (DDS) to generate an FMCW signal which important roles on the annual asymmetry, however, the noon hours. In contrast to this, the EIA crest in the summer decreases with time (down-chirp) and has a center frequency detailed physical mechanisms of how the geomagnetic hemisphere becomes stronger and this transition occurs around of 670 MHz. The sweep bandwidth is 50 MHz which gives the I. INTRODUCTION configurations effects the annual asymmetry were still noon time, (ii) the December solstice exhibits a much pronounced high range resolution of 3 m. Considering the cost, 2.4 GHz ionizationA weather in both hemispheresradar usually and enhancedmeasures EIA meteorological than the June unexplained. There were no detailed studiessignal used on as effects a reference of thermospheric clock input of neu thetral DDS winds is split on and the solsticesconditions at of all over the a longitudinalwide area at sectors,a high altitude. (3) this Because local time it used for a local oscillator (LO). the FMCW signal is dependency of hemispheric asymmetry and annual asymmetry annual asymmetry. Hence, the main objective of this paper is observes weather phenomena in the area, it is mainly used for transmitted toward raindrops with the power of only 100 mW. has been consistently observed throughout the ascending phase of to study the local time, longitudinal and solar activity weather forecasting. However, blind spots exist because an Beat frequency which has data of the range and the radial theordinary solar cycleweather 24 and radar possible scans mechanism horizontally, are in whichvestigated. results Based in variations of annual asymmetry and its responsible neutral and on our detailed analysis, location of subsolar point, offset between electrodynamicvelocity of raindrops mechanisms. is carried by 60 MHz and applied to the difficulties in obtaining information on rainfall at higher and input of the Rx baseband module. In the Rx baseband module, geographiclower altitudes and geomagneticthan the specific equator altitude. and the Therefore, declination a weatherangle of magnetic field are playing important roles in the annual quadrature demodulationII. DATA is ANDperformed RESULTS by a digital down radar that covers the blind spots is required. converter (DDC). Thus, detectable range can be doubled than asymmetry. FORMOSAT–3/COSMIC is a constellation of 6 satellites, A weather radar that scans vertically could solve the usual. Two Dimensional-Fast Fourier Transform (2D-FFT) is primarily dedicated to GPS radio occultation experiment to Keywords:problem. This Annual kind Asymmetry;of weather radar, Thermospheric so called an Winds; atmospheric GPS – performed by two FPGAs. Because the 2D FFT is performed study the Earth’s atmosphere and ionosphere. To study the profiler, pointsRadio towardsOccultation the sky and observes meteorological with 1024 beat signals, the radar can have high resolution of annual asymmetry, we used NmF2 data from the vertical conditions according to the height [1]. Also, because the the radial velocity. Finally, data of raindrops are transferred to electron density profiles provided by UCAR atmospheric profiler I.usually INTRODUCTION operates continuously at a fixed a PC with local LAN via the an UDP protocol. TABLE I. (http://www.cosmic.cdar.edu). A 41-day period that cantered position,Globally, it could the catch hemispheric the sudden averaged change NmF2of weather values in the in shows main specification of the system. on June 21 and December 21 is used to represent the solstices Decemberspecific area. solstice are significantly higher than those of June and December, respectively, from 2008 to 2012. at June solstice at all longitudes. Four types of anomalies, Solstice differences of 41-day mean F10.7 values equatorialIn this paper, ionization K-band anomaly, rain radar winter which or has seasonal low transmitted anomaly, from 2008 to 2012 are -0.9405, 3.6317, 6.6449, 39.4982 and semiannualpower and high anomaly resolutions and of annual the range anomaly and the orvelocity annual is 22.897 respectively (+ve means December is more). asymmetry,introduced. areThe often frequency found in modulated the F2 layer. continuous Apart from wave the (FMCW) technique is used to achieve high sensitivity and above four anomalies, recently Liu et al. (2009) and Chen et Fig. 1 shows the local time and latitudinal variation of the reduce the cost of the system. In addition, meteorological al. (2010) reported two different anomalies. Those are zonally averaged NmF2 and asymmetry index. During the results are discussed. Reflectivity, a fall speed of raindrops Weddell Sea Anomaly (WSA) and Mid-latitude summer solstices, EIA crest in the winter hemisphere is stronger than and Doppler spectrum measured when it rained are described, night-time anomaly (MSNA). All the above anomalies are the summer hemisphere during the morning to noon. and characteristics of the melting layer are analyzed as well. well understood except annual anomaly. Few studies on However, around noon to early afternoon hours, the EIA crest annual asymmetry can be found in the literature [Yonezawa, in summer hemisphere become stronger than the winter EIA 1971;II. SuD EVELOPMENT et al., 1998; OF Rishbeth K-BAND and RAIN Muller RADAR Wodarg, SYSTEM 2006; crests and higher electron density in summer hemisphere is Mendillo et al., 2005, Liu et al., 2007, Zhen Zeng et al., 2008]. maintained throughout the afternoon to midnight. This distinct AllA. Antennathe above studies show that the asymmetry has significant hemispheric behaviour is consistently observed throughout the localTo time,suppress longitudinal side-lobe levels and solarand increase cycle an variations. antenna gain, One ascendingFig. 1. Manufactured phase of antenna SC 24and seinpdicatingaration wall. that this feature is pooffsetssible dual mechanism reflector isantennas the varying are used Sun- E[2].arth Also, distance separation (about independent of solar flux changes. The transition of strong

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Fig. 1: Local time and latitudinal variations of the zonally averages NmF2 during June and December solstices (left and middle panels) and the Asymmetry Index (AI) (right panel) from 2008 to 2012. Solid black lines in right panels indicate the contour line corresponding to zero AI.

EIA crest from winter to summer hemisphere is not same in than June) the AI values are significantly smaller than the June and December. In fact, the transition is much earlier in years 2008 – 2011. This decrease in AI can be attributed to the the persisted throughout the morning time. If we compare decrease in F10.7 flux values from June 2012 (134.8 sfu) to December and June solstices together, December NmF2 is Dec 2012 (111.9 sfu). With a view to examine the detailed significantly higher than June, which is known as annual latitudinal and longitudinal distributions of the observed noon asymmetry. In order quantify the annual asymmetry, the and midnight enhancements in the annual asymmetry, we asymmetry index suggested by Rishbeth et al. (2006) by estimated the solstice difference (Dec - Jun) of NmF2 values averaging the corresponding summer and winter hemispheric (∆NmF2 = NmF2 – NmF2 ) during noon and mid-night. NmF2 values is computed using equation-1. Dec Jun Figure 2 shows the longitudinal and latitudinal variation of NmF2Dec(N + S)avg−NmF2Jun(N + S)avg 퐴퐼 = (1) ∆NmF2 values during noon and mid night. The large positive NmF2Dec(N + S)avg+NmF2Jun(N + S)avg values of ∆NmF2 at noon time indicate that the overall ionization in December is significantly larger than in June The right panels of Fig.1 show the local time and solstice throughout the globe with a major part of noon-time latitudinal variations of AI during the years 2008 to 2012. asymmetry comes from EIA crest regions due to inter-solstice Overall positive AI values indicate that the global NmF2 differences in the EIA crests. Further, one can observe the values in December solstice are significantly higher than June high values of ∆NmF2 at south Atlantic (~-600 to 00), Pacific solstice, confirms the existence of Annual asymmetry (~-1200 to -600) and European-Asian (~00 to 1200) longitudes. throughout the ascending phase of solar cycle 24. The local Solstice difference values show the large negative values in time variation of AI further indicates that the annual the northern hemisphere during night, perhaps due the mid- asymmetry is significantly enhanced during noon and midlatitude summer night time anomaly (MSNA). This observed o o night with two distinct peaks, one is around 20 – 50 MLAT longitudinal and local time variations in the annual asymmetry during noon and another stronger mid-night peak at low- will be discussed through the geomagnetic field configuration, latitudes during the year 2008. This distinct hemispheric inter-hemispheric neutral winds, longitudinal variability of behavior is consistently observed throughout the ascending vertical ExB drifts and Weddle sea anomaly using detailed phase of solar cycle 24. In the year 2011, AI values are model simulations. substantially larger, which could be due to larger F10.7 in December-2011 than in June-2011. During the year 2012, III. DISCUSSION though the overall asymmetry is positive (December is larger

Fig. 2: Longitudinal and Latitudinal variation of the solstice difference (Dec - Jun) of Nmf2 (units of elec./m3) during noon (a-e) and mid-night (f-g) from 2008 to 2012 (Zero contour line is represented by black solid line).

terms of effective winds. An effective wind describes the overall effect of winds, for different declination and A. Role of ExB and thermospheric winds during noon time inclination values, can be calculated by using the following Thermospheric winds are very important in the F layer formula [Titheridge, 1998] dynamics and transport mechanisms. During day time, winds are mainly driven by the EUV heating. Wind blows from one Weff = (MW cos D ± ZW sin D) cos I sin I (2) hemisphere to other hemisphere during solstices, due to the high pressure bulge at subsolar point, can significantly alter Where MW and ZW are meridional (equatorward positive) the F region electron densities. During daytime, westward and zonal (eastward positive) winds respectively. Here + and – zonal winds, driven by differential temperature at dawn and signs apply in the southern and northern hemisphere. We dusk, can also push the F region density to higher altitudes at divided the entire globe into three major regions to explain the specific longitudes where the magnetic declination is physical mechanism during noon. Those are, region (a), covers westward. Zonal electric filed and the horizontal magnetic the longitudes from 00 to 1800 and from -1800 to -1600 where field creates an upward push of plasma at the equatorial the dip equator is located in the northern hemisphere, region ionosphere, known as equatorial fountain effect. This fountain (b), covers the longitudes from -1600 to -600, where the is strong during noon time, which is the primary cause of magnetic declination is eastward and region (c), covers the equatorial ionization anomaly. There is always strong longitudes from -600 to 00, where the magnetic declination is coupling between the ExB and inter-hemispheric winds. westward. Three major peaks of the solstice difference during During December solstice, subsolar point is located noon are associated with the above selected regions. at 23.5o S geographic latitude, creates an interhemispheric Let us consider the dip equator is 60 away from the wind towards North Pole. This interhemispheric winds can geographic equator and it is further away from the subsolar effectively push the plasma along the magnetic field lines at point during December solstice. This is an ideal case to mid latitudes in southern hemisphere and drag the plasma explain the annual asymmetry in region (a). Winds can along the filed lines into the northern hemisphere. At the same effectively push the plasma along the magnetic field lines time, equatorial fountain pushes plasma up at the equator and because of inclined magnetic field lines. If the subsolar point then plasma slides down along the magnetic field lines due to is close to the dip equator, winds are less effective because of gravity and pressure gradient forces in both hemispheres. But, nearly zero inclination. Magnitude of the effective winds are in southern hemisphere, plasma transport due to winds and important to counteract the effect of EXB. When EXB and ExB are in opposite directions, which can significantly reduce effective winds are strong enough, then it prevents the loss of the loss by maintain the plasma at higher altitudes. Whereas, ionization more effectively. During December solstice, in northern hemisphere, effects due to ExB and winds both are distance between subsolar point and dip equator is more than in same direction, which enhances the loss by pushing the June. Hence, the annual asymmetry is may be due to the good plasma to lower altitudes. This is the primary cause of the counter balance between EXB and effective winds in solstice asymmetry of EIA. Same mechanism can explain the December solstice than June solstice in region (a). In region June solstice also. But why December NmF2 is greater than (b), even though it is away from the subsolar point, zonal wind June? Here comes the importance of the offset between suppress the further uplift of plasma due to eastward geographic and geomagnetic equator. This can be explained in

98 declination. But, in region (b), large upward wind can be anomaly, is also important, which are the primary causes of possible during day, compared to all other longitudes due to the annual asymmetry during mid-night. westward declination, enhance the December NmF2 value in this region. Acknowledgment B. Role of WSA and MSNA in the annual asymmetry during We sincerely acknowledge the UCAR/CDAAC team for mid-night providing the COSMIC ionPrf data. Another interesting observation is that a very large NmF2 can be seen at southern higher latitudes in December References solstice (See Fig. 2). This intense peak, also known as [1] Lei, J., X. Dou, A. Burns, W. Wang, X. Luan, Z. Zeng, and J. Xu (2013), Weddell Sea Anomaly (WSA), is the primary cause of night Annual asymmetry in thermospheric density: Observations and simulations, J. Geophys. Res. Space Physics, 118, 2503–2510, doi:10.1002/jgra.50253. time high latitudes enhancement of the AI (left most panel of figure 1). This was extensively studied by several authors and [2] Lin, C. H., C. H. Liu, J. Y. Liu, C. H. Chen, A. G. Burns, and W. Wang they concluded that the large thermospheric wind and (2010), Midlatitude summer nighttime anomaly of the ionospheric electron geomagnetic field configuration is the primary cause for the density observed by FORMOSAT 3/COSMIC, J. Geophys. Res., 115, A03308, doi: 10.1029/2009JA014084. WSA. Our HWM07 simulation results (not shown here) also support the previous studies (Liu et. al., JGR, 2010). Solstice [3] Liu, H., S. V. Thampi, and M. Yamamoto (2010), Phase reversal of the difference values show the large negative values in the diurnal cycle in the midlatitude ionosphere, J. Geophys. Res., 115, A01305, northern hemisphere during night, perhaps due the mid- doi: 10.1029/2009JA014689. latitude summer night time anomaly (MSNA). In fact, WSA is [4] Mendillo, M., C.-L. Huang, X. Pi, H. Rishbeth, and R. Meier (2005), The much stronger than the MSNA, may be due to the hemispheric global ionospheric asymmetry in total electron content, J. Atmos. Sol. Terr. difference of the upward winds. Hence, we can conclude that Phys., 67, 1377–1387. the WSA is the primary causes for mid-night enhancement of [5] Rishbeth, H., and I. C. F. Muller-Wodarg (2006), Why is there more AI. ionosphere in January than in July? The annual asymmetry in the F2-layer, Ann. Geophys., 24, 3293–3311. IV. CONCLUSIONS [6] Su, Y. Z., G. J. Bailey, and K. I. Oyama (1998), Annual and seasonal A detailed investigation is carried out by using a long term variations in the low-latitude topside ionosphere, Ann. Geophys., 16, 974– data set of COSMIC GPS-RO during 2008 – 2012 to 985. understand the physical mechanism of annual asymmetry. The complex processes that involved in the annual asymmetry of [7] Titheridge, J. E. (1995), Winds in the ionosphere—A review, J. Atmos. Terr. Phys., 57, 1681–1714. NmF2 were discussed through detailed analysis of observations and modelled simulations. This study makes a [8] Tulasi Ram, S., S.-Y. Su, and C. H. Liu (2009), FORMOSAT-3/COSMIC decent attempt to explain the possible interlink between observations of seasonal and longitudinal variations of equatorial ionization thermospheric neutral winds and the complex electrodynamics anomaly and its interhemispheric asymmetry during the solar minimum period, J. Geophys. Res., 114, A06311, doi: 10.1029/2008JA013880. to explain the local time and longitudinal variability of annual asymmetry. Varying Sun – Earth distance contributes to the [9] Yonezawa, T. (1971), The solar-activity and latitudinal characteristics of global annual asymmetry but the observed longitudinal the seasonal, non-seasonal and semi-annual variations in the peak electron structures is due to the thermospheric winds, which plays an densities of the F2-layer at noon and at midnight in middle and low latitudes, J. Atmos. Sol. Terr. Phys., 33, 887–907. important role in the longitudinal variability of annual asymmetry due the offset between geographic and [10] Zeng, Z., A. Burns, W. Wang, J. Lei, S. Solomon, S. Syndergaard, L. geomagnetic equator. Role of other anomalies, such as Qian, and Y.-H. Kuo (2008), Ionospheric annual asymmetry observed by the Weddell Sea Anomaly and mid-latitude summer night time COSMIC radio occultation measurements and simulated by the TIEGCM, J. Geophys. Res., 113, A07305, doi: 10.1029/2007JA01289.