Application of GPS Radio Occultation Data for Studies of Atmospheric Waves in the Middle Atmosphere and Ionosphere

Transcription

1 Journal of the Meteorological Society of Japan, Vol. 82, No. 1B, pp , Application of GPS Radio Occultation Data for Studies of Atmospheric Waves in the Middle Atmosphere and Ionosphere Toshitaka TSUDA Radio Science Center for Space and Atmosphere (RASC), Kyoto University, Uji, Japan and Klemens HOCKE Communications Research Laboratory (CRL), Koganei, Japan (Manuscript received 6 April 2003, in revised form 28 October 2003) Abstract GPS radio occultation (RO) measurements from a low-earth orbiting (LEO) satellite can determine profiles of atmospheric temperature in the troposphere and stratosphere with high vertical resolution. The RO technique can also provide electron density perturbations in the ionospheric E region. We discuss in this review the application of GPS occultation data for the studies of the dynamical structure of the troposphere, stratosphere and ionosphere. By analyzing RO data obtained by the GPS/MET (GPS/ Meteorology) experiment, the detailed thermal structure near the tropical tropopause has been described. The GPS/MET temperature data have also been used to determine the global distribution of atmospheric gravity wave energy in the stratosphere. These studies indicate enhanced wave activity over regions of tropical convection, in particular, around the Indonesian Archipelago. In addition, orographic generation of atmospheric waves is recognized over the Andean mountain range, whose effects reach the ionosphere, producing the sporadic E layers. 1. Introduction Signals from GNSS (Global Navigation Satellite System) satellites, such as GPS and GLONASS, received on a low earth orbiting (LEO) satellite, are used for an active limb sounding of the atmosphere and ionosphere. During a rising or setting of a GPS satellite (occultation), the radio rays between the GPS and LEO satellites successively scan the ionosphere, stratosphere and troposphere from the LEO orbit height down to the surface. A refractive index profile can be retrieved from the Corresponding author: Toshitaka Tsuda, Radio Science Center for Space and Atmosphere (RASC). Kyoto University, Uji, Kyoto , Japan. tsuda@kurasc.kyoto-u.ac.jp ( 2004, Meteorological Society of Japan time variations of the ray bending angles by assuming local spherical symmetry of the atmospheric layers. The first GPS radio occultation (RO) measurements of the Earth s atmosphere were realized in the GPS/ MET (Global Positioning System/Meteorology) experiment conducted by UCAR (University Corporation for Atmospheric Research) (Ware et al. 1996). GPS/ MET provided a high quality data set of refractivity profiles between one and sixty kilometers in altitude from April 1995 to February 1997, from which temperature and water vapor profiles were obtained (Rocken et al. 1997). Taking advantage of the calibration-free characteristics and nearly all-weather capabilities, the RO technique is expected to provide valuable datasets for assimilation into numerical weather

2 420 Journal of the Meteorological Society of Japan Vol. 82, No. 1B prediction models and for climate monitoring and research. In addition, RO has become a powerful tool for studies of atmospheric dynamics, including atmospheric waves. The RO temperature profiles are characterized by very good vertical resolution ( m), which has not been achieved by conventional satellite measurements. The RO technique can also observe electron density irregularities in the ionospheric E region. Using GPS/MET RO data, we have studied the tropopause structure, temperature fluctuations in the stratosphere caused by atmospheric gravity waves and electron density perturbations in the ionospheric E region. In the following sections we summarize these results. The German CHAMP (CHAllenging Minisatellite Payload) and Argentine SAC-C (Satelite de Aplicaciones Cientificas-C) satellites were launched on July 15, 2000 and November 21, 2000, respectively. They carry an improved GPS receiver designed by JPL (Jet Propulsion Laboratory), and they have achieved significant progress in measuring water vapor and temperature profiles in the troposphere and stratosphere. The datasets are now produced and made available by UCAR, GFZ (GeoForschungsZentrum) and JPL to a wide scientific community. These new datasets are expected to help clarify the detailed behavior of the atmosphere and ionosphere. 2. Structure of the tropical tropopause The tropopause in the equatorial region plays a key role in connecting the tropical troposphere globally to the middle atmosphere through dynamical coupling and transport/ mixing of minor constituents; therefore, the tropical tropopause is of great scientific interest. The main thermal structure of the troposphere is dynamically controlled, while the radiative effects become dominant in the stratosphere. Holton et al. (1995) described the tropical troposphere as significantly affected by the extra-tropical stratosphere through a waveinduced forcing, called a global scale suction pump or a downward control. We show in Fig. 1 an example of a GPS/ MET temperature profile in comparison with a nearby radiosonde sounding (Nishida et al. 2000). In the upper troposphere, between Fig. 1. Comparison of temperature profiles between GPS/MET (solid line) and radiosonde observations (dotted line) in Indonesia (6.9 S, E). Title denotes the observation time in UT and location of the GPS/MET profile. Time lag between the GPS/MET event and the balloon launch was 3 h, and the horizontal separation was 450 km (Nishida et al. 2000). 10 km and the temperature minimum, the two profiles agree very well, with a standard deviation of less than about 1 K. (Note that below about 10 km GPS/ MET profiles deviate from the radiosonde profile, because the temperature profiles were derived by assuming a dry atmosphere and purposely ignoring the effects of water vapor on the refractive index.) Figure 1 indicates that the tropical tropopause is characterized by a low temperature and high altitude associated with a sharp inversion and step-wise increase of temperature lapse rate in a thin layer around the temperature minimum. The enlarged profile in Fig. 1 indicates that the RO sounding achieves a high-resolution measurement of the detailed temperature structure around the tropopause. Using GPS/ MET data, Nishida et al. (2000) analyzed seasonal variations of the tropopause temperature and the corresponding height over the western Pacific. They reported that the GPS/ MET results generally agree well with the climatological behavior of the tropical tropopause as well as nearby radiosonde observations in Indonesia.

3 March 2004 T. TSUDA and K. HOCKE Fig. 2. Global distribution of the tropopause temperature determined from GPS/MET data in the northern winter months (from December 1996 to February 1997) (Nishida et al. 2000). Fig. 3. The black body temperature, TBB ðkþ inferred from the OLR data from December, 1996 to February, Cold TBB at low latitudes corresponds to tall clouds (Nishida et al. 2000). Fig. 4. Distribution of potential energy ðepþ of mesoscale (<10 km vertical scale) temperature fluctuations from the GPS/MET data at km in November February in The Ep value is averaged in an area extending 10 and 20 in latitude and longitude, and the center coordinates are shifted every 1 and 2, respectively (Tsuda et al. 2000). Figure 2 shows a global distribution of the tropopause temperature from the GPS/MET data in three months from December 1996 to February Figure 3 shows the black body temperature ðtbb Þ inferred from OLR (Outgoing Long wave Radiation) data for the same Fig. 5. Latitude-height section of Ep estimated from the temperature fluctuations in the GPS/MET data in November February (top), September October (middle), and May August (bottom). These contour plots are reproduced by using the results in Fig. 8 of Tsuda et al. (2000). 421

4 422 Journal of the Meteorological Society of Japan Vol. 82, No. 1B period (Nishida et al. 2000). Note that the WMO-defined tropopause temperature in Fig. 2 is close to the minimum temperature in the tropics. There is a clear boundary between the tropical and middle latitude regions at around north and south. Figure 3 suggests that tall clouds (deep convection) existed over South America, Africa, and the Indonesian Archipelago at low latitudes. The cold tropical tropopause in Fig. 2 appeared over South America, Africa, and the western to central Pacific. A global distribution of the temperature structure, as in Fig. 2, will be very useful in understanding the role of the tropical tropopause. The detailed behavior of the tropopause is modulated by Kelvin waves, showing periodic variations of the minimum temperature and the corresponding height with an oscillation period of days (Tsuda et al. 1994; Shimizu and Tsuda 2000). These waves significantly affect the linkage processes between the troposphere and stratosphere. Therefore, highresolution measurements of the temperature structure in the vicinity of the tropopause are important. Using GPS/ MET temperature profiles, Randel and Wu (2003) recently studied the thermal structure and variability of the tropical tropopause, and found that the characteristics of the cold-point temperature around the tropopause and the corresponding height are considerably affected by transient convection, whose behavior was independently inferred from satellite OLR data, and atmospheric gravity waves and Kelvin waves. 3. Global distribution of the atmospheric gravity wave activity in the lower stratosphere From Fig. 1 we see that in the stratosphere there are significant temperature fluctuations, probably caused by atmospheric waves with vertical wavelengths of several kilometers. The general characteristics of these fluctuations are consistent between the GPS/ MET and radiosonde results. Comparison studies reported that the RO soundings are capable of describing detailed height structure of atmospheric gravity waves with a height resolution comparable to in-situ and remote-sensing techniques, such as a balloon-borne radiosondes and wind profiler radars, respectively (Tsuda et al. 2000; Tsuda and Hocke 2002). Extracting mesoscale temperature perturbations ðt 0 Þ with vertical wavelengths shorter than about 10 km in the lower stratosphere from individual GPS/ MET profiles, Tsuda et al. (2000) estimated potential energy per unit mass, Ep ¼ð1/2Þðg/NÞ 2 ðt 0 /TÞ 2, where g and N are the gravitational acceleration and buoyancy frequency, respectively. Figure 4 shows the latitude-longitude distribution of Ep at km in winter months in the northern hemisphere (from November to February in ). Large Ep values are detected at low latitudes (about 25 from the equator), and they are particularly enhanced over Indonesia, Africa, and South America where there are areas of tall cumulonimbus clouds (cold T BB ) in Fig. 3. Because cumulus convection is active in the tropics, we can expect generation of various waves, such as equatorial Kelvin waves, atmospheric tides, and gravity waves. Energy and momentum are transported upward by these vertically propagating waves. Wave-wave and/ or wave-mean flow interactions are crucially important for the understanding of dynamical processes in the equatorial atmosphere, including the formation of peculiar long-term variations such as quasi-biennial oscillation (QBO) and semi-annual oscillation (SAO). In particular, the effects of gravity waves have been highlighted in recent studies (e.g., Dunkerton 1997). The results in Figs. 3 and 4 confirm that atmospheric waves are generated by convection in the tropical regions. It is noteworthy that Ep in Fig. 4 is enhanced at low latitudes over the Atlantic Ocean where neither convection nor orographic effects are expected for the generation of atmospheric waves. A recent study by Kawatani et al. (2003) using a numerical model, however, has identified that in this longitude sector gravity waves are generated over a moist region at low latitudes in the northern hemisphere (Africa), and they can propagate southward across the equator. The model results help explain the observed Ep distribution in Fig. 4. Figure 5, reproduced from Tsuda et al. (2000), shows a latitude-height section of Ep values during the three prime times of GPS/ MET. Note that the number of the GPS/MET data samples becomes significantly smaller at

5 March 2004 T. TSUDA and K. HOCKE 423 latitudes higher than about 70, so that the Ep values are less significant there. Below about 30 km Ep became the largest at low latitudes, and the width of the enhancement was wider in November February. The enhancement of the gravity wave energy at low latitudes has been explained reasonably well by using a linear gravity wave model (Alexander et al. 2002). Above 30 km the enhanced peak of Ep near the equator tends to disappear. Instead, the large Ep values appear at middle and higher latitudes. In the lower stratosphere (20 30 km), the Ep values near the equinox (September October) Fig. 6. Comparison between E k at km observed with the MU radar (top) and the monthly average value of Ep with the GPS/MET data (bottom). The dashed line indicates a leastsquare fit. The mean ratio between E k and Ep was about 0.67, which is very close to a theoretical prediction (Tsuda et al. 2000). are nearly the same in the middle latitudes between the northern and southern hemispheres. But the Ep is larger in the winter hemisphere, which is evident in May August. This tendency from the GPS/MET data is consistent with earlier ground-based observations of gravity waves (Tsuda et al. 2000; Nastrom et al. 2000). That is, the gravity wave energy in Fig. 6 (kinetic energy from MST radars; E k and Ep from GPS/ MET) shows a clear annual cycle, with larger values in winter months. It is noteworthy that the new RO measurements from CHAMP and SAC-C, having a much higher data rate than GPS/MET, are now available, and they are being extensively used to analyze the global distribution of the gravity waves (Christian Marquardt 2003; Madineni Venkat Ratnam 2003; personal communications). 4. The effects of upward propagating waves on generation of ionospheric irregularities 4.1 Orographic generation of mountain waves GPS/ MET observations indicate a large enhancement of stratospheric gravity waves over the Andean mountain range, which is likely an indication of an interaction between the meanflow and topography (Hocke et al. 2002). The mountain waves thus generated seem to propagate upward in the stratosphere and mesosphere, eventually reaching the ionospheric E region. Electron density perturbations in the ionospheric E region (sporadic E) are also determined from RO measurements. That is, when the radio waves pass through a thin ionization layer or a plasma irregularity at the Earth s limb, the phase path excesses of the GPS signals show strong fluctuations. Due to ionospheric dispersion of the L1 and L2 radio waves, the difference of the L1 and L2 phase path excess is proportional to the number of free electrons along the ray path. Thus, the small-scale variations of the observed path difference are approximately proportional to the small-scale plasma fluctuations at the Earth s limb (Hocke and Tsuda 2001b). Figure 7 summarizes various parameters of the atmosphere and ionosphere obtained with GPS RO measurements in October, 1995 (Hocke and Tsuda 2001a). Electron density ir-

6 424 Journal of the Meteorological Society of Japan Vol. 82, No. 1B Fig. 7. Longitude distributions of various parameters of the atmosphere and ionosphere at 35 S 55 S. From bottom to top panels, illustrated are the number of GPS/MET data, mean topography, stratospheric gravity wave activity, and ionospheric irregularities (sporadic E), respectively (Hocke et al. 2002). Fig. 8. Electron density irregularities (vertical scales < 7 km) in the lower ionosphere at km observed by GPS/MET. A dot corresponds to a radio occultation event. The dot radius is proportional to the electron density enhancement of the sporadic E layer. Red dots are observed in June/July 1995, green dots are in October 1995, and blue dots are in February 1997 (Hocke and Tsuda 2001b). regularities (sporadic E) in Fig. 7 were frequently detected at a longitude region over the Andes. A good correlation between the stratospheric gravity wave intensity and sporadic E occurrence suggests that the fluctuations of electron density occurred due to the combined neutral wind shear associated with gravity waves and geomagnetic effects. A global distribution of sporadic E events is analyzed in Fig. 8 by using the GPS/MET data during some prime times (Hocke and Tsuda 2001b). The results clearly indicate frequent occurrence of sporadic E phenomena over the Andean mountains in the southern hemisphere in summer months. 4.2 Vertical coupling processes of the atmosphere and ionosphere in the tropics Because of the concentrated topography at southern middle latitudes, the effects of upward-propagating gravity waves on the production of electron density irregularities are clearly seen in Fig. 7. Similar dynamical coupling processes are also expected at low latitudes. The longitudinally inhomogeneous distribution of topography in the equatorial region results in zonal asymmetry of wave sources, which produce large longitudinal variations in the equatorial stratosphere and ionosphere. During February 1997 at southern, tropical latitudes (5 S 25 S), we analyzed in Fig. 9 various parameters similar to those in Fig. 7. Figure 9 indicates a remarkably high correlation among these parameters, showing maxima over South America (Brazil), Africa, Indonesia/ Australia, and the Pacific Ocean (Hocke and Tsuda 2001a). Upward propagating gravity waves, excited by active tropical convection, reach up to the ionosphere and generate sporadic E layers by an interaction between the waves and background geomagnetic fields (i.e., the wind shear mechanism). These studies suggest that the stratospheric waves propagate upward into the ionosphere and produce electron density irregularities by the combined effects between wind motions of waves and geomagnetic fields, as well as through various instabilities. The coupling of the ionosphere with the lower and middle atmosphere through these dynamical and electro-

7 March 2004 T. TSUDA and K. HOCKE 425 Fig. 9. Various parameters of the southern tropics (5 S 25 S) as a function of geographic longitude, during GPS/MET prime time (2 16 February 1997): (a) maximum of small-scale (vertical scales < 7 km) fluctuation amplitude of electron density at km; (b) normalized temperature variance in the stratosphere (solid line for km, dotted line for km); (c) water vapor pressure averaged over 4 6 km altitude (solid line for GPS/MET, dotted line for ECMWF); (d) surface topography; (e) number of occultation events. The curves in (a), (b), (c), and (e) are averaged by a sliding window with a length of 10 in longitude. Negative (positive) longitude corresponds to west (east), respectively (Hocke and Tsuda 2001a). dynamical effects is a topic of great interest in equatorial regions. 5. Summary and concluding remarks In this paper we have reviewed studies that show that the GPS radio occultation technique can quantitatively evaluate the dynamical structures of the troposphere, the middle atmosphere and ionosphere. Below we summarize the main results obtained by using the GPS/MET datasets: (1) The vertical resolution of the GPS RO observations is superior to other satellite techniques, and it can describe the detailed temperature structure in the vicinity of the tropical tropopause, which is characterized by the sharp variations in the temperature gradients in a thin layer. (2) The GPS RO observations are capable of detecting mesoscale temperature fluctuations in the stratosphere caused by atmospheric gravity waves, and we have used these observations to analyze the global morphology of gravity wave activity at km. (3) The gravity wave energy in the stratosphere is largely enhanced over the region of active convection in the tropics, such as Indonesia, South America, and Africa, indicating that convection is the major source of gravity waves at low latitudes. (4) Over the Andean mountain range, we have detected large amplitude waves that are presumably excited by orographic effects. Moreover, we have found a correlation between the stratospheric gravity waves and the electron density perturbations in the ionospheric E region. Such correlation is also present in the equatorial region. We anticipate that the follow-on RO missions, such as SAC-C, CHAMP, COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) and EQUARS (Equatorial Atmosphere Research Satellite) will further clarify the behavior of the wave dynamics. Acknowledgements We greatly appreciate Dr. C. Rocken and his colleagues at UCAR for providing us the GPS/ MET occultation data. This study is supported by the Japanese GPS Meteorology project of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The GPS/ MET program is sponsored primarily by the National Science Foundation (NSF). We deeply thank Dr. R. Anthes for his valuable suggestions and careful reading of the manuscript. References Alexander, M.J., T. Tsuda, and R.A. Vincent, 2002: Latitudinal variations observed in gravity waves with short vertical wavelengths. J. Atmos. Sci., 59,

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