Kelvin waves as observed by Radiosondes and GPS measurements and their effects on the tropopause structure: Long-term variations

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Tsuda Research Institute for Sustainable

1 Kelvin waves as observed by Radiosondes and GPS measurements and their effects on the tropopause structure: Long-term variations M. Venkat Ratnam and T. Tsuda Research Institute for Sustainable Humanosphere (RISH) Kyoto University Japan III International SOWER meeting, July, 2006

2 Outlook Brief Introduction Data Base and analysis procedure Kelvin waves observed with radiosonde and associated effects on tropopause structure Global morphology of Kelvin waves with CHAMP/GPS observations - Longitude height section - Time-longitude section Comparison between Radiosonde and CHAMP GPS RO Generation and propagation of Kelvin waves - Correlation with OLR (proxy for deep convection) - Response to the background wind Long-term variations of Kelvin waves, and effects on tropopause structure Summary and conclusions

3 Equatorial Kelvin Waves (KWs) Introduction: One of the most dominant atmospheric waves in the tropics with a planetary scale [Wallace and Kousky,1968] Kelvin waves are Eastward propagating waves haveing periods of 3-20 days, and vertical wavelength 5 12 km and are trapped within 15 from the equator [Andrews et al., 1987]. Kelvin waves are centered over the equator with symmetric Gaussian latitudinal structure, with a typical e-folding scale of degree latitude [Mote et al., 2002]. Fluctuations in the U, T, p coupled to cloudiness fields Importance: Convective coupled Significantly affect the behavior of Tropopause and influence the Kelvin waves dehydration and cirrus formation [Fujiwara et al., 1998] and occurrence Free mode of turbulence [Fujiwara et al., 2003] Kelvin waves have been found coupled to Convection (SCCs) first by Takayabu and Murakami [1991] and in the observational data by Wheeler and Kiladis [1999], Straub and Kiladis [2002] and Yang et al., [2003]. These convectively coupled waves have typical periods of 5-10 days, S=2-3, phase speed of 15m/s.

4 Introduction (continued..) Free mode Kelvin waves are primarily forced by deep convection which donot propagate coherently with convective centers in troposphere. They mainly depend on space-time patterns of convective forcing along with background winds and temperature. Such waves are observed by Radiosonde [Wallace and Kousky, 1968; Tsuda et al., 1994; Sato et al., 1994; Holton et al., 2001; Shimizu and Tsuda, 1997; Fujiwara et al., 2001] and MST radar [Sasi and Deepa, 2001]. These have periods days, S=1-2 and phase speed of 20-30m/s. Vertically propagating KWs have enhanced amplitudes near tropopause and also in lower Stratosphere (depending upon background wind). Global Observations: High period Kelvin waves in lower Stratosphere is also observed by CLAES satellite observations [Shiotani et al.,1997]. Although 5-10km Vertical wavelength is poorly resolved by these satellites. Faster Kelvin waves (5-10 days, vertical wavelength >10km ) in middle troposphere and lower stratosphere are better sampled with Nimbus-7 LIMS satellite observations [Salby et al., 1984], MLS observations [Mote et al., 2002], CRISTA [Smith et al., 2002]. Using high vertical resolution (<1km) and high accuracy (0.5 K) temperature measurements by CHAMP and SAC-C, for the first time Tsai et al. [2004] shown the evidence of KW in UTLS region using one year data. Randel and Wu [2005] extended the study with special emphasis on how these KWs are influenced by Background stratospheric winds and also how these are linked with tropical deep convection using limited data set.

Data Base CHAMP/GPS : May 2001 June 2006 Latitude Selection : +/- 10 degrees Height coverage : 10-30 km Height resolution : 200 m Field of observation : Temperature Singapore Radiosonde data: Jan

5 Data Base CHAMP/GPS : May 2001 June 2006 Latitude Selection : +/- 10 degrees Height coverage : km Height resolution : 200 m Field of observation : Temperature Singapore Radiosonde data: Jan 2001 June 2006 (background wind) Height coverage : upto 30 km Height resolution : 200 m Field of observation : Zonal wind (for QBO) Intensive Radiosonde campaigns: Nov.2002, CPEA 1 (April-May 2004), CPEA2 (Nov. Dec.2005) Location : over equator around 100E longitude Height coverage : upto 30 km Height resolution : 100 m Field of observation : Temperature, zonal velocity 10 0 *

6 Kelvin waves Analysis procedure for Radiosonde data Nov.2002: 4-8 launches per day Large perturbations in T are observed particularly between 13 km and 17 km and seem to be connected to the downward phase propagating fluctuations from the lower stratosphere. Monthly mean profiles were deducted from individual profiles to get fluctuation component Low pass filter with cutoff at 3 days is then applied to the fluctuation component. Profiles were further smoothed out in vertical applying Low pass filter cutoff at 2 km vertical wavelength. The time-height section of filtered profiles of T during the radiosonde campaign show very different behavior between the troposphere and stratosphere. Similar to the results reported earlier, vertically propagating waves (downward phase propagation with height) were observed to be dominant in T in the UTLS.

7 Global Morphology of Kelvin waves using CHAMP GPS RO Analysis procedure Latitude Selection : +/- 10 degrees Height coverage : km Height resolution : 0.2 km Step 1: 30 day median profiles (15 days before and after) were removed to get temperature fluctuations Step 2: Profiles are further smoothed in altitude with cut off at 2 km using 2 Chebyshev lowpass filter Step 3: To isolate dominant wave numbers, a fitting function, 2 f ( x) = A sin ( ) s s x φ s s= 1 mixing with wave 1 and wave 2 components is then applied to best fit the Ts data with the amplitude As and phase fs for all longitudes x solved by means of a nonlinear least-square method. Since CHAMP GPS is polar orbiting, less number of occultation in equatorial region, hence RO are accumulated for 3 days (±1 day) and treated those variations to the central day.

8 Longitude-Height of temperature fluctuations-typical example May 3-5, 2004 (During CPEA 1 campaign) T profiles were enhanced around Tropopause height. Fitted curve (WN1&2) T F Original T 20km The profiles occurring at proximate locations are very similar, indicating GPS are of high accuracy. Performing this data processing over long-term will reveal the time evolution of zonal wave propagation Tsuda et al., 2006, JMSJ (In Press)

9 Time-Longitude variations of fitted T s (during CPEA 20km Koto Tabang (100 o E)

10 Comparison between CHAMP GPS RO and Radiosonde observed wave structure November 2002 Detailed comparison: Ratnam et al., 2006(a), JMSJ

11 Time-longitude section of OLR, T, TF, and TR at 16km T and TF are constructed every three adjacent days and are vertically shifted to the middle date daily. Each curve is shifted 4 K vertically. OLR: 10 o N and S TR = TF T Ratnam et al., 2006 (a), JMSJ

Comparison between CHAMP GPS RO and Radiosonde observed wave structure Height - time sections of (a) TF observed by GPS RO data at 1000E longitude, (b) interpolated T (combination of TF and TR )

12 Comparison between CHAMP GPS RO and Radiosonde observed wave structure Height - time sections of (a) TF observed by GPS RO data at 1000E longitude, (b) interpolated T (combination of TF and TR ) observed at the same longitude, (c) low-pass filtered (3 day in time and 2 km in height) temperature variations observed by radiosonde at Koto Tabang (100.32oE), during November 2002 campaign. Detailed comparison:

13 Longitude-height section of TF, TR, and Lat-long distribution of OLR WN1&2 Residual OLR WN1&2 Residual OLR

Generation: Correlation of Kelvin wave amplitudes near tropopause with OLR Note: For clarity 15 day running mean is applied Correlation coefficient R = -0.29, SD=0.

14 Generation: Correlation of Kelvin wave amplitudes near tropopause with OLR Note: For clarity 15 day running mean is applied Correlation coefficient R = -0.29, SD=0.23, N=1550 (Between wave amplitudes and zonal mean OLR) R = -0.52, SD=0.21, N=1550 (Between wave amplitudes and OLR( o E) ) Peak amplitudes of Kelvin waves are mostly linked to tropical convection either with wave numbers 1 and/or 2 (convectively couples waves), although they do not always match exaclty.

Long-term behavior of Kelvin waves and effects on tropopause structure using GPS data Kelvin waves response to the background wind Strong relation to background wind -

15 Long-term behavior of Kelvin waves and effects on tropopause structure using GPS data Kelvin waves response to the background wind Strong relation to background wind - Thermal damping (Shiotani and Horinouchi, 1993) Extended figure of Randel and Wu, JGR,2005 (with additional 3.5 years data) Ratnam et al., 2006 (b), Ann. Geophys., (In Press)

16 Cross correlation between Kelvin wave amplitudes, OLR and background wind May 2001-October 2005 KW amplitudes Vs OLR: A reasonable correlation (-0.52) is observed as expected around tropical tropopause KW amplitudes Vs Zonal wind: Zonal wind plays a significant role only above 23 km height, with peak around 27-28km, the region of maximum shear In order to estimate the impact of different background conditions at different heights on the observed KW amplitudes a cross correlation has been performed.

17 Anomalies from Climatological Mean (4 and 1/2 years) R=0.56 S TP T Δz T T T Δz TP +Δz TP TP TP Δz =

18 Summary and conclusions We have studied vertical and temporal characteristics of equatorial Kelvin waves and their effects on tropical tropopause by using both ground based and satellite data. These different techniques reveal vertically propagating waves with downward phase progression in the UTLS region in both temperature and zonal velocities. The period of these waves is between 10 and 15 days with vertical wavelengths of ~5-8 km. Similar to the earlier reported results, the tropopause temperature and height were also significantly affected. This modulation of tropopause is thought due to global scale Kelvin waves, as the major properties match the results reported earlier [ex. Shimizu and Tsuda (1997; 2000)]. However, due to lack of simultaneous high-resolution global measurements, previous studies do not indicate whether these perturbations are localized or have a global feature. In the present case, simultaneous CHAMP GPS RO measurements are used to observe the global variability of these waves and found existence of higher zonal wave number components. These higher wave number components, which were estimated as about 4, could also be responsible for the modification of the tropopause structure. Thus, caution is advised in relating radiosonde observed perturbations and associated effects on the tropopause with global scale Kelvin waves (particularly near the source region).

19 ummary and conclusions.(continued) Most of these higher wave number components were probably filtered out near the tropopause and only components of wave numbers 1 and 2 appear to propagate into the stratosphere, because a reasonably good agreement is found between radiosonde observations and CHAMP GPS RO measurements fitted for wave number 1 and 2 alone. Reasonably good correlation between OLR averaged between o E and Kelvin waves with WN 1 & 2 is observed near tropopause. A clear month-to-month coincidence between Kelvin waves and tropopause variation is observed within the annual variation using long-term data set from GSP RO. At higher heights, response of Kelvin waves to the background wind (QBO) is clearly observed with higher vertical wavelengths during strong eastward shear consistent with earlier reported results. A clear signature of QBO is also noticed in the Tropopause structure with higher height colder temperatures, and sharp tropopause during strong Kelvin wave activity (or during strong eastward shear). Next Step: To get detailed structure of higher zonal wave number component near the source region (tropopause) by using forthcoming COSMIC data which will be available from this month end. Thanks for your attention!

Tropical temperature variance and wave-mean flow interactions derived from GPS radio occultation data

Tropical temperature variance and wave-mean flow interactions derived from GPS radio occultation data Bill Randel 1, Mijeong Park 1, Peter Hitchcock 1 and Joowan Kim 2 1 NCAR, Boulder, CO USA 2 Kongju