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Characterization of the Sources of Gravity Waves Over the Tropical Region Using High Resolution Measurements

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Characterization of the Sources of Gravity Waves Over the Tropical Region Using High Resolution Measurements Characterization of the sources of Gravity Waves over the tropical region using high resolution measurements Debashis Nath & M. Venkat Ratnam National Atmospheric Research Lab. Gadanki, India Outline of the Talk • Brief introduction and significance of the study • Data description • Characteristics of gravity waves of different scales and possible source mechanisms • Seasonal characteristics of Inertia gravity waves • Wave-mean flow interaction • Summary and Conclusions Geographical Location of the Observational site (13.45 N, 79.2 E) OVERVIEW Mesosphere 40km Stratosphere VHF Radar Radiosonde Kelvin waves LIDAR, GPS RO - Eastward propagation Gravity waves (UTLS region) - Downward phase Rossby waves Spatial and Temporal Gravity waves Trapped by - Westward propagation variations using GPS vertical shear RO and SABER VHF Radar Radiosonde LIDAR, GPS RO Tropopause Vertical structure of fast Tropical Easterly Jet H H and Ultra-Fast waves L Turbulence 10km Generation mechanism Tropospheric Geostropic Adjustment Tropospheric disturbances - Eastward-propagating cloud system Regional convections - Low-level convergence (OLR, GPS-RO, WV) ISM VHF Radar ABL Radiosonde, GPS RO Orography Provide a favorable condition for development 0km Data Description GPS radiosonde (Väisälä RS-80, RS-92 and Meisei) April 2006 to March 2009 around 1730 LT (LT=UT+0530h), 926 upper air soundings. During campaign period, balloons are launched for every 6 hours. Indian MST Radar Beams : 6 beams (East, West, ZenithX, ZenithY, North, South). Pulse Width:16 micro sec, Coded Pulse. IPP : 1micro sec. 10 mints/hour 10 mints/hour Every 6 hours (5 days) 72 hours (15-07-2008 to 18-07-2008) Satellite observations of equivalent black body brightness temperature (TBB) Hourly cloud top equivalent blackbody temperature, called Brightness Temperature (BT) from MTSAT-1R (Multi-functional Transport SATellite) data provided by the Japan Meteorological Agency (JMA) through Kochi University, Japan. GPS RO COSMIC (2006-2008) temperature profiles OLR daily composites data from NCEP ECMWF zonal wind and Potential Vorticity data Background meteorological and cloud conditions Bandpass Filtering Amplitude Phase Source Region for Gravity Wave due to wind shear? Source Region for Gravity Wave due to convection ? • Height of constant phase, source region, Wind shear at upper tropospheric height generate the spectrum of waves starting from short scale to inertial period. • Localized convection contributes to the generation of short and medium scale waves. Hodograph analysis for the extraction of Gravity wave parameters UPara /UPerp =-i(f/Ω). Where Ω is the wave intrinsic frequency and f is the Coriolis frequency.Coriolis period over Gadanki f ~ 52 hrs. Dispersion Relations 2 2 2 2 2 2 K zonal + K meridional = K vertical (Ω –f ) / N (low freq.) K (medium freq.) Ω =N horizontal K vertical 2 2 2 2 NKK()zonal + meridional (high freq.) Ω = 2 2 2 KKK()zonal+meridional + vertical kZonal,kMeridional and kVertical are the zonal, meridional and vertical wavenumbers. Quasimonochromatic wave components are extracted by a harmonic fitting to the fluctuations C = f / K , C = f / K and C = f / K components as the following equation: Z Zonal M Meridional V Vertical Where CZ , CM and CV are the Zonal, Meridional and U= A sin ((2 Π / λz )Z + φ). the Vertical Intrinsic phase speed. Where U=[u, v, t] the fluctuation components and A is the amplitude, λz is the vertical wavelength. The hodograph analysis with the sine fitted data i.e. the plot of U’ and V’ is an ellipse. Gravity wave Parameter Estimation Dominant periods: SGWs: 30mints – 2 hours MGWs: 14-18 hours IGWs : 50-52 hours (IGWs) Dominant amplitudes: SGWs: 4-6 m/s, K MGWs: 0.75-1.25 m/s, K IGWs : 5-9 m/s, K Dominant wavelengths: SGWs: 5-7 km, 50-60 km. MGWs: 4-5 km, 80-100 km IGWs : 3-5 km, 100-200 km Dominant Phase Speed: SGWs: 1-5 m/s MGWs:~2.5m/s IGWs : 0-40 m/s Horizontal Phase Propagation Direction UT N W E S LS • Waves are propagating mainly eastward and westward at lower stratospheric and upper tropospheric heights respectively. • The westward propagating waves are mainly filtered out due to large westward wind shear. Vertical Phase Propagation Direction Monsoon 0-10 km 10-14 km 18-25 km 2008 Rotation Clock Anti-clock Clock Anti-clock Clock Anti-clock Total 1-90 day 55.5 % 44.5% 40% 60% 94% 6% Event 1 5- 25 day 57.15% 42.8% 47.6% 52.4% 94.7% 5.26% Event 2 30-45 day 43.75% 56.25% 37.5% 62.5% 93.3% 6.66% Event 3 50-75 day 61.5% 38.5% 46.2% 53.8% 96% 4% Event 4 75-90 day 68.75% 31.25% 12.5% 87.5% 93.75% 6.25% Wave is propagating mainly upward in lower stratosphere and downward in upper troposphere in all the four events, confirming wind shear as the dominant source for the generation of IGW. Investigation of Exact Source mechanisms E = 1/2 [U’2 +V’2 ], E = 1/2 (g/N)2(T’/T )2 •Deep convection with large shears (event 1) k P 0 •Medium convection with medium shear (event 2) Magnitude of Shear at 17 km mismatches •Deep convection with small shears (event 3) with energy at several occasions. Why? •Low convection with strong shears (event 4). Horizontal Wind Imbalance ECMWF Model Wind Data at 100 hpa High High Low Strong horizontal shear, plausible source for GW generation due to Geostropic Adjustment The flow become unbalanced in the regions where the Lagrangian acceleration become comparable to the Coriolis force. It will represent the regions where the flow is close to the state of balance other than geostropic balance. (Plougonven et. al. 2003) Geostropic Adjustment ECMWF ERA-40 Interim Potential Vorticity Data at 100 hpa & 320 K isothermal level V Dv Dt ag Lagrangian Rossby number ~ 1 for gravity wave RoL= = f v v Vag is the ageostropic velocity and v is the wind vector Investigation of Exact Source mechanisms June 2008-August 2008 •Deep convection with large shears (event 1) •Medium convection with medium shear (event 2) 2 2 2 2 Ek= 1/2 [U’ +V’ ], EP= 1/2 (g/N) (T’/T0) •Deep convection with small shears (event 3) •Low convection with strong shears (event 4). Background Meteorological conditions Vertical and Horizontal Propagation directions Eastward and Westward propagation of IGW at lower stratosphere and upper troposphere irrespective of any season. This confirms earlier campaign results. Debashis Nath et.al., JGR 2008 Season 0-14 km 0-10 km 10-14 km 18-25 km Rotation Clock Anti- Clock Anti- Clock Anti- Clock Anti- Similarly the dominant clock clock clock clock source mechanism i.e. strong wind shear is also Monsoon 22 % 78 % 57 % 43 % 43 % 57 % 89 % 11 % being confirmed. Post-monsoon 30 % 70 % 61 % 39 % 49 % 51 % 79 % 21 % Winter 28 % 72 % 58 % 42 % 56 % 44 % 75 % 25 % Kinetic and Potential Energy Source Mechanisms Month (April 2006-March 2009) Month (April 2006-March 2009) •Annual and Semiannual oscillation in GW activity is significant at lower stratosphere and •Strong zonal and meridional wind shear are upper troposphere respectively. significant at upper troposphere during monsoon and winter season respectively. •Winter enhancement in GW activity is noticeable, which may be due to meridional wind shear. Debashis Nath et.al., JGR 2008 Summarized source mechanisms for GW in different seasons Convection (Short/medium period, lower troposphere) Summer (Monsoon) Wind Shear due to TEJ/ Geostropic Adjustment (Short, medium, long period GW) UTLS Convection Postmonsoon Wind Shear (UTLS) Orography Winter Meridional Wind Shear (UTLS) Wave Mean Flow Interactions O O O O Time-Height section of monthly mean EP over a spatial band of (a) (-5 S-5 N&90 -120 E) using COSMIC & monthly mean zonal wind over Singapore (b) (10O-15ON&60O-90OE) using COSMIC & monthly mean zonal wind over Gadanki (c) (13.45ON&79.18OE) using Meisei radiosonde & monthly mean zonal wind over Gadanki. The solid and dashed line indicates the westerly and easterly wind. Summary and Conclusions • GW’s, are propagating mainly eastward and westward at lower stratosphere and upper troposphere respectively. The westward propagating waves are filtered out due to strong wind shear at upper tropospheric heights. • Dominant source mechanisms for short and medium scale waves are convection and wind shear during monsoon, whereas for IGW it is zonal wind shear/Geostropic adjustment during monsoon and meridional wind shear during winter. • Annual and semi-annual oscillation in the GW activity are observed at lower stratosphere and upper troposphere respectively. • Enhancement in tropospheric total kinetic energy during winter season is significant, which is due to meridional wind shear. • The magnitude of EP from COSMIC, matches fairly with ground based radiosonde measurements (~ 14 J/kg). • The wave activity is more prominent in the eastward shear phase of QBO due strong wave mean flow interaction. Thank You.
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