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

Transcription

1 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 National University, Gongju, Korea Thanks: support from NASA GNSS Science Team

2 Objectives: Exploratory data analysis: evaluate long record of tropical temperature zonal variances using high vertical resolution GPS radio occultation measurements Quantify large- and small-scale variability, and relationships to background winds, stability, and deep convection GPS radio occultation technique Occulting GPS high quality temperature retrievals over ~10-35 km vertical resolution ~ 1 km Ionosphere Neutral atmosphere Occulting LEO dense observations after 2006 ~2000 occultations/day Earth

3 focus on tropical measurements (10 o N-S) ~ 200 occultations / day zonal structure over 10 o N-S example GPS profiles simple gridding strong variability near tropopause 18 km gridding = Gaussian weighting in longitude zonal variance: total T total 2 = S (T i T) 2 gridded T grid 2 = S (T grid T) 2 residual T residual 2 = T total 2 - T grid 2

4 total zonal variance in tropics (10 o N-S) ❸ increased variance > 30 km 16 QBO winds (contours) 2016 QBO disruption tropopause ~ 17 km ❶ maxima near tropopause with an annual cycle ❷ asymmetry between easterly and westerly shear zones suggest asymmetric wave source spectra Kelvin waves and gravity waves

5 Separating resolved (gridded) vs. residual T 2 variance 7.0 resolved (gridded) variance small-scale (residual) variance 10.5 similar patterns; large fraction of variance in small scales similar to SABER Ern et al 2014 JGR

6 resolved variance is primarily Kelvin waves wavenumber-frequency spectra at 18 km example Kelvin waves for 2013 Apr 1 logarithmic contours Mar 1 Feb 1 Scherrlin-Pirscher et al 2017 ACP Jan 1

7 Phase-speed spectra for tropical waves zonal wavenumber phase speed spectra frequency spectra wavenumber-frequency spectra wavenumber-phase speed spectra can then sum over wavenumber to get total power as a function of phase speed

8 eastward >> westward (well-known result) 4.0 mostly Kelvin waves eastward traveling waves 4.0 westward traveling waves

9 T 2 power as function of phase speed and latitude T 2 at 18 km July 2011 equatorially-centered Kelvin wave zonal wind

10 T 2 power as function of phase speed and altitude QBO easterlies zonal wind large amplitudes near tropopause and in QBO westerly shears

11 T 2 power as function of phase speed and altitude QBO easterlies QBO westerlies zonal wind large amplitudes near tropopause and in QBO westerly shears weak vertical propagation for QBO westerlies

12 Kelvin wave evolution through QBO phase T 2 power as function of phase speed and altitude 1 2 zonal wind 3 no critical lines (c=u) for fast Kelvin waves variance maxima in region of strong shear (du/dz) slow C gz and thermal dissipation vertical group velocity for Kelvin waves: C gz = (c-u) 2 *k/n

13 wave variance vs. background shear T 2 power as function of phase speed and month 28 km

14 T 2 power as function of phase speed and month wave variance vs. background shear 28 km 20 km maxima not tied to du/dz

15 Kelvin waves in the tropical tropopause region irregular time variations, no obvious links to U, N 2 or deep convection (OLR) forcing mechanisms are poorly understood Scherrlin-Pirscher et al 2017 ACP

16 seasonal variations near the tropopause average seasonal cycle for small-scale (residual) variance Kelvin wave variance residual variance follows tropopause, modulated by annual cycle

17 global behavior: November 2007 total variance gridded interesting structures related to background winds and large-scale waves residual

18 Key points: gridded variance mostly Kelvin waves 7.0 In the tropics: residual > gridded T 2 variance persistent maxima near tropopause strong links to QBO over ~20-30 km, asymmetric max. for westerly shears forcing of Kelvin waves near tropopause is poorly understood work in progress! small-scale residual variance 10.5 interesting annual cycle

19 extra slides

20 pressure pressure Simulating Kelvin wave propagation/dissipation in stratosphere (from Peter Hitchcock) W E W observed 100 W E W WKB solution, origin at 18 km, Fels 1982 thermal damping 100

21 temporal evolution of resolved and residual variance strong coupling to QBO over ~20-30 km 24 km annual cycle for residual, not correlated to resolved waves 17 km in general, residual > resolved

22 quasi-stationary waves = total (eastward + westward) linked to high latitudes annual cycle near tropopause