WRF MODEL STUDY OF TROPICAL INERTIA GRAVITY WAVES WITH COMPARISONS TO OBSERVATIONS. Stephanie Evan, Joan Alexander and Jimy Dudhia.

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1 WRF MODEL STUDY OF TROPICAL INERTIA GRAVITY WAVES WITH COMPARISONS TO OBSERVATIONS. Stephanie Evan, Joan Alexander and Jimy Dudhia.

2 Background Small-scale Gravity wave Inertia Gravity wave Mixed RossbyGravity wave Planetary scale Kelvin wave Wave period A broad spectrum of waves is needed to drive the middle atmospheric circulation. Intermediate-scale inertia gravity waves can have an important role in the dynamics of the upper atmosphere. The properties of these waves and their effect on the mean flow are not well documented. Radiosonde data acquired in the tropics have been commonly used to study IGWs. Good vertical resolution but limited horizontal information. In addition to observational data, analysis data can also be used to study gravity waves. Coarse vertical resolution in the stratosphere but provide global information on the wave structure. Use a flexible model such as the WRF model to understand the misrepresentation of inertia gravity waves in analyses by combining mesoscale model and GCM approaches.

3 TWP- ICE Experiment The Tropical Warm Pool International Cloud Experiment was conducted in 2006 in the area of Darwin to study convection. 3-hourly radiosonde data from 21 January to 12 February ECMWF Data: ECMWF analyses at 00, 06, 12, 18 UTC and forecasts at 03, 09, 15, 21UTC from 21 January to 12 February Spatial resolution of 1 x 1 with 21 levels from the surface up to 1hPa

4 Wave analysis Vertical and temporal interpolations for U, V and T. The perturbations are defined by removing a temporal linear trend to the time-series of U, V and T at each altitude. For ECMWF perturbations are defined by removing a linear trend at each latitude/longitude/altitude point. A wavelet analysis (the S-Transform) is used to analyze the perturbations and to compute the wave properties. Evan and Alexander (2008) : Intermediate-scale tropical IGW observed during the TWPICE campaign. JGR.

5 Time-frequency spectrum of the perturbations ECMWF RADIOSONDE ~1K ~2K Period =2.5days ~2.3m/s ~4m/s ~2.3m/s ~4m/s

6 ECMWF vs Radiosondes & ECMWF captures similar structure of the 2-day wave but with a longer vertical wavelength. The differences between the observed wave and the simulated one could be a consequence of the coarse vertical resolution of the ECMWF model in the stratosphere.

7 Final locations of the rays in the upper troposphere Ray tracing analysis Mean travelling time of ~ 7 days. The wave was observed in the stratosphere after January 28 and was generated in the upper troposphere around January 20. MTSAT Infrared Brightness temperature averaged from 01/18 to 01/27

8 WRF Simulations WRF: Weather and Research Forecasting model developed by NCAR. WRF is a flexible modeling framework extensively used both as a weather forecast tool and as a regional climate model. Boundary and Initial conditions provided by the 2006 operational ECMWF analyses and forecasts. The simulations are carried out globally in a tropical channel with latitudinal boundaries at ±42.

9 WRF Simulations WRF dx=37km, similar to operational ECMF in Simulations with different initializations times: 18, 20, 22, 24 and 26 of January. The model is run in a free running mode until February 11 to overlap the period of the wave generation and propagation up to the stratosphere. Tests with 2 cumulus schemes Kain-Fritsch (mass flux) vs Betts-MillerJanjic (adjustment scheme). Altitude (km) 84 vertical levels from the surface to 1hPa. ECMWF WRF Vertical grid-spacing (km)

10 Model Results : Precipitation Comparison of simulated (right) versus GPCP and ECMWF (left) daily mean rainfall (mm/day) from 18 January to 28 January 2006.

11 Model Results : Zonal Wind Latitude-pressure cross section of zonal wind averaged over 18 January-6 February WRF zonal winds are interpolated to ECMWF pressure levels.

12 Evolution of diabatic heating from the cumulus scheme. Time-height sections of heating rate from the BMJ scheme for the period of wave generation. Average over (10S, 100E-5N, 140E) Active monsoon : Jan Suppressed monsoon : 26Jan-02 Feb

13 Time frequency spectrum Radiosondes WRF U~ 2.5 m/s V~ 2.5 m/s

14 Wave generation mechanism WRF zonal wind at 13km averaged from 01/18 to 01/27 WRF 3-hourly precipitation rates averaged from 10S to 5N In the simulation top of convection around 12km and λz = 14km in upper troposphere.

15 Wave packet ECMWF WRF 01/18 WRF 01/20 WRF 01/22 WRF 01/24 WRF 01/26 Squared amplitude of the 2-day wave inferred from the quadrature spectrum of u' and v' averaged between 30 and 20hPa. Average over 28 January to 6 February.

16 Vertical structure WRF 01/18 Radiosonde vertical wavelength=6km ECMWF vertical wavelength=9.3km WRF 01/26 Vertical profiles of phase differences and coherences for u for a period of 2.5 days. Reference level is 23km

17 Vertical structure WRF 01/18 dz=500m WRF 01/18 dz=1.5km

18 Summary and conclusions. Good agreement between the simulated and observed wave structure which suggests WRF ability to reproduce the observed 2-day wave event. The WRF model gives comparable results for the horizontal structure but shows better skill than ECMWF to resolve the vertical structure of the wave. This study also evaluates WRF performance in the tropical stratosphere and shows that WRF has some ability to resolve gravity waves generated by convection in the tropics. Evan, Alexander and Dudhia: Model study of intermediate-scale tropical IGW and comparison to TWP-ICE campaign observations, submitted to JAS.

19 Evolution of the zonal wind in WRF and ERA-INTERIM First 4 days of the simulation Average over 4 days during the middle of the simulation Last 4 days of the simulation

20 Space-time spectrum of wave acceleration 2006 EP Flux divergence at 26-27km (m/s/day/wvn/cpd) 2007 EP Flux divergence at 26-27km (m/s/day/wvn/cpd)

21 Wave forcing