Reprint 676

The Impact of Moisture Data in the Numerical Simulation of

Tropical Cyclones using a Non-hydrostatic Model

V.B. Malano*, W.K. Wong & S.T. Lai

ESCAP/WMO Typhoon Committee Annual Review 2005

* Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA)

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The Impact of Moisture Data in the Numerical Simulation of Tropical Cyclones using a Non-hydrostatic Model

Vicente B. Malano1, W.K. Wong2 and Edwin S.T. Lai2

1 Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA),

Science Garden, Agham Rd, Diliman, Quezon City-1101, Philippines

2 Hong Kong Observatory, 134 A, Nathan Road, Tsim Sha Tsui, Kowloon, Hong Kong, China

Abstract

Moisture analyses based on input from satellite observations were ingested into the initial field of the

Hong Kong Observatory (HKO) Non-hydrostatic Model (NHM) to study their impacts on the prediction of movement, intensity and precipitation in tropical cyclones (TC). The moisture analysis was performed using the Local Analysis and Prediction System (LAPS) through the assimilation of visible albedo and infra-red brightness temperature data. Experiments for two selected TC cases were conducted. The results showed that additional moisture information in the initial field of NHM could have positive impact in the short-term prediction of movement, although forecasts at 24 hours or beyond were very much controlled by the synoptic-scale forcing imposed by the lateral boundary conditions. In intensity trend prediction, NHM demonstrated improved skills as a result of moisture data injection. The use of satellite data in the moisture field could also produce positive impacts in simulating the precipitation patterns around the cyclone centres.

1. Introduction

The evolution of a tropical cyclone depends on many factors. Development effort to improve skills in modelling TC track, intensity and related weather phenomena is partly hindered by limitations in the representation of the physics in the atmosphere. Assumptions and approximations are often made.

The main objective of this paper is to study the impact of moisture data from satellite

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observation in the simulation of tropical cyclones using a non-hydrostatic model, in particular the effects on the movement, intensity change, and the precipitation structure of TCs. Similar studies were conducted by Jian and McGinley (2005) who incorporated conventional data, radar data, as well as GOES-9 infra-red and visible data, into a non-hydrostatic model through a diabatic data assimilation system; the results reported were mostly encouraging. 2. Methodology and TC Cases 2.1 NWP Model

The numerical experiments were performed using the HKO Non-hydrostatic Model

(NHM), originally developed by the Japan Meteorological Agency (Saito et al., 2006). The horizontal resolution was 10 km with 221 grid-points in both zonal and meridional directions. The number of vertical levels was 40.

Initial and boundary conditions were extracted from the experimental MPI-RSM (Message

Passing Interface version of the Regional Spectral Model) (NPD/JMA, 2002) at HKO. The horizontal resolution was 20 km, with 40 hybrid sigma levels in the vertical. Observation data, including TC bogus profiles and rainfall estimation from satellite infra-red brightness temperature data, were assimilated by the 3-dimensional optimal interpolation (3D-OI) and physical initialization (PI) schemes (Lam and Yeung, 2003).

In the NHM, the Kain-Fritsch (KF) convective parameterization scheme and the cloud

microphysics process were enabled in an attempt to better resolve the grid-scale and convective precipitation of TC. To alleviate the spin-up problem of moisture variables during the model integration, the specific humidity of the hydrometeor fields in the NHM was initialized using the cloud and moisture analysis output from the Local Analysis and Prediction System (LAPS) adapted from the NOAA FAB/ESRL (Albers et al., 1996). Satellite data, including albedo in visible (VIS) channel, brightness temperature in infra-red (IR1 and IR2) as well as water-vapour (WV) channels, were ingested in the experiments. 2.2 Design of numerical experiments

NHM simulations were carried out to obtain 30-hour forecasts for two TC cases over the

South China Sea (SCS): (1) Tropical Storm Kompasu (0409) in July 2004, and (2) Typhoon Damrey (0518) in September 2005. In each case, three experiments were conducted for selected model runs as described in the table below:

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Experiment Description (i) Control experiment (CTRL) Initial fields were interpolated from MPI-RSM.

Only specific humidity of water vapour was initialized using output of MPI-RSM; other cloud microphysics variables were set to zero initially.

(ii) Experiment A (EXPT-A) All moisture fields in NHM were initialized by HKO-LAPS with VIS, IR1, IR2 and WV data ingested.

(iii) Experiment B (EXPT-B) All moisture fields in NHM were initialized by HKO-LAPS, but with only IR1 data ingested.

3. Discussion of results in numerical experiments 3.1 Tropical Storm Kompasu (0409)

Two model runs with initial time at 00 and 06 UTC on 15 July 2004 were carried out, giving a total of 6 experiment data sets for this case. 3.1.1 Prediction of track and intensity. After Kompasu entered the SCS in the early morning of 15 July 2004, forecast tracks produced by the three sets of NHM experiments initialized at 00 UTC (Figure 1) and 06 UTC (figure not shown) that day followed closely the best-track. They also showed slight improvement compared with the MPI-RSM. The inclusion of satellite data (EXPT-A and EXPT-B) in initial moisture fields resulted in better tracks up to T+12 hr forecast (Figure 2), larger errors in the range of 18 – 24 hours, and then improved performance again thereafter up to T+30 hours. Compared with the CTRL and MPI-RSM, overall position errors were reduced by more than 20% and 35% respectively in both EXPT-A and EXPT-B.

For intensity prediction, we can see from Figure 3 that NHM, with a higher resolution than

MPI-RSM, could reduce the error in the intensity prediction. However, improvement in intensity prediction when additional satellite moisture data were included in EXPT-A and EXPT-B was not obvious, and the impact could well be perceived as slightly negative if compared against the results of CTRL.

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3.1.2 Precipitation Forecast Figure 4 shows the 1-hr accumulation of precipitation for T+12 hr forecast initialized at 00 UTC 15 July 2004. The CTRL predicted some weak rainbands extending outwards over the eastern flank of Kompasu’s circulation. With the inclusion of satellite data into the initial moisture fields in EXPT-A and EXPT-B, the weak rainbands were effectively removed and intense convection was overall more confined within a range of 100 km from the centre of Kompasu, which agreed better with the extent of overcast cold cloud as seen in the IR imagery.

In the T+24 hr forecasts initialized at 00 UTC (Figure 5) and 06 UTC 15 July 2004 (figure not shown), all three experiments could predict the asymmetric structures of precipitation fields around the centre of Kompasu. They were generally consistent with the radar echo patterns. The precipitation intensities produced by all three experiments were of similar order of magnitudes. In particular, the use of satellite data in the initial moisture field was able to forecast the semi-annular rain-free area over the eastern part of the TC circulation. 3.2 Typhoon Damrey (0518)

Three sets of model runs, namely (i) 00 UTC, (ii) 06 UTC on 23 September, and (iii) 06 UTC on 24 September 2005, were undertaken to investigate the prediction of Typhoon Damrey in response to satellite data ingested through LAPS into initial moisture fields of NHM.

3.2.1 Prediction of track and intensity.

Typhoon Damrey underwent a slight southward deflection when moving across the SCS on 24 September. This was not forecast by the MPI-RSM in its model runs on 23 September. For predictions using NHM, the forecast tracks with satellite data ingested in the initial conditions showed a slight improvement throughout the 30-hour integration period. Figure 6(a) compares the forecast tracks from the MPI-RSM and from the three NHM experiments. Initially, in the first 12 to 18 hours of forecasts, tracks using satellite data in the moisture analysis were very similar to the CTRL and the MPI-RSM. After that, the EXPT-A and EXPT-B forecast tracks remained more to the south and were closer to the best track by about 50 km. However, the apparent improvement was offset by a faster-than-actual zonal movement. This was also the case for the other two sets of experiments initialized at 00 UTC and 06 UTC 24 September. As a net result, in terms of position errors, the use of additional moisture data did not have any positive impact in improving the overall track forecast.

For intensity prediction, the NHM showed some improvement in predicting the trend of

minimum pressure (see Figure 6(b)). But due to NHM’s faster westward movement, earlier

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landfall over Hainan Island rendered comparison of subsequent intensity forecasts inconclusive. 3.2.2 Precipitation Forecast Selected rainfall patterns forecast by the three NHM experiments were compared with satellite IR imagery and TRMM rainfall estimates. While the spiral rainbands around Damrey were reproduced in all the experiments, results from EXPT-A and EXPT-B correctly suggested that the more intense rainbands would be located on the southern part of Damrey’s circulation. For the T+12 forecast initialized at 06 UTC 24 September (Figure 7), the ‘opening’ of the eye towards the northwestern quadrant was well predicted by both EXPT-A and EXPT-B, but less so by the CTRL. 4. Conclusion

A preliminary study on the use of HKO Non-hydrostatic Model was conducted to understand the impact of moisture data in the numerical simulation of TC. Two cases in 2004 and 2005 with different intensities were selected: Tropical Storm Kompasu and Typhoon Damrey. Numerical experiments were performed to compare the track, intensity change and precipitation forecast using different initial fields. The following observations were deduced from the results obtained: (a) the use of high resolution NHM gave some improvement in the short-term (within 24

hours) forecasts of movement and intensity change; but model performance beyond the first 24 hours was governed more by synoptic forcing imposed by the lateral boundary conditions obtained from coarser resolution model; and

(b) satellite moisture data had positive impact in the precipitation prediction for TC in the first

24 hours in terms of precipitation intensity and the distribution of rainbands around the cyclones.

While it is difficult to be conclusive given the limited number of cases studied, EXPT-A

and EXPT-B did produce some promising initial results, particularly in terms of short-term motion and rainfall distribution. Based on the results of this study, more numerical experiments on TC cases will be conducted to investigate more systematically the impact of satellite data and the use of high resolution NHM for TC prediction. The performance of quantitative precipitation forecast in terms of errors in intensity and location will also be studied further.

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Acknowledgement

This research project was made possible through the ESCAP/WMO Typhoon Committee

Research Fellowship Scheme with financial and logistic support from the Hong Kong Observatory

(HKO) of the HKSAR Government. The leading author would like to thank in particular all

members of the Forecast Development Division of HKO for their assistance, especially Mr. C.K.

Chow for his support in data preparation and graphical processing. Thanks were also due to the

valuable comments on the draft of this paper by Mr. C.Y. Lam and Dr. M.C. Wong.

References Albers, S., J. McGinley, D. Birkenheuer, and J. Smart, 1996: The Local Analysis and Prediction System (LAPS): Analyses of clouds, precipitation, and temperature. Weather and Forecasting, 11, 273-287. Jian, G.J. and J. McGinley, 2005: Evaluation of a Short-Range Forecast System on Quantitative Precipitation Forecasts Associated with Tropical Cyclones of 2003 near Taiwan. J. Met. Soc. Japan, 83, 657-681. Lam, C. C. and H.Y. Yeung, 2003: Impact of Radar Data on Model Forecast of Heavy Rain Associated with Landfalling Tropical Cyclone. Workshop on NWP Models and Heavy Rain, Tokyo, Japan, 4-6, February 2003. NPD/JMA, 2002: Outline of the Operational Numerical Weather Prediction at the Japan Meteorological Agency, Japan Meteorological Agency, March 2002. Saito, K., T. Fujita, Y. Yamada, J. Ishida, Y. Kumagai, K. Aranami, S. Ohmori, R. Nagasawa, S. Kumagai, C. Muroi, T. Kato, H. Eito and Y. Yamazaki, 2006: The operational JMA nonhydrostatic mesoscale model. Mon. Wea. Rev., 134, 1266-1298.

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Figure 1 - Forecast tracks by all three NHM experiments and MPI-RSM initialized at 00 UTC 15 July 2004.

Figure 2 - Temporal variation of average position errors of the numerical experiments with initial time at 00 UTC and 06 UTC on 15 July 2004.

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(a) 00 UTC 15 July 2004

(b) 06 UTC 15 July 2004 Figure 3 - Intensity forecast of Kompasu with initial time at: (a) 00 UTC 15 July 2004; and (b) 06 UTC 15 July 2004.

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Figure 4 - T+12 hr forecasts of hourly precipitation field (colour shading) with MSLP initialized at 00 UTC 15 July 2004 by: (a) CTRL; (b) EXPT-A; and (c) EXPT-B. IR imagery at 12 UTC 15 July 2004 is shown alongside in (d) for validation.

25°N

20°N

15°N

115°E 120°E

(a) CTRL

(b) EXPT-A

(c) EXPT-B (d) IR imagery

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(a) CTRL (b) EXPT-A

(c) EXPT-B (d) radar imagery

Figure 5 - T+24 hr forecasts of hourly precipitation field (colour shading) with MSLP initialized at 00 UTC 15 July 2004 by: (a) CTRL; (b) EXPT-A; and (c) EXPT-B. Radar imagery at 00 UTC 16 July 2004 is shown alongside in (d) for validation (circles plotted in 100-km interval).

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(a) forecast tracks

(b) time-series of intensity forecast

Figure 6 - Comparison of the three NHM experiments results (CTRL, EXPT-A and EXPT-B), initialized at 06 UTC 23 September 2005 against MPI-RSM prognosis initialized at the same time: (a) forecast tracks; (b) time-series of intensity forecast.

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(a) CTRL (b) EXPT-A

(c) EXPT-B (d) TRMM rain rate

Figure 7 - T+12 hr forecasts of hourly precipitation field (colour shading) with MSLP initialized at 06 UTC 24 September 2005 by: (a) CTRL; (b) EXPT-A; and (c) EXPT-B. TRMM rain rate at 1706 UTC 24 September 2005 is shown alongside in (d) for validation.