Forecasts of Tropical Cyclogenesis with a Global Mesoscale Model: Modulation of Six Tropical...

Forecasts of Tropical Cyclogenesis with a Global Mesoscale Model:Modulation of Six Tropical Cyclones by the MJO in May 2002

B.-W. Shen1, W.-K. Tao2, R. Atlas3, Y.-L. Lin4, O. Reale2,5, J.-D. Chern2,5, C.D. Peters-Lidard2, K.-S. Kuo2,6

1:UMCP/ESSIC; 2:NASA/GSFC; 3:NOAA/AOML; 4:NCSU; 5:UMBC/GEST; 6:GEST and Caelum Research Corp.

1. INTRODUCTION: Over the past several decades, tropical cyclone (TC) track forecasts have been steadily improving, but intensity and genesis forecasts have lagged behind. A major challenge in TC genesis prediction is, among others, the accurate simulation of complex interactions across a wide range of scales, from the large-scale environment (deterministic), to mesoscale flows, down to convective-scale motions (stochastic). Global circulation models (GCMs) have been used to study TC genesis and inter-annual variability, but their insufficient grid spacing and limited physics parameterizations are known limiting factors. Recent advances in supercomputing have made it possible to deploy ultra-high resolution GCMs, obtaining remarkable TC track and intensity forecasts. One of the important questions to be answered is: are high-resolution GCMs capable of predicting realistic TC genesis? In this work, TC genesis is investigated with the aid of a global mesoscale model (GMM), used to conduct numerical experiments on the genesis of six consecutive TCs in May 2002, including two pairs of twin TCs in the Indian Ocean, Supertyphoon Hagibis in the West Pacific Ocean and Hurricane Alma in the East Pacific Ocean. It has been hypothesized that the Madden-Julian Oscillation (MJO) may play an essential role in the formation of this kind of TCs, and therefore its occurrence could represent a crucial precursor to TC genesis. In this study, we will show that our real-data simulations are the first of this type of global model experiment to support this hypothesis by demonstrating the transition from larger-scale convectively driven systems to smaller-scale TCs.

2.THE MODEL and NUMERICAL APPROACH:

The GMM, previously called the high-resolution finite-volume GCM (fvGCM), has three major components:1)finite-volume dynamics, 2)NCAR CCM3 physics, and 3) the NCAR Community Land model (e.g., Lin 2004, Atlas et al. 2005, Shen et al. 2006a).Dynamic initial conditions (ICs) and sea surface temperature (SST) are derived from the GFS T254 (~ 55km) analysis data and 1o optimum interpolation SSTs from the National Centers for Environmental Prediction (NCEP). No vortex initialization (e.g., bogusing) scheme is applied in the initial fields. Our previous studies with more than sixty 5-day runs have shown the 1/8 degree model with disabled cumulus parameterizations (CPs) was able to produce remarkable forecasts of track and intensity associated with intense Atlantic hurricanes in 2004 and 2005.Realistic vertical structure of TC (e.g., Katrina) was also simulated, including maximum horizontal winds near the top of the boundary layer, a narrow eyewall, and an elevated warm core (e.g., Shen et al. 2006a,b).

4. CONCLUDING REMARKS: In this study, we present some preliminary simulations of cyclogenesis for 6 TCs that occurred in May 2002 with a global mesoscale model. It is found that the model is capable of predicting the genesis of five of these TCs about two to three days in advance and their subsequent movements as well. The only exception is the genesis forecast for TC Errol. The occurrence of a large-scale MJO and its accompanying wind bursts appears to dictate the location and timing for TC genesis, but further analyses on the detailed transition processes among different scales are still needed. Based on our results, it can be stated thatimprovement in long-term simulations of the MJO could enhance the model's prediction of TC genesis. Moreover, improved representations of cloud-scale moist processes are also crucial, due to the nature of multi-scale interactions between the MJO and TCs. On-going model development, showing promising results along this direction, includes a multi-scale modeling framework (e.g., Randall et al. 2003;Tao et al. 2007). In modeling the formation of Hurricane Diana (1984), Davis and Bosart (2001) found strong sensitivities to model configurations.Therefore, it is important to perform a systematic study by varying the model configurations and selecting more cases.The performance of boundary/surface layer parameterizations, which might have caused the over-intensification of weak storms (e.g., TCs 01A and 02B), is being investigated. While short-term TC (track and intensity) forecasts with GCMs have generally not addressed issues of TC genesis,long-term TC climate studies do not emphasize the accuracy of TC genesis at small temporal and spatial scales (e.g., the timing and location of TC genesis). We anticipate thatimproved models enabled by modern supercomputers will be eventually able to bridge the ``gap'' between the former and latter.

References (selected) and Notes:• Atlas, et al. 2005: GRL, 32, L03801,2004GL021513. • Lin, S.-J., 2004: MWR, 132, 2293-2307. • Moncrieff, M. W., M. A. Shapiro, J. M. Slingo, and F. Molteni, 2007: WMO Bulletin, 56 (3), 1-9.• Shen et al. 2006a: GRL, 33, L13813,2006GL026143.• Shen, Tao, Atlas, Lee, Reale, Chern, Lin, Chang, Henze, Li, 2006b: AGU 2006 Fall Meeting. • Shen, Tao, Atlas, Lin, Reale, Chern, Peters-Lidard, Kuo, 2007: Forecasts of Tropical Cyclogensis with a Global Mesoscale Model. Submitted.• Tao et al., 2007: BAMS. (revised)• We thank Dr. D. Anderson under the NASA Cloud Modeling and Analysis Initiative (CMAI) program, Dr. J. Entin under NASA Energy and Water System (NEWS), Dr. A. Busalacchi of UMCP/ESSIC, and NSF Science and Technology Center for the support for this study. We would like to thank Mr. Steve Lang for proofreading this. Acknowledgment is also made to Dr. Tsengdar Lee at NASA HQ, NASA Advanced Supercomputing Division and NASA Center for Computational Sciences for computer time used in this research.

Figure 1: MJO-organized convection over the Indian Ocean at 0630 UTC 1 May 2002 (a). When the MJO moved eastward, two pairs of twin TC appeared sequentially on 6 May (b) and 9 May (c). (see also Moncrieff et al. 2007)

Figure 2: Forecasts of the genesis of the first pair of twin TCs from a 10-day simulation initialized at 0000 UTC 1 May 2002. Panels (a)-(i) show simulated total predictable water (TPW, km/m2) at Day 1 - Day 9.The BEST TRACKs with marks at 0600 UTC are plotted with black lines.

Figure 3: Forecasts of two pairs of twin TCs from a 10-day simulation initialized at 0000 UTC 6 May 2002. Panels (a)-(f) show simulated TPW at Day 1, 2, 3, 4, 6, and 8, respectively.

Figure 4: Forecasts of the genesis of Supertyphone Hagibis from a 10-day run initialized at 0000 UTC 11 May 2002. TPW at (day-1, day-3, day-4) are shown in the first row, at (day-5, day-6, day-7) in the second row. The BEST TRACKs are plotted and marked at 0000 UTC with black lines.

Figure 5:Forecasts of the genesis of Hurricane Alma from a10-day run initialized at 0000 UTC 22 May 2002.TPW at (day-1, day-2, day-3) are shown in the first row, at (day-4, day-6, day-8) in the second row. The BEST TRACKs are plotted and marked at 0000 UTC with black lines.

3. RESULTS:

•

TC 01ATC 01A

KesinyKesiny

TC 02B

Errol

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