NRL Monterey and Stennis Researchers Study Tropical Cyclones


8/20/2009 - 64-09r
Contact: Donna McKinney, (202) 767-2541


The hurricane season has arrived and the Naval Research Laboratory's Marine Meteorology Division (NRL-MRY) in Monterey, CA, and Physical Oceanography Division in Stennis Space Center, MS (NRL-SSC), are working on multiple aspects of tropical cyclone research. NRL-MRY participated in a field program that is providing them with a wealth of unique and critical Western Pacific typhoon data sets that will help improve operational forecasts with enhanced tools and ultimately protect assets throughout this large operations area. NRL-MRY and NRL-SSC will also be participating in a follow-on study in 2010 with a greater focus on aspects of the ocean response to typhoons, which is critical not only to typhoon strength forecasts for ship and aircraft protection, but also to acoustics and anti-submarine warfare.

NRL-MRY participated in an ONR and NRL-sponsored field program called Tropical Cyclone Structure (TCS-08) to study tropical cyclone genesis, structure and structure change, extra-tropical transition, and targeted observations. This Guam and Japan-based project in August and September 2008 focused on northwest pacific typhoons, since aerial reconnaissance stopped in 1987 and few in-situ observations exist to verify today's satellite and numerical weather prediction (NWP) products. The project was collaborative with a related international effort called THORPEX (The Observing System Research and Predictability Experiment) Pacific Asian Regional Campaign (T-PARC) involving jet aircraft from Taiwan and Germany, and French driftsonde balloons launched from Hawaii. The U.S. aircraft were the NRL P-3 with a Doppler wind lidar developed by the Army Research Laboratory (ARL) and integrated for flight by ONR-sponsored scientists and engineers and the ELDORA Doppler weather radar, which is used to map the 3-dimensional (3-D) rain and wind structure; and two WC-130J Air Force "Hurricane Hunter" aircraft that penetrated three typhoons and several tropical storms multiple times in order to study the eyewall and inner structure. Both aircraft released dropsonde probes to map the 3-D wind, thermal and moisture properties down to the surface while operating at 30,000 to 8,000 feet.

Satellite Monitoring

NRL-MRY's tropical cyclone web page (http://www.nrlmry.navy.mil/TC.html) was adapted to support the TCS-08 effort in real-time.

Figure 1: 85 GHz horizontal polarization image from the Tropical Rainfall Mapping Mission (TRMM) microwave imager (TMI) for Super Typhoon Jangmi on September 26, 2008 at 2000 UTC highlighting rainbands (red/yellow/green shades) and eyewall structure. The WC-130J planned flight track is overlain and drifting buoy locations are annotated with symbols.
The mission operation center at the Naval Postgraduate School provided NRL with specific areas of disturbed weather, enabling researchers to focus their best suite from 38 available satellite sensor data sets on each potential genesis region. Access to 13 passive microwave imagers/sounders that can sense through non-raining clouds enabled NRL researchers to monitor rain cells, organization into convective clusters, and eventually rainbands during TC genesis. While most previous studies in the Atlantic and Eastern Pacific focused on mature storms, this study is one of only a few to focus on genesis as this aspect of storm life is critical to improving extended range forecasts. A better understanding of genesis is critical to extended range forecasts needed for improved maneuver of ships and military assets to mitigate frequent disruption caused by typhoons. Satellite products included scatterometer surface wind vectors; visible, infrared and water vapor imagery; and microwave derived rainrates, wind speeds, and total precipitable water values. These near-real-time products (1 to 3 hour latency) were distributed via web pages, viewed collaboratively during web discussions and sent via x-chat to the NRL P-3. Satellite products were also displayed via Google Earth, enabling the overlay of real-time aircraft flight tracks, onboard measurements, and/or deployed drifting buoys as illustrated in figure 1. The drifting buoys were deployed by a WC-130J aircraft ahead of two typhoons to measure the air-sea response to typhoon forcing, which was a pathfinder for more extensive ocean moorings, drifters, and profilers to be deployed in 2010 as part of the ONR sponsored program called Interaction of Typhoons and the Ocean Pacific (ITOP).

Mesoscale Modeling: COAMPS-TC

A new version of the Navy's Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®) has been developed specifically for forecasting tropical cyclones (COAMPS-TC).

Figure 2: COAMPS-TC real time forecast initialized on 0000 UTC 26 September 2008 for Super Typhoon Jangmi. The model forecasted radar reflectivity at 2 km resolution, valid at 00Z 28 September (48 h forecast), is shown in color on the 5 km grid mesh (higher values/warmer colors are more intense).
The COAMPS-TC system is comprised of data quality control, analysis, initialization, and atmosphere and ocean forecast model sub-components. The atmospheric module of the COAMPS-TC system was demonstrated in real-time during T-PARC/TCS-08 using horizontally nested grids (45, 15, and 5 km resolution). The inner two grid meshes are adaptive in that they follow the TCs, enabling the tropical cyclone convection to be explicitly resolved on the finest mesh efficiently. An example of a real-time forecast from the highest-resolution mesh of COAMPS-TC for Super typhoon Jangmi is shown in figure 2.

Targeted Observations

NRL produced real-time targeting products based on global-scale and storm-scale forecast models and their adjoint systems to support the targeted observing objectives. Adjoints allow for the calculation of the sensitivity of tropical cyclone forecasts to changes in the initial state in a mathematically rigorous, computationally feasible manner. These sensitivity products provided information on features and processes that influence the tropical cyclone track and intensity forecasts, and the products were instrumental in determining resource allocations and deployments (when and where aircraft should take additional observations to improve forecasts). NRL uniquely contributed both global and storm-scale sensitivity products, providing critical information on both large-scale remote influences and the sensitivity of the forecast to fine-scale structures within the storm itself.

Figure 3. The COAMPS® adjoint sensitivity fields for the (a) vorticity (m2 s-1) and (b) vertically integrated total energy (J kg-1) valid at 1200 UTC 10 September 2008 for typhoon Sinlaku. The WC-130J flight track is shown by the magenta line and the red dashed box indicates the response function location. The dropsonde deployment locations are shown by the turquoise balloon symbols.

Global-scale guidance was produced using the Navy Operational Global Atmospheric Prediction System (NOGAPS) forecast and adjoint models twice daily for five fixed regions, plus additional products during high-interest periods, at 150 km resolution. Computational resources for these products were provided through the DoD High Performance Computing Program. Storm-scale targeted observing products were produced using the COAMPS® forecast and adjoint system twice daily, centered on storms of interest, at 40-km resolution. A unique aspect of this nonhydrostatic adjoint system is that an exact adjoint to the explicit microphysics has been developed and used. Results indicate that forecasts of tropical cyclone formation in the western Pacific are very sensitive to the initial state. For example, sensitivity fields for typhoon Sinlaku valid at 1200 UTC 10 September 2008 are shown in figure 3. The sensitivity of the final time kinetic energy in the box shown in the figure to the initial vorticity at 2 km indicates a highly structured pattern with anticylonically curved sensitivity maxima (figure 3a). The perturbation total energy (figure 3b) maximum area is well sampled by the WC-130J aircraft soundings deployed (small turquoise dots).

TC Genesis

Accurately forecasting typhoon genesis will greatly aid TC warning centers, ship routing, targeting applications and the placement of high resolution mesoscale model runs (like COAMPS-TC). NRL's ability to accurately meet any five day forecasting requirement implies that TC genesis is included. Fortunately, our studies indicate disturbed areas of weather that evolve into tropical cyclones contain specific characteristics for atmospheric variables within a 20 by 20 degree box centered on the incipient system. However, these attributes (low to mid-level moisture, vertical shear, large scale convergence, sea surface temperature, and storm speed) differ between Atlantic and western Pacific tropical cyclones. A genesis index was created using a regression formula based on these meteorological parameters and outputs the probability the system will become a TC.

Figure 4. The tropical cyclone genesis index was applied to each of the disturbances highlighted in the smaller enclosed boxes (NOGAPS 850 hPa vorticity field). The values of the genesis index for the disturbance in each box are indicated at the top.

Using seven predictors containing pattern and magnitude of the 850 hPa vorticity and divergence, relative humidity and speed of disturbance, a preliminary 24-hour prediction formula was constructed for the western north Pacific. Using the test cases, a threshold of 0.3 was identified as the optimal value above which a tropical disturbance has a high probability of forming a TC in the next 24 to 48 hours. The hit rate was 66% and the false alarm rate was 25%, while the corresponding miss rate and correct rejection rate were 34% and 75% respectively. This regression formula was applied to NOGAPS fields in real-time during TCS-08 to identify genesis cases, with a resulting success rate of ~ 75%. An example is shown in figure 4, in which the disturbance contained by the small green box (left-most box) has a genesis index of 0.4 and became typhoon Kalmaegi after 24 hours.

The TCS-08 field program data set provides a wealth of coincident model, satellite, aircraft, and in-situ data that will permit NRL and collaborators to make progress in understanding western Pacific tropical cyclones that routinely impact Navy and DoD resources and operations. TCS-08 research will directly benefit the Joint Typhoon Warning Center and it's ability to forecast tropical cyclone genesis, monitor structure changes and predict extratropical transitions while also enabling progress in tropical cyclone modeling, targeting and air-sea interaction efforts.



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