The stormWatch index

 Index overview

The StormWatch index consists in an identification of all storm events (including hurricanes, typhoons but also high latitude storms) in the observations collected by the satellite embedded scatterometers since 1991. Here is the list of products and related time coverage parsed to build this index.

Methodology for the identification of storm events

Review of storm events characteristics

A review of the hurricane force extratropical cyclones observed using QuikSCAT is presented as a base of the knowledge used to identify storms events in scatterometers datasets. The authors are :Joan Von Ahn. STG/NESDIS ORA/NOAA Ocean Prediction Center, Joe Sienkiewicz, NOAA Ocean Predication Center, Greggory McFadden, SAIC/NOAA Ocean Prediction Center

Forecasters routinely use QuikSCAT winds along with ship and buoy observations to determine wind warning categories of GALE (34 to 47 kts), STORM (48 to 63 kts), and HURRICANE FORCE (64 kts or greater)

For most cyclones, HF (HURRICANE FORCE) conditions were observed to occur at or near the time of minimum central pressure (the mature phase of the cyclone). We have found that HF conditions on average last less than 24 hours. This is relatively short-lived compared to the average life span of 5 days for ocean storms

Illustration 1 : An alternative model of frontal-cyclone evolution (Shapiro and Keyser, 1990): incipient broad-baro-clinic phase (I), frontal fracture (II), bent back front and frontal T-bone (III), and warm core frontal seclusion (IV). Upper: sea level pressure (solid), fronts (bold), Figure and cloud signature (shaded). Lower: temperature (solid), and cold and warm air currents (solid and dashed arrows)

In tropical cyclones, HF winds tend to be found close to the center on the periphery of the eye wall. Where are HF conditions observed in mid-latitude cyclones? To answer this question, lets first take a look at the life cycle of a typical ocean storm. Illustration 1 shows the evolution of an ocean storm as depicted by Shapiro and Keyser2 (1990). The cyclone begins as an open frontal wave with a warm front and cold front (I). As the cyclone intensifies, the frontal wave begins to amplify. The cold front pushes eastward (south of the low) and the temperature gradient tightens to the west of the low center (II). The front associated with this tightening temperature gradient west of the low is referred to as the bent back front or occluded front. The wave continues to amplify (III) as the bent back (occluded) front and associated temperature gradient swings eastward to the southwest of the low center. The strongest temperature gradient in phase III is associated with the continuous warm to bent back front and not in association with the cold front to the south. Phase III is referred to as the frontal T-bone. Phase IV shows the mature cyclone or warm core frontal seclusion. At this point the very strong temperature gradient (or front) has encircled the surface low center. A shallow pocket of relatively warm air has migrated to the low center and become cut off or secluded (thus the term warm seclusion). Within the warm seclusion the air is very unstable and convection may occur. An arc of very strong temperature gradient surrounds this pocket of warmer air with cold air found to the exterior of this temperature gradient. A very strong pressure gradient exists on the cold side of the temperature gradient (south of the low). It is in this area of strong pressure gradient that HF conditions are often observed.

Illustration 2 : Composite of maximum winds observed by QuikSCAT for open ocean HF cyclones for the months of October - April for the three year period from 2001 - 2003 in (a) the North Pacific (II cyclones) and (b) the North Atlantic (6 cyclones). Wind speed (kts) is shown in solid contours (color bar in center of figure.) In both (a) and (b) the area of HURRICANE FORCE winds is a red crescent shaped area to the south of the low center

To determine where HF conditions occur most frequently relative to the center of the mature cyclone, QuikSCAT winds were used to create composites of the maximum winds for 17 open ocean HF cyclones (11 in the North Pacific and 6 in the North Atlantic). These composites are shown in Illustration 2. All cyclones used in these composites were near maturity or close to minimum central pressure. The Green shading depicts GALE FORCE winds, Yellow-STORM FORCE, and Red-HURRICANE FORCE. The occluded fronts are drawn in purple. Both composites show large crescent-shaped areas of HF winds to the south of the composite cyclone center. The Atlantic composite shows the band of extreme winds from 120 to 180 nmi from the center of the cyclone. The Pacific composite is slightly different with HF winds from approximately 60 to 150 nmi from the center of the cyclone composite. Is this the only location to expect HF winds? No! The cyclones used for these composites were chosen carefully to eliminate any possible land influence. QuikSCAT winds have also revealed HF conditions to the northeast of low centers in advance of (or north of) the accompanying occluded front and near mountainous coasts such as Greenland, Alaska, and the Kamchatka Peninsula.

Illustration 3 : Mature phase of the Shapiro-Keyser cyclone model. The red shaded area to the south of the Low center is the area of HF winds

Illustration 4 : QuikSCAT pass from 1600 UTC 01 December 2004. Wind speed in kts (color bar in upper right). The HF winds are depicted as red wind barbs. Note the location is to the south of the Low center

Illustration 3 is a conceptual model that shows the sea-level pressure, fronts, and area of HF winds of the mature phase of an ocean cyclone. The red-hatched area extending from southeast to nearly west of the low center illustrates where QuikSCAT frequently observes HF winds. Illustration 3 illustrates where to anticipate HF winds in a mature cyclone. The QuikSCAT image from a mature North Atlantic cyclone from December 1, 2004 (Illustration 4) reinforces this point. Note the large area of HF winds (RED wind barbs) to the south of the low center in agreement with the conceptual model in Illustration 4.

Identification of storm events

"Feature tiles" extracted from QuikSCAT scatterometer orbit files (swath) over hurricane Floyd (1999)

As stated in the review above, the identification of an extreme event on scatterometer data is primarily based on the high wind velocity detection. However care must be taken since high wind velocities retrieved from scatterometer measurement can come from contamination by rain or the presence of sea ice.

Therefore, it is of primary importance to check the quality of the scatterometer measurement and apply the required corrections prior to any detection.

One the scatterometer winds can be trusted, the first step of the identification of a storm event can be based on a threshold wind speed. However, since we know that scatterometer winds are significantly underestimated in the high wind range, the threshold wind speed cannot be based on the actual Hurricane force wind threshold, for instance, that would lead to missing most of the storm events on scatterometers datasets. Therefore, the wind threshold for the identification of storm events on scatterometer datasets can be adjusted to a smaller value determined for instance by the minimum wind speed of the 1% highest quality checked wind speed recorded by a given scatterometer over a period of 1 year. By doing such, the storm event criterium can be considered largely independent on the scatterometer model used in the wind vector retrieval.

Extraction of storm events characteristics

Storm position and extension

The storm position is set to be the position of the highest wind speed associated with the identified storm event.

The extension of the storm event is set as the location where the wind speed decreases continuously from the maximum recorded wind but still remains higher than a minimum threshold wind speed. This threshold is configurable and set by default to 15m/s based on experience.

When searching, in atm. model output, the location of a storm that has been detected on scatterometer data, a window of 5*5 degrees around the detected storm position is used to search in the model wind speed with no minimum threshold on the wind speed. Then, no window limitation is applied for the estimation of the storm extension. But the same minimum threshold is applied to the estimation of the storm extension.

Storm center

Scatterometer data

The storm center is estimated as the location where the wind speed is maximum. This convention is in line with the possible use of StormWatch results to initiate tacking of Strom generated waves whose main source is the higher wind area of the Storm.

External reports

When available, the closest reported position of the storm center is indicated.

Storm intensity

The storm intensity is estimated by the total wind power over the detected storm area. The wind power is the square root of the wind speed times the individual wind cell size.

 Extreme parameters

The maximum wind speed together with the area where the wind speed is detected above the scatterometer extreme wind threshold are considered as the dominant extreme parameters to be extracted together with the maximum wind vorticity.

Detailed description of StormWatch tags