The Dvorak Technique

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One of the primary goals of this course is for you to develop the ability to comprehend the discussions, advisories, and forecasts issued by the National Hurricane Center. For example, the references to SFMR measurements that you just learned about probably wouldn't make sense to a member of the general public, but you now have an appreciation of how this passive remote sensor works. This knowledge allows you to integrate such references into your overall understanding of the current or future status of a tropical storm or hurricane. Let's look at another commonly referenced term found in many NHC discussions -- the Dvorak Technique.

At 1445Z on September 8, 2003, satellite imagery revealed that Hurricane Isabel already had an impressive eye, even though the storm was still over the eastern Atlantic (as evidenced by this full-disk water-vapor image). At this time, Isabel was clearly out of range of aircraft reconnaissance; yet forecasters at the National Hurricane Center were still able to estimate that Isabel had a maximum sustained wind speed of 100 knots. Below is an excerpt from NHC's discussion at 15Z on September 8 (note the bold portion in particular):

ZCZC MIATCDAT3 ALL
TTAA00 KNHC DDHHMM
HURRICANE ISABEL DISCUSSION NUMBER 10
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
11 AM EDT MON SEP 08 2003
 
ISABEL HAS CONTINUED TO RAPIDLY INTENSIFY. THE INITIAL INTENSITY IS
INCREASED TO 100 KT BASED ON SATELLITE INTENSITY ESTIMATES OF 115
KT FROM TAFB AND AFWA...102 KT FROM SAB...AND 102 KT/T5.5 3-HOUR
OBJECTIVE DVORAK INTENSITY ESTIMATES. THE 100 KT INITIAL INTENSITY
IS ALSO CONSISTENT WITH THE LATEST AMSU INTENSITY ESTIMATES OF 100
KT AND 960 MB.

As you can see, there are several techniques for remotely sensing the estimated intensity of a tropical cyclone. In this section, I'll kick-off the discussion about remote sensing by focusing on the Dvorak Technique so that you can interpret the T5.5 Number referenced in the Isabel discussion. In a nutshell, the Dvorak Technique is an analysis procedure for estimating the intensity of tropical cyclones based on cloud patterns on satellite imagery. The technique is named after Vernon Dvorak, who pioneered the technique with his research in the 1970's and early 1980's.

How does the Dvorak Technique work? In a nutshell, it's really just a statistical system that combines observed cloud patterns on satellite imagery with a set of established guidelines (based on years of observations) to estimate the intensity of a tropical cyclone. These estimates are called T Numbers, which range from 1.0 to 8.0 (check out the Dvorak scale). Please note that the scale refers to a "CI Number" (Current Intensity Number) and not, specifically, a "T Number". However, the two are usually highly similar. Forecasters arrive at a T Number (which estimates a tropical cyclone's intensity) by comparing cloud patterns on a single satellite image (sometimes referred to as the "satellite presentation") to a set of statistical guidelines. Once forecasters determine a T Number, they can then modify it in an attempt to preserve the continuity of past (recent) estimates and account for recent trends in the satellite presentation (indicative of intensification or weakening). The final value, after any modifications, represents the Current Intensity (CI) Number.

Manually conducting a complete Dvorak analysis to arrive at a specific T Number (and adjust to a CI Number) is a fairly complex process, which requires a great deal of experience to perform well. Don't worry, you won't be asked to perform such detailed analyses in this course, but if you're interested in seeing some more details, you may be interested in some of the links in the Explore Further section below. Still, it probably won't come as a surprise to you that some subjectivity exists when forecasters attempt to classify cloud patterns, which is one drawback to the technique. More recently, forecasters at NHC have relied on objective computer analyses that have been developed to take the subjective element out of Dvorak estimations. If you're interested in learning more about this evolution and the details of these objective schemes, check out the Explore Further section below. One standard objective technique is the Advanced Dvorak Technique (ADT), which attempts to achieve the accuracy of the original Dvorak Technique without the subjective limitations. Importantly, the ADT can be applied to any tropical cyclone across the globe, in any phase of its life-cycle (previous objective techniques weren't applicable during certain parts of the life-cycle).

When monitoring tropical cyclones, you can access ADT estimates and imagery at the Cooperative Institute for Meteorological Satellite Studies (CIMSS) ADT page, from National Environmental Satellite, Data and Information Service (NESDIS), and from the Regional and Mesoscale Meteorology Branch of the Cooperative Institute for Research in the Atmosphere (RAMMB-CIRA). As an example of the type of data available from CIMSS, check out the time series below, which plots "adjusted T Numbers" and CI numbers for Super Typhoon Haiyan in early November 2013. You can think of the "Adjusted T Numbers" as the objective counterpart to human-derived T Numbers.

A time series of T and CI Numbers for Super Typhoon Haiyan in early November 2013. You may treat the Adjusted T Number (in green) as the "objective" counterpart to a human-derived T Number. Note that on November 7, Haiyan "maxed out" the Dvorak Scale according to ADT analysis.

Credit: CIMSS

By examining the time series above, you can get a sense of just how close T and CI Numbers typically are. You can also see that Super Typhoon Haiyan "maxed out" the Dvorak Scale according to ADT analysis, reaching the highest CI Number possible (8.0). To achieve such tropical cyclone "perfection" is very rare, indeed, and not surprisingly, Haiyan's satellite presentation was absolutely stunning, with a wide, symmetrical ring of deep convection (marked by very cold cloud tops) surrounding the eye (check out this enhanced infrared image of Haiyan at 0930Z on November 7). In fact, a statement from the Satellite-Services Division of NOAA stated at the time that the Dvorak Technique "makes no allowance for an eye embedded so deeply in cloud tops as cold..." as those seen around Haiyan's eye. In fact, you can tell from the time series above that the adjusted T numbers actually went slightly off the scale for a brief time!

Today, most forecasters use automated and objective Dvorak analyses, and there are many advantages to using the ADT, but performing subjective analyses manually still has value. Indeed, analysts and researchers still regularly conduct manual Dvorak analyses. While you won't have to do complete Dvorak analyses in this course, conducting some basic Dvorak classifications can still help you become "one with the atmosphere" so that you can really be in tune with how a particular storm is evolving. As your experience grows in tropical weather forecasting, you will discover that tropical cyclones appear in a variety of sizes and shapes on satellite imagery. A major component of the Dvorak Technique hinges on forecasters classifying the shape and pattern of clouds they observe on visible and infrared satellite imagery into four basic categories (which you should be sure to know):

1. Curved-band pattern: Often observed in the early stages of tropical cyclone development, this pattern is characterized by a band of dense cloudiness that begins to curve around the center of the storm. In weak hurricanes, the band coils entirely around the center of the storm. For example, Check out this infrared image that shows the curved-band pattern associated with Tropical Storm Jeanne at 1030Z on September 20, 2004. At the time, the maximum sustained winds were 60 miles per hour, and the curved band wrapped around most of the center of the storm.
2. Shear pattern: Typically observed in the formative stages of a tropical cyclone or during weakening, the shear pattern is characterized by deep convective clouds moving to one side of the storm's center. For example, check out this satellite image of a sheared Tropical Storm Nicholas at 1145Z, October 21, 2003. On this particular satellite image, low clouds are marked in yellow, while higher clouds are in bright whites and faint blue shadings. Note that the center of low-level circulation lies to the west of the deep convection, indicative of the relatively strong westerly shear between 850 mb and 200 mb. Recall that a tropical cyclone is in a weakened state when upper-level winds push deep convection away from the storm's low-level circulation.
3. Central Dense Overcast (CDO) pattern: The CDO pattern describes the region of dense cirrus clouds that shrouds the core of a tropical cyclone, which is sometimes observed in stronger tropical depressions, tropical storms, and weak hurricanes. For example, this satellite image showing the tropical-depression stage of Hurricane Alex at 1155Z on August 1, 2004 (at the time, the maximum sustained wind speed was 30 miles per hour) displays a CDO pattern. Prior to a tropical cyclone attaining a maximum sustained wind speed of 64 knots, the CDO appears fairly homogeneous (uniformly cold cloud-top temperatures on infrared imagery). In other words, no eye is readily apparent.
   
   I say "readily" here because an embryonic eye may have already "secretly" formed. As a tropical cyclone intensifies, an eye typically starts to develop near the center of the tightening spiral associated with the cyclone's primary curved band. But, the CDO typically masks most of this emerging pattern from the view of conventional satellite imagery (high cloud tops shield lower-level features from detection by visible and infrared imagery). Forecasters do have tools for detecting these "secret" eyes, which we'll explore later in the lesson, but forecasters continue to use the Dvorak CDO pattern until an eye appears on conventional satellite imagery.
4. Eye Pattern: Once an eye is evident on conventional satellite imagery, an "eye pattern" exists, although I should note that a large portion of the "CDO cloud" remains, as with this enhanced infrared image of Hurricane Emily from July 17, 2005. Clearly, the eye appears as an oasis of relative warmth within the cold CDO. Eye patterns can be somewhat subtle like the example from Emily to very obvious as in the case of Super Typhoon Haiyan.

Eye patterns can characterize tropical cyclones of widely varying intensities. For example, a storm that has an eye could be a Category 1 or a Category 5 hurricane. That's a huge difference, but both would fall under the eye pattern! To further help forecasters refine their assessments based on eye patterns, they look at specific characteristics of the eye. For example, recognizing the eye of a hurricane is banded helps meteorologists to better estimate the intensity of the storm as in this satellite image of Hurricane Jeanne at 1815Z on September 22, 2004. Essentially, a curved band had coiled entirely around the center of the storm (forming a "banded eye"), suggesting that it was a weak hurricane. Tropical forecasters also look at a specially enhanced infrared satellite image called a Dvorak image to help them distinguish between various eye patterns (see below). You can access the latest Dvorak imagery for storms around the globe, if you're interested. Forecasters use Dvorak imagery to determine the radiating temperature of the eye and compare it to the radiating temperatures of the surrounding cloud tops. As a general rule, the larger the difference in temperatures between the eye and the surrounding cloud tops, the stronger the hurricane.

A Dvorak image of Hurricane Emily at 1115Z on July 16, 2005. Forecasters look at Dvorak imagery to help them determine the difference in the radiating temperatures between the eye and the surrounding cloud tops.

Credit: NOAA

After classifying the cloud pattern and looking at satellite-derived temperatures, forecasters completing the Dvorak Technique manually would take into account other factors such as trends in the cloud pattern that indicate a weakening or intensification and assign a T Number and CI Number, which range from 0 to 8 in increments of 0.5. Officially, T Numbers and CI Numbers appear in a coded format, which you may be interested in if you're into tracking tropical cyclones in real-time. But, how do these numbers translate to storm intensity? Below is a chart that links the estimated CI number with the basic patterns of clouds that I described above. Current Intensity Numbers have also been calibrated against aircraft reconnaissance of tropical cyclones in the Northwest Pacific and Atlantic Oceans. On average, the CI Numbers correspond to the specific wind speeds and central barometric pressures also shown in the graphic below.

The range of Dvorak Intensity Numbers as a function of the basic cloud patterns associated with tropical cyclones. Along the bottom of the image, note the corresponding minimum central pressures (in mb) and maximum sustained wind speeds (in knots). A word of caution about accepting the wind speeds associated with a given Dvorak Intensity pressure as gospel--remember that the pressure gradient (not central pressure alone) largely governs wind speed.

Credit: David Babb

In case you're wondering, the reason for the basin differences in central pressures at a fixed CI Number is that the overall mean sea-level pressures are lower in the Northwest Pacific (more details later in the course). So, given a central pressure and maximum sustained wind speed associated with an Atlantic tropical cyclone, the central pressure of storm in the Northwest Pacific must essentially be lower for it to generate the same wind speed. Remember, it's the pressure gradient that largely determines wind speed, which is why small tropical cyclones (such as Andrew in 1992) can generate stronger winds than a larger cyclone (such as Floyd in 1999) with the same minimum central pressure (see a comparison of these two hurricanes).

As you track tropical cyclones in real-time, you'll regularly see references to T Numbers and CI Numbers in discussions from NHC and JTWC. With what you now know about the Dvorak Technique, you should be able to interpret those references and understand what they suggest about a tropical cyclone's current status. The Dvorak Technique, however, is far from the only way that satellites are used in tropical cyclone forecasting. We'll explore another intriguing use of satellite data on the next page with a discussion of "Cloud-Drift Winds."