A Little on BD and MW

As I promised, here is some background on the two more complex portions of the tropical cyclone (TC) summaries:

 

The Dvorak Method:

Here is how NOAA explains it, which is probably far clearer than I would:

“                  The Dvorak Technique Explained

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The Dvorak technique is a method using enhanced Infrared and/or visible satellite imagery to quantitatively estimate the intensity of a tropical system. Cloud patterns in satellite imagery normally show an indication of cyclogenesis before the storm reaches tropical storm intensity. Indications of continued development and/or weakening can also be found in the cloud features. Using these features, the pattern formed by the clouds of a tropical cyclone, expected systematic development, and a series of rules, an intensity analysis and forecast can be made. This information is then standardized into an intensity code.

Full Satellite Intensity Estimate Code:

 

EXAMPLE:     Ttt/cc/Lvv/xxhrs

     Where:

     T  - Tropical  (ST is used for Sub-Tropical systems)

     tt – Satellite Derived T-Number

     cc – System Current Intensity T-Number

     L  - Past Change 

          D – Developing

          W – Weakening

          S – Little or No change (Same)

     vv – Amount of Past change in T-Number

     xx – Hours over which the change was observed

 

 

EXAMPLE:     T3.5/3.5/D1.0/24hrs

Translation: The system appears on satellite imagery to be T-Number 3.5 storm. This value matches the Current intensity generating about 55 knots of sustained winds. The storm has developed a full T-Number in the past 24 hours increasing the wind speeds from 35 to 55 knots. The atmospheric pressure in the center of the storm has also lowered.

EXAMPLE:     T4.5/5.5/W1.0/24hrs

Translation: The system appears on satellite imagery to be T-Number 4.5 storm. The Current intensity is still 5.5 and the system is still generating sustained winds of about 100 knots. As a forecast tool, the Dvorak technique is showing a weakening trend and the likelihood of a lowering CI within the next 24hrs.                                                                                                                                                                                                                                          ”

                              (Source: http://www.ssd.noaa.gov/PS/TROP/dvorak.html , NOAA Satellite and Information Service, 08/26/2011)

 

And here is a handy table to translate these numbers into wind speed and pressure values:

 

  CI          MWS         MWS            MSLP           MSLP           Saffir-Simpson   

Number  (Knots)      (MPH)       (Atlantic)    (NW Pacific)         Category     

 1          25 KTS       29 MPH                                               (Approximate)

 1.5       25 KTS       29 MPH      

 2          30 KTS       35 MPH      1009 mb       1000 mb

 2.5       35 KTS       40 MPH      1005 mb        997 mb

 3          45 KTS       52 MPH      1000 mb        991 mb

 3.5       55 KTS       63 MPH       994 mb         984 mb

 4          65 KTS       75 MPH       987 mb         976 mb        1  (64-83 KTS)

 4.5       77 KTS       89 MPH       979 mb         966 mb        1  (64-83 KTS); 2  (84-96 KTS)

 5          90 KTS       104 MPH     970 mb         954 mb        2  (84-96 KTS); 3  (97-113 KTS)

 5.5       102 KTS     117 MPH     960 mb         941 mb        3  (97-113 KTS)

 6          115 KTS     132 MPH     948 mb         927 mb        4  (114-135 KTS)

 6.5       127 KTS     146 MPH     935 mb         914 mb        4  (114-135 KTS)

 7          140 KTS     161 MPH     921 mb         898 mb        5  (136+  KTS)

 7.5       155 KTS     178 MPH     906 mb         879 mb        5  (136+  KTS)

 8          170 KTS     196 MPH     890 mb         858 mb        5  (136+  KTS)

CI   -- Current Intensity

MWS  -- Mean Wind Speed

MSLP -- Mean Sea Level Atmospheric Pressure in Millibars

 

                              (Source http://www.ssd.noaa.gov/PS/TROP/CI-chart.html, NOAA Satellite and Information Service, 10/31/2009)

The type of satellite image used in this technique is commonly referred to as the Basic Dvorak (BD) image. These images are really the same as the common infrared (IR) image one might see on TV, however, the grey scale is different and designed to highlight various features using high contrast shades. The relative amount and organization of these shades allows analysts to derive the Dvorak Classification.

 

 

Microwave Imagery:

     Along with the geostationary satellites that provide a wide angle view of the same area of the earth, there are many polar orbiting satellites that circle the earth in an inclined orbit at a much closer distance to the earth. These satellites’ images consists of swaths made along their track, the width of these swaths depend on the satellite and instrument. On board, these satellites carry many different instruments to do everything from make standard visible imagery to detect lightning. Another common purpose is to produce microwave (MW) wavelength imagery. The great thing about MW images is that they can “look” through clouds to see what’s going on inside them, or beneath them. In the TC summaries I’ll be posting MW images made from 85GHz and 37GHz frequencies. 85GHz images highlight frozen particles (hail and graupel), so they show the structure of the upper portions of storms (but can see through the cirrus that often obscures the tops of TCs). 37GHz images allow the lower structure of storms to be seen, since they respond most to liquid water (rain). I include these images because often they do a really good job of showing the structure of a rainband or the formation of a TC’s eyewall.

 

 

 

I’ll be trying a new format for the TC summaries. They contain mostly the same data, but rearranged. The satellite images have been replaced by a single image with four panels. These images are produced by the NRL Monterey Marine Meteorology Division (http://www.nrlmry.navy.mil/TC.html) and allow multiple types of images to be compared side-by-side.