**Radiosonde Replacement System WMO Levels Generation for Coded
Messages**

**Radiosonde Replacement System WMO Coded Message Length Changes**

The implementation of the Radiosonde Replacement System at field sites
commencing with Sterling, Virginia, on or about 1 August, 2005 will
introduce longer coded messages. The format of the messages will be the
same WMO format for coded upper air messages used with the MicroART
legacy system. The number of levels in the coded messages will be two to
three times greater for the TTBB and TTDD parts. The number of levels in
the TTAA, TTCC, PPBB, and PPDD parts will be relatively unchanged. These
changes reflect updated coding practices and higher resolution level
selection criteria. The maximum size limits of the parts of the coded
messages follow:

TTAA TTCC TTBB TTDD PPBB PPDD

15 levels 10 levels 135 levels 40 levels 40 levels 40 levels

In addition, the 31313 message indicator associated with the message
parts TTAA, TTBB, TTDD, and TTCC parts of the message will be included.
This 31313 group includes information on the solar radiation correction
used for the radiosonde, the radiosonde type, and the ground system as
well as the group for radiosonde time of release. With the legacy system
this correction was applied by the National Centers for Environmental
Prediction. With the Radiosonde Replacement System and Software, it will
be applied at the field site. For additional information on the message
requirements please reference the WMO 306, Manual on Codes,
International Codes, Volume 1.1 Part A---Alphanumeric Codes and WMO 306,
Manual on Codes, Regional Codes and National Coding Practices, Volume
II.

**WMO Guidance for Coded Message Parts for Section 5, Coded message
parts TTBB and TTDD Significant Thermodynamic Levels and Coded Message
Part for Section 4 Significant Winds Levels Parts PPBB and PPDD**

**[Significant Thermodynamic Levels Parts TTBB and TTDD]{.underline}**

The reported significant levels alone shall make it possible to
reconstruct the air temperature and relative humidity profiles within
the limits of the criteria specified. (WMO Manual on codes number 306,
Paragraph 35.3.1.1.)

Levels which are necessary to ensure that the temperature obtained by
linear interpolation (on a T-log P or essentially similar diagram)
between adjacent significant levels shall not depart from the observed
temperature by more than 1 degree C below the first significant level
reported above the 300-hPa level or the first tropopause, whichever,
level is the lower, or by more than 2 degrees C thereafter. (WMO Manual
on codes 306, Paragraph 35.3.1.3 (a))

Levels which are necessary to ensure that the relative humidity obtained
by linear interpolation between adjacent significant levels shall not
depart by more than 15 percent from the observed values. (The criterion
of 15 percent refers to an amount of relative humidity and not to the
percentage of the observed value, e.g. if an observed value is 50
percent, the interpolated value shall lie between 35 percent and 65
percent.) (WMO Manual on codes 306, Paragraph 35.3.1.3 (b)).

**[Significant Winds Levels parts PPBB and PPDD]{.underline}**

The reported significant data alone shall make it possible to
reconstruct the wind profile with sufficient accuracy for practical use.
Care shall be taken that:

> \(a\) The direction and speed curves (as a function of the log of
> pressure or altitude) can be reproduced with their prominent
> characteristics.
>
> \(b\) These curves can be produced with an accuracy of at least 10
> degrees for direction and 5 meters per second for speed.

\(c\) The number of significant levels is kept strictly to a necessary
minimum.

To satisfy these criteria, the following method of successive
approximation is recommended, but other methods of obtaining equivalent
results may suit some national practices better and may be used:

\(1\) The surface level and the highest level attained by the sounding
constitute the first and the last significant levels. The deviation from
the linearly interpolated values between these two levels is then
considered. If no direction deviates by more than 10 degrees and no
speed by more than 5 meters per second, no other significant levels need
be reported. When one parameter deviates by more than the limit
specified in paragraph (b) above, the level of greatest deviation
becomes a supplementary significant level for both parameters.

\(2\) The additional significant levels so introduced divide the
sounding into two layers. In each separate layer, the deviations from
the linearly interpolated values between the base and the top are then
considered. The process used in paragraph (1) above is repeated and
yields other significant levels. These additional levels in turn modify
the layer distribution, and the method is applied again until any level
is approximated to the above mentioned specified values.

**[Missing Wind Data]{.underline}**

If altitude is given in geopotential units in Parts B of D, a layer for
which data are missing shall be indicated by reporting the boundary
levels of the layer and a level in between with a height value somewhere
in between the boundary heights and a group ddfff or solidi (/////) to
indicate the layer of missing data, provided that the layer is at least
1500 geopotential meters thick. The boundary levels are the levels
closest to the bottom and the top of the layer for which the observed
data are available. The boundary levels are not required to meet
"significant level" criteria. (WMO Manual on codes number 306, Paragraph
32.3.1.5.1.).

**MicroART Legacy System Software**

**[Thermodynamic Significant Levels]{.underline}**

Selection of thermodynamic levels is a variant of the WMO procedure.
Departures from linearity are determined between the consecutive
standard levels or exceptional levels from flight initiation to
termination. The WMO procedure selects the significant levels
irrespective of the standard levels and the exceptional levels. This is
accomplished from MicroART the six-second data. The level selection
criteria for MicroART are also less stringent than for the Radiosonde
Replacement System. Consequently, fewer levels are selected by MicroART
than in the Radiosonde Replacement System.

**[Wind Significant Levels]{.underline}**

Levels selection techniques for winds are a variant of the WMO
procedures for selecting significant wind levels. Wind speed and
direction are selected at one-minute intervals from the flight
initiation to flight termination. The data from one-minute winds files
are then used to provide winds for standard pressure levels in the TTAA
and TTCC message parts including maximum winds and coded in the PPBB and
PPDD message parts of the coded message for the fixed regional levels
(WMO Region 4). Winds are provided for the standard pressure levels in
message Parts TTAA and TTCC to include the tropopause level(s) and the
maximum wind levels.

**Radiosonde Replacement System Radiosonde Workstation Software**

**[Significant Levels Selection]{.underline}**

Coded levels selection techniques by the RWS software follow the WMO
departures from linearity procedure to select significant wind levels
and thermodynamic levels. The significant levels are selected
irrespective of the standard levels and the mandatory significant
levels. This procedure is applied to one-second data-rate data and uses
more stringent level selection criteria than suggested by the WMO and as
used by MicroART. The significant levels portions of the Radiosonde
Replacement System coded messages can be longer than those with the
MicroART legacy system.

**[Thermistor Radiation Error Correction]{.underline}**

The radiation correction algorithm in the Radiosonde workstation is a
departure from the model used by the National Centers for Prediction and
from active correction models in software at sites that make corrections
on site. Current thermistor radiation correction models use solar
elevation angle, balloon ascent rate, and thermistor coating
absorbtivity and reflectivity characteristics to apply corrections.to
the radiosonde temperature readings. The model used for the Radiosonde
Replacement System Mark IIA GPS radiosondes includes current approaches
from older methods but also includes flight-time cloud and sky cover
information. The inclusion of cloud information for overcast conditions
can increase temperature correction in the stratosphere by 0.3 degrees
Celsius greater than that under clear conditions. Corrections over an
entire flight from launch for a midday flight can range from near zero
at the surface to approximately 1.3 degrees Celsius at 30 km altitude.

**Criteria for Significant Levels Selection for Coded Messages**

Significant levels selection criteria for the Radiosonde Replacement
System, MicroART, and WMO for the coded message transmitted over the
Global Telecommunications System and for the data archived at the
National Climatic Data Center are contained in the following table:

Table 1. Significant Level Selection Criteria

+---------+----------+--------+---------+----------+--------+---------+
| Li      | Coded    |        |         | NCDC     |        |         |
| nearity | Message  |        |         | Archive  |        |         |
| C       |          |        |         |          |        |         |
| riteria |          |        |         |          |        |         |
+---------+----------+--------+---------+----------+--------+---------+
|         | Tem      | Re     | Wind    | Tem      | Re     | Wind    |
|         | perature | lative |         | perature | lative |         |
|         |          | Hu     |         |          | Hu     |         |
|         |          | midity |         |          | midity |         |
+---------+----------+--------+---------+----------+--------+---------+
| WMO     | 1.0 ^ο^C | 15     | 5 mps   | N/A      | N/A    | N/A     |
|         | surface  | p      | de      |          |        |         |
|         | to 300   | ercent | parture |          |        |         |
|         | hPa (or  | for    | for     |          |        |         |
|         | Trop)    | entire | wind    |          |        |         |
|         |          | flight | speed   |          |        |         |
|         | 2.0^ο^ C |        | and 10  |          |        |         |
|         | 300 hPa  |        | degrees |          |        |         |
|         | (or      |        | de      |          |        |         |
|         | Trop) to |        | parture |          |        |         |
|         | flight   |        | for     |          |        |         |
|         | ter      |        | di      |          |        |         |
|         | mination |        | rection |          |        |         |
+---------+----------+--------+---------+----------+--------+---------+
| M       | 1.0^ο ^C | 10     | S       | 0.5^ο^ C | 5      | One     |
| icroART | surface  | p      | tandard | surface  | P      | minute  |
|         | to 300   | ercent | and     | to 100   | ercent | winds   |
|         | hPa (or  | for    | fixed   | hPa (or  | for    | from    |
|         | Trop)    | entire | r       | Trop)    | entire | surface |
|         |          | flight | egional |          | flight | to      |
|         | 2.0^ο^ C |        | levels  | 1.0^ο^ C |        | term    |
|         | 300 hPa  |        | det     | less     |        | ination |
|         | (or      |        | ermined | than 100 |        |         |
|         | Trop) to |        | from    | hPa to   |        |         |
|         | flight   |        | one     | flight   |        |         |
|         | ter      |        | -minute | ter      |        |         |
|         | mination |        | winds   | mination |        |         |
+---------+----------+--------+---------+----------+--------+---------+
| RWS     | 0.5^ο^ C | 5      | 5 mps   | 0.5^ο^ C | 5      |         |
|         | surface  | P      | de      | surface  | P      |         |
|         | to 100   | ercent | parture | to 100   | ercent |         |
|         | hPa      | for    | for     | hPa      | for    |         |
|         |          | entire | wind    |          | entire |         |
|         | 1.0^ο^ C | flight | speed   | 1.0^ο ^C | flight |         |
|         | less     |        | and 10  | less     |        |         |
|         | than 100 |        | degrees | than 100 |        |         |
|         | hPa to   |        | de      | hPa to   |        |         |
|         | flight   |        | parture | flight   |        |         |
|         | ter      |        | for     | ter      |        |         |
|         | mination |        | di      | mination |        |         |
|         |          |        | rection |          |        |         |
+---------+----------+--------+---------+----------+--------+---------+