Input Reflectivity Profiles
You can define the reflectivity profile with the following parameters:
- Reflectivity gradient above the melting layer (7 dBZ/km (11.3 dBZ/mi))
- Reflectivity gradient below the melting layer (1 dBZ/km (1.6 dBZ/mi))
- Melting layer height (in tenths of kilometers)
- Melting layer thickness (1 km (0.6 mi))
- Melting layer peak intensity (7 dBZ)
The melting level is attached to the ingest file. In a network of several radars and a separate IRIS Analysis machine, the profile information is given to the IRIS instance at each radar site.
 |
The melting layer height is given in IRIS . You can change the value without restarting IRIS in the Setup utility. This means you can adjust the reflectivity profile frequently based on external data sources such as numerical model or temperature soundings. |
Getting Reflectivity Profile Input Data
The most accurate way to determine the melting layer height is to study measurement from the same radar at the same time as when the profile correction is needed.
You can see the melting layer nicely in IRIS VVP and XSECT products of either reflectivity or, even better, vertical velocity. However, this is not always available: when the precipitation area is approaching the radar, there is data only from the upper parts of the profile.
Numerical weather prediction models and balloon soundings are good sources of temperature profiles.
A bulk method is to take average temperature (daily or even monthly averages) and calculate the melting level height assuming the moist adiabatic lapse rate 6.5 °C / km. This lapse rate is a good estimate during the precipitation event.
If nothing else is available, start with the following table.
| Month | Polar climate | Mid latitudes | Tropics |
|---|---|---|---|
| January | -1 | 1 | 2.5 |
| February | -1 | 1 | 2.5 |
| March | -1 | 1.5 | 3 |
| April | 0.2 | 2 | 3.5 |
| May | 1.3 | 2.5 | 4 |
| June | 2.2 | 3 | 4.3 |
| June | 2.5 | 3.5 | 4.5 |
| August | 2.0 | 3.5 | 4.5 |
| September | 1.5 | 3 | 4.3 |
| October | 0.5 | 2.5 | 4 |
| November | -1 | 1.5 | 3.5 |
| December | -1 | 1.0 | 2.5 |
What if There Is No Melting Level ?
When it’s snowing, there is no melting layer (-1 in the table above). Then the profile becomes a simple descending line, defined by parameter set.
- Reflectivity gradient above the melting layer 7 dBZ/km (11.3 dBZ/mi)
- Reflectivity gradient below the melting layer 0 dBZ/km (0 dBZ/mi)
- Melting layer height 0 km (0 mi)
- Melting layer thickness 0 km (0 mi)
- Melting layer peak intensity 0 dBZ
The most demanding task is to get the profile right when the melting layer is close to surface. Even small errors misplace the bright band and thus lead to severe overestimation and underestimation close to the radar. If you cannot change the profile frequently, and the temperature fluctuates below and above 0 with bright band appearing and disappearing, Vaisala recommends you apply the snow profile as described above. You still have the bright band overestimation problems, but at least you fix all the weakening above the bright band.
 | Make sure you give the altitude information from external data source referring to the same reference height (sea level, antenna level) as defined in the IRIS setup. Be careful when the bright band is close to ground. |