Precipitation is the liquid or solid condensation products of water vapor that fall from clouds or from the air to the ground. It includes rain, hail, snow, dew, frost, hoarfrost, and fog. The total amount of precipitation that reaches the ground in a given period of time is expressed in terms of the vertical depth of water (or water equivalent in the case of solid precipitation) at a horizontal projection of the cover of the Earth's surface. Snowfall is also expressed by the depth of freshly fallen snow and the newly covered horizontal area./1/
A precipitation gauge is used to determine the amount of precipitation falling on the ground at that location per unit area and time interval. By taking a series of such measurements at the same location, and assuming a continuous areal distribution, it is possible to obtain an estimate of the total amount of precipitation water deposited.
This quantity is an important element in hydrology, climatology, and meteorology. It is obvious that the accuracy of the estimation depends on two parameters:
- The first influencing factor is the proximity of the rain gauge, which measures the amount of precipitation, to the precipitation field. That is, if the amount of precipitation per unit area would have fallen on the ground at the measuring point of the rain gauge (horizontal projection) if the rain gauge had not been present and
- Second, how well a series of rain gauges across the area of a measuring network can represent the total mass of rainwater. /2/
Wind speed and precipitation type are major influencing parameters in determining precipitation amounts relative to the horizontal projection. Thus, precipitation measurements taken with an unprotected rain gauge at a mountain station will result in lower readings.
To correct the wind parameter, various wind protection systems have been developed over the past decades, and numerous versions of these systems are in use worldwide.
2.Wind protection systems
The following are the most common basic types used in Europe:
In the Anglo-American measurement networks, wind protection systems based on the age (1937) of versions 1 and 2 are used. The ICOS measurement network is an exception. The ICOS measurement network operated in Europe is equipped with wind protection systems based on the age of version 2.
The Tretyakov windbreak is recommended as the standard windbreak by the WMO.
In the measurement networks of European meteorological services, including Russia, the Tretyakov wind protection system is used in modified versions.
- The Tretyakov wind protection system
Tretyakov (1952) developed a shield similar to the Nipher based on wind tunnel tests.
The Tretyakov windbreak ring is also equipped with vertically circular metal slats. The metal slats are horizontally movable and conically converge at the base of the rain gauge. The individual slats are connected to each other by a chain, giving the Tretyakov windbreak system excellent aerodynamic properties.
1. Variants of the Tretyakov wind protection system for use with automatic rain gauges
A distinction is made in the application between the classic Tretyakov windbreak and the Tretyakov windbreak for automatic rain gauges.
If we consider the different variations of the wind protection system according to Trtejakov, the following distinguishing criteria can be formulated:
- CHMI
The CMHI is currently using the Tretyakov wind protection system at the measuring heights
- 100.0 cm
- 200.0 cm.
The system consists of 21 lumbar slats, each slat is cm long.
To prevent snow accumulation, the slats are shortened.
The material used is aluminum or V2A.
- IMGW
The IMGW is currently using the Tretyakov wind protection system at the measuring heights
- 100.0 cm
- 200.0 cm.
The system consists of 21 lumbar slats, each slat is cm long.
To prevent snow accumulation, the slats are shortened.
The material used is aluminum or V2A.
- DWD
The DWD is currently implementing the Tretyakov wind protection system at the measuring heights
- 102.5 cm
- 152.5 cm
- 202.5 cm.
The system consists of 21 slats. Each slat is cm long.
The slats are designed to prevent snow deposits.
The material used is V4A stainless steel.
The increase of the measuring height from 100 cm to 102.5 cm is based on the requirements of the ICOSNET
- KNMI
The KNMI is currently using the Tretyakov wind protection system at the measuring heights
100.0 cm.
The system consists of 21 slats, each slat is cm long.
To prevent snow accumulation, the slats are shortened.
Aluminum is used as the material.
- ROSHYDROMET
ROSHYDROMET is currently using the Tretyakov wind protection system at the measuring heights
- 100.0 cm
- 200.0 cm
- 300.0 cm.
The system consists of 21 molded slats. The slats adhere to strict WMO recommendations. Each slat is 1 cm long.
This is a strict windbreak according to Tretyakov.
Aluminum is used as the material.
- Error considerations
Measuring height
The aim is to carry out error analyses relating to the influence of the installation height of the wind protection system and the catch ratio of the rain gauge.
Cases:
|
Wind protection |
Rain gauge |
|
97.5 |
100 |
|
100 |
100 |
|
102.5 |
100 |
|
147.5 |
150 |
|
150 |
150 |
|
152.5 |
150 |
|
197.5 |
200 |
|
200 |
200 |
|
202.5 |
200 |
- Dynamic behavior
Consideration of behavior at high flow velocities.
With the Tretyakov windshield, representative measurement results can be achieved if the installation instructions are followed.
An ideal windbreak should:
- (a) laminarize the air flow at the rain gauge collecting opening
- (b reduce local turbulence at the rain gauge intake
- (c reduce the wind speed at the rain gauge, regardless of the installation height
- (d) avoids injections
- (e) must not be covered with snow./2/
The points are fulfilled by the system.
In the Tretyakov system offered here, the slats are connected with a tensioning strap and must be adjusted to a funnel shape at an angle of approximately 70°./3//4/ Due to the design and if the installation instructions are followed, the dynamic processes occur in a wave-like manner.
The dynamic processes in the wind protection system are shown here using the age system as an example: https://www.youtube.com/watch?v=ri1bXHV6VYI
Based on the dynamic effects (turbulence) demonstrated by the windshield according to Alter and the requirements formulated in /2/, Tretyakov developed the windshield. The goal was to improve the points ac.
The effects of the desired height changes on the measurement results are shown in the following table:
|
Position of the slats in relation to the collecting surface
|
Mod. Age (g) |
Modified Age I(g) |
|
1 inch below |
8.5 |
9.2 |
|
1/2 inch below |
8.8 |
9.3 |
|
At the same height |
8.9 |
9.4 |
|
1/4 inch above |
8.9 |
10.4 |
|
1/2 inch above |
8.3 |
9.9 |
|
1 inch above |
6.7 |
8.9 |
Comparative wind tunnel test with sawdust from shielded models of the Sacramento rain gauge at various installation heights of the blades above the gauge's collecting surface. /5/
With an average elevation of 2.5 cm (0.98 inch), a slat length of 19 cm and the catch area of the Sacramento Gauge results in an estimated undercatch of approximately -0.5 to --1.0 mm.