Design and set up of a network of disdrometers


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Rain keeps still as one of the most difficult meteorological variables to measure nowadays. There are several ways of performing point measurements, and for some applications precision obtained compared with manual and human driven techniques is more than reasonable. One of the problems with rain is its inherent spatial variability. One can have very good measurements at a point but when compared in a daily basis with a close location, important differences are found. Obviously these differences dilute when compared in a climatic basis, long periods, since they tend to compensate.

Nevertheless for certain applications is necessary to have daily precipitation fields and unless a huge number of point measurements are taken, other techniques need to be used. These techniques are RADAR and Satellite measurements. With some obvious differences, these techniques estimate the rain curtain falling or not falling to ground from the echo of an electromagnetic wave on the drops. Since this is not a direct measurement and considering that sometimes not all these drops end in the ground, some calibration exercises are needed.

Two factors affect in the kind of echo that a curtain of rain produces when a radio beam hits it, one is the size of the droplets, other is the velocity they have. Knowing these two factors for an enought time and at the right place are valuable information for RADAR and satellite rain detection calibration. This project deployed more than 16 instruments able to detect the size and velocity of droplets with a 1 minute temporal resolution and almost in real time in a central server. This was fun.


The objective of this project was to have 1 minute samples of size and velocity droplets at 8 locations with redundant information all in an area of less than 8 Km2 and have this information in a central server at 250 Km away.


For this project we used OTT Parsivel disdrometer due to their well known performance. Version 1 of this disdrometer was not specially designed for remote operation, so we had to work hard on the solar power unit. For datalogging and transmission we went for our own design and prototype. Risky but it worked close to perfection. We had to transmit a matrix of wind velocity and size of droplets of more than 1064 cells every minute and coming from 16 disdrometers. We decided to go for an ad-hoc development in order to have everything under control.


The main reason of success with this project was the design of an intense maintenance program. Disdrometers windows get dirty, animals love to scratch their back on then and spiders love those nice arms in the middle of the field. Besides this minor problems, this project went really smoothly. Some international publications and a Ph Thesis came out of this campaign.




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