Bas Crezee

Oct 092012

We have not yet experienced extreme weather here in France, but the two places we visited this sunday are both closely related to extreme weather. First, we visited Mt. Aigoual (1567m), which is one of the locations in France with the most extreme winds. After this we went to the east, to visit the X-band radar, which was built thanks to cooperation of three american institutes: the National Severe Storms Laboratory (NSSL), NOAA and Oklahoma University. This radar generally operates in severe weather conditions when doing measurements in severe thunderstorms. In this blogpost I will describe the day by presenting the pictures.

For this sunday, over Southern france calm and generally sunny weather was predicted with some high and mid-level clouds possible. When heading north from Montpellier we already had some nice views on the sky. It is clear that the front separating the nice weather in southern France (~25 degrees) from the cold weather in The Netherlands is not too far away.

Heading north, cloudiness starts to increase. Getting closer to the Mt. Aigoual, the roads become more narrow and there are chestnut trees all around us. Patches of fog passing by reveal that we are getting close to the cloud base already. And soon, we arrive at the top of Mt. Aigoual at 1567m above sea-level, no nice view on the surrounding, but totally cloudy and a lot of wind! This morning gusts up to 117 km/h have been recorded, and at the moment we arrive it is still quite windy. As you can see in the picture below, we were well prepared for this weather with some warm clothes.

The meteorological observatory has a nice visitor center, with a lot of information about cloud types and weather prediction and also a nice collection of historical instrumentation and weather maps.

As written above, Mt. Aigoual is known for its extreme wind speeds. There are over 200 days per year where the wind gusts reach over 100 km/h, while the highest gust ever recorded is 335 km/h on november 6, 1966. In the image below you will find more about climatology and records at Mt. Aigoual.

Because of the extreme wind conditions often combined with snow during winter, this site is very useful for testing meteorological instruments. If you want to put your instruments to the test, you should mount them on top of the tower located at this platform.

We were guided through the observatory and learned that the station was not only used for meteorological observations, but also for ecological research by the university of Montpellier. The students walked by foot from Montpellier to mt. Aigoual, which is approximately 90 km!
When leaving Mt. Aigoual, it was still covered in clouds, just a few hundred meters below the top we enjoyed the nice hills in autumn colors which combined very well with the clouds in the sky. (click on HD for highest resolution)
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On our way towards the X band radar, we saw some nice wavy cloud patterns. These clouds are called lenticularis clouds, which literally means ‘shaped like a lens’ in Latin. While most of the clouds in the sky drift along with the upper level winds, these clouds stay at the same place. They are actually lee clouds and are caused by a mountain induced wave in the air flow.

After about one hour driving we arrived at the X band radar. This radar is a dual polarization radar operating at a wavelength of 3 cm. By using dual polarization – just like the TARA radar that we use – it is possible to determine the shape of the particles. Also this radar uses doppler shift to determine particle velocities.

A close-up of the dish of the radar. The tubes on top are waveguides, they literally guide the radar waves towards the transmitter.

Near the radar there was also one antenna of the Lightning Mapping Array (LMA). This instrument is impressively simple in design, while producing very nice data. The Lightning Mapping Array is able to reconstruct three dimensional paths of lightning strikes. Lightning emits strong radiation in the radio frequency band. The LMA consists of eleven antenna’s (of which one is shown in the picture below) which can receive this radiation, every antenna measures in three directions. Next to the antenna is a GPS receiver which records the time at which the signal was received, precisely within less than one millionth of a second. After this the signals of the different antenna’s can be combined to construct the three dimensional path of the lightning strike over time. Combining the lightning data from the LMA with the data from radars like this X band radar or TARA can provide more insights into cloud electrification and how it is related to cloud microphysical processes.

That is it for now! We are looking forward to thursday on which we might perform the first measurements on some showers or thunderstorms. Models are not too optimistic yet for our region, but things might change, so let’s hope!

Oct 072012

Most people travelling from The Netherlands to southern France hope to get some nice sunny weather. Not for us! Since we are here for the HyMEX project we hope for some heavy rains. HyMEX stands for Hydrological cycle in the Mediterranean Experiment and it aims at a better understanding of this hydrological cycle. For this reason many scientists have come together in Spain, France and Italy to observe and model interesting weather systems. This campaign brought together a lot of scientific instrumentation to perform detailed observations of the atmosphere. These observations are used to get a better understanding of the physical processes related to the formation of clouds and precipitation.
The contribution of TU Delft in this project is the TARA radar. It is an Transportable Atmospheric RAdar and operates in the S Band at 3.3 GHz. Utilizing the doppler effect, this radar can retrieve a vertical profile of wind velocities. Also by making use of dual polarization, the radar can distinguish between different precipitation types.

The TARA radar during foggy conditions at the measurement site in Candillargues.

The TARA radar was transported to France at the beginning of the HyMEX campaign, the beginning of september. Our flight to France was on the 1st of october. We left the Netherlands in rainy weather, during flight we had a fantastic view on the sunset. Arriving in France, we immediately noticed the mild temperatures.
The next morning we visited the Hymex Operational Center (HOC), this is the location where each morning most people involved in the campaign meet. Meteorologists of Meteo France give the weather briefings and the observation strategy is planned. Which radars will be operated? Where will the most intense rain or thunderstorms occur according to the models? Are we going to operate the two Falcon research aircraft for measurements? By using a video connection, also the other operational centers in Spain and Italy are involved in this planning.

Picture taken at the Hymex Operational Center (HOC). From left to right: Igor, Ning, Kyriaki, Bas

After our visit to the HOC, we went to the nearby small airport of Candillargues where the TARA radar is located. We had a look at the other instrumentation near our site. It was quite impressive to see the instruments together. Next to the TARA radar there is a wind profiler from Meteo France. Further there are two LIDAR instruments which can measure vertical profiles of aerosols, clouds and atmospheric gases. There was much more instrumentation, worth to mention is an instrument that can measure rain droplet charges related to cloud electrification and lightning.
The rest of the day was dedicated to learning how to change the elevation of the radar. It is not a difficult operation, but some precautions have to be made since the dishes are quite large and heavy. Soon a small guide with instructions and pictures will be made available on how to change the elevation for TARA.
Now that we can change the elevation, we are ready to learn taking observations. We are looking forward to this!

Posing in front of TARA. From left to right: Bas, Igor, Kyriaki and Ning.