Estação Meteorológica da Amareleja [IM] (20/08/2008)

Tópico em 'Instrumentos Meteorológicos' iniciado por Daniel Vilão 21 Ago 2008 às 12:00.

  1. J.S.

    J.S.
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    Fil e Daniel,

    Em Ingles agora..

    I am not trying to be arrogant, so excuse me for my direct wording...

    The topography, the size of the trees or whether or not the station is in accordance with the reghion simply does not matter. The standards are very clear: no trees allowed in a radius of 100 meters. Period. With a very good reason btw. The trees introduce all kinds of anomalities, in this case I wonder if even sunshinehours are measured correctly, something I have never witnessed on a official station in my life BTW. And I have seen a few of them.

    Now 15 meters in a circle in a pine forest is really not even close. This open area surrounded by trees introduces a artificial valley effect. Resulting in cooler nights allround and warmer days (in low wind situations). First of all: the air will be more still than when it would have been an area according to WMo standards. So there is an easier build up of cold and warm layers (nights and days resp). second point is that the air will inevitably be more still, which means that when solar irradiation reaches 800 to 1000 W/m2, selfheating of the sensorshield rapidly picks up giving way to (large) errors.

    That is why buildings and trees are not allowed within 100 meters and outside of this radius only when they do not reach 1/10 of the distance to the station. As I already said: a 15 meter high tree must be outside a 150 meter radius. Even one tree....Shrubs are allowed only starting from 100 meters.

    This standards have been set because of research about effects of trees, shrubs and buildings.

    In Holland, there also many stations which do no comply to WMO standards. I have pictures of two (out of three) in my province. Vlissingen KNMI is totally wrong, too much to talk about. Westdorpe KNMI has a cornfield during the three summer motnhs of 2+ meters height at 4 meters from the sensorshields over a lentgh of hundreds of meters... I siply went there because I found the readings suspicious, it was so clear....I knew it had to do with crops...I have taken a lot of pictures if this...

    This makes your initiative all the more interesting BTW!

    As far as I can tell, the station of Herdade dos Lameiroes (www.cotr.pt) seems to be in better accordance than nearby Amareleja. At least, the meteorlogist told me it measures according to WMO standards and the pictures seem to indicate this, but still some trees nearby there also...
     
  2. Fil

    Fil
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    Obrigado J.S., informações preciosas as tuas. Realmente as árvores podem facilmente perturbar a leitura da insolação, especialmente durante o inverno quando o sol fica baixo. Não sabia do raio de 100 m, assim sendo poderá haver muitas outras estações do IM que não obedecem inteiramente aos standards da WMO. Mas para mim 100 m sem uma única árvore à volta parece-me um pouco exagerado e assim torna-se bastante difícil arranjar locais adequados.

    Sei que a estação das Penhas Douradas também tem bastante arvoredo à volta. A estação daqui de Bragança tem uma casa onde fica o observador a apenas alguns metros da estação. Talvez seja por estas regras rígidas da WMO que o IM coloca muitas das suas estações num local alto em relação às redondezas de modo a ultrapassar estas dificuldades?...
     
  3. J.S.

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    Hej Fil,

    If you study things closely, you can pinpoint the effect of some trees at much larger distances. I believe it is not allowed to have a forest within a
    400 m radius, according to the WMO. So a single tree between 100-400 m is oke, but a forest is a different matter. So a forest at 15 meters is really not what we want at all. You cannot compensate by placing it somewhere higher, the only thing you do is making things more difficult to assess. Those standards are there so we really can compare climates.

    BTW: you notice effects of a few trees etc at less than 100m in some situations, not all. So although the averages of a few trees at 100 m would not be noticable (may be), when the wind blows form their direction, the effect is clear.

    I have seen the effect of drifting snow in Holland in strong winds. More than 200 meters away from a barn, upwind, there was no snow at all while in other places snowdrifts piled up many meters..You could see where the wind could blow and by how far the wind (we are talking about 7 to 8 B!) was blocked by the barn....Now suppose your sensorshield was in the snowfree area (which otherwise would be snowcovered), the nights become clear (what exctly happened in that situation) and temperature drops to -25 in places (which happened exactly in that night, 4 nights in a row < -20 C). You will measure much higher temperatures because you're not having the effects of the reflecting snow at night.....You can imagine other effects as well.

    Anyway: this station is substandard. High temperatures should be reviewed with caution. In Holland, I know of many hobbyists simply measuring under much better circumstances and because of using calibrated thermometers, have better total readings than this and some other official stations.

    Now I hope we see more pictures of Portuguese stations and may be you can ask the IM for a comment.
     
  4. Dan

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  5. J.S.

    J.S.
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    Yes, well, again this is not according to standards I am afraid. Trees (like the blossoming one on one picture) nearby. Some houses also nearby. Thats just the way it is. Intercomparison between other stations, that do measure according to standards becomes difficult if not impossibe, because other non climatological factors interfere.

    The way it should be, is like this (Wilhelminadorp KNMI).

    [​IMG]

    (why does the picture not load btw?).

    A link is here

    http://picasaweb.google.com/simoes.jacinto/PlaatsingWeerstation692008#5245849328324175890

    The barn at the back is 300 meters or more away. In all other directions, distances to shrubs and trees are bigger.
     
  6. belem

    belem
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    Boa noite
    Já agora que foram lá: por acaso tiveram atentos ao relevo da zona e viram alguma depressão ou vale, como foi descrito pelo JS?
    Amareleja, conhece locais mais baixos e potencialmente mais quentes em seu redor?
     
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  7. J.S.

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    The depression is caused by the trees and it is clearly visible. But we can discuss it all we like. Let's see what the WMO says about the site of a meteorolgical station, be it for climatological or agricultural purposes.

    "Reference: Guide to Climatological Practices (WMO No. 100; 2001 edition)

    The Siting of Climatological Stations

    The precise exposure requirements for specific instruments used at climatological stations, aimed at optimizing the accuracy of the instrumental measurements, are discussed in the Manual on the Global Observing System, Part III (WMO No. 544), and in the Cuide to Meteorological Instruments and Methods of Observation (WMO No. 8). However, the representativeness and/or homogeneity of the climatological record are closely related to the choice of location of the observing site. A station sited on, or near a steep slope, ridge, cliff, hollow, building, wall or other obstruction, is likely to provide data which are representative of only the site. A station which is or will be affected by the growth of vegetation, including even limited tree growth near the sensor, growth of tall crops or woodland nearby, by the erection of buildings on adjacent land, or by increases (or decreases) in road or air traffic including that due to change in use of runways or taxiways will provide neither representative nor homogeneous data."

    and

    "Ordinary and Principal Climatological Stations should be sited on a level piece of ground covered with short grass; the site should be well away from trees, buildings, walls, and steep slopes and should not be in a hollow."

    and

    for agricultural meteorology:

    "Under no circumstances should the site be concrete, asphalt, or crushed rock. Obstructions such as trees, buildings, and nearby shrubs should not be closer to the instruments than eight to ten times their height No obstructions should cast shadows during the greater part of the day; brief periods of shadows near sunrise and/or sunset are sometimes unavoidable."


    for climatological meteorology:

    "The location of the observing posts (meteorological instrument area) should be typical of the physico-geographical conditions of the surround¬ing area and protected from the influence of industry. It is therefore necessary to locate a meteorological instrument area in an open site far from any constructions or woods. The minimum distances from constructions and groups of trees should be greater than 10 times and 20 times their heights respec¬tively. The site should also be farther than 100 m from bodies of water, except where coastal measurements are required."

    That is why I think that Amareleja does not even come close.
     
  8. belem

    belem
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    A questão que coloquei no meu último post, mantem-se...:D
    E porquê?
    Porque só quis saber se há locais potencialmente mais quentes, na zona.
    O google earth, diz-nos que há, mas eu gostaria de ouvir a opinião de quem lá foi.
    Uma estacão meteorológica deve ter características padrão de uma determinada região, concordo.
    Mas, isto é feito, tendo em conta a posição dos agrupamentos urbanos, que no caso do Alentejo, situam-se quase sempre nas zonas mais frescas e altas.
    Por isso, os valores do Alentejo em termos de médias de Julho ou Agosto, por vezes enganam, pois parecem demasiado amenos. Na verdade, se uma aldeia se situasse num vale ou encosta amplamente favorecido em termos caloríficos, como em algumas partes do Guadiana, Tejo, Douro (etc...), as estações locais teriam também que obedecer a essas condições e os valores seriam claramente mais significativos.
    Por isso e voltando novamente ao importante da questão, a minha curiosidade, era somente direccionada, para a dúvida sobre o tipo de relevo da zona e em caso de haver depressões ou encostas, só para ter conhecimento disso mesmo.
     
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  9. Daniel Vilão

    Daniel Vilão
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    Quando visitei a zona da Amareleja tentei reparar nisso mesmo.
    A Amareleja em si deve estar a uma altitude que ronda os 200 m, mas o relevo é muito pouco acidentado e quase não há encostas ou depressões na zona.
    A zona é, só por si, caracterizada pela sua baixa altitude, mas nada de depressões muito significativas, por isso penso que dificilmente os dados seriam muito diferentes mesmo que a estação mudasse de sítio, pois já são bastante representativos da zona.
     
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  10. belem

    belem
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    Obrigado, Daniel Vilão.:D
    É uma zona sem grandes declives então... Penso que isso explica a côr uniforme no mapa que coloquei no tópico dos 50ºc.
     
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  11. Daniel Vilão

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    Sim, os declives existentes são mínimos e a variação de temperatura entre eles é mínima, pois nada justifica variações muito grandes neste caso concreto.
    É certo que a estação se encontra num local um pouco mais fresco, mas todas as estações do IM se localizam em locais mais frescos do que os envolventes, perto de matas ou em descampados, em locais altos.
    Esta estação não é excepção e acho o local apropriado e bastante aconselhado para a localização da estação.
    Os desvios de temperatura entre essa mata e a freguesia da Amareleja não devem ser maiores do que 0,5 ºC a 1,0 ºC, o que reforça a ideia de que se trata de um bom local.
    O único senão é o facto de as ávores serem altas e poderem impedir variações repentinas na temperatura, humidade e um melhor arejamento dos sensores, mas nada de especial, dado que as árvores envolvem a estação num raio de cerca de 10 a 15 metros, portanto a desvantagem é mesmo a altura das árvores, que ainda é bastante significativa e quase apanham o anemómetro, podendo ter uma influência ínfima nos registos de vento e até nas leituras do piranómetro, especialmente no Inverno, que é quando o sol se encontra mais baixo e mais perto da linha do horizonte.
    De resto, a estação não fica num local tão mau assim e a qualidade dos seus registos é aparentemente elevada.
     
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  12. J.S.

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    Well, the standards are not there for nothing and frankly you cannot judge, by sight, what those influences are. You simple cannot assess them just like that, you need to measure in a location nearby in a situation according to WMO standards.

    Now trees at 15 meters with substantial height (say 10 meters??) away should have been at least 100 meters away. The big problem here is not only the anemometer nor the piranometer, it is very clearly the thermometer. I do not know why you dismiss so easily the WMo standards. I will repeat them:

    "A station which is or will be affected by the growth of vegetation, including even limited tree growth near the sensor, growth of tall crops or woodland nearby, by the erection of buildings on adjacent land, or by increases (or decreases) in road or air traffic including that due to change in use of runways or taxiways will provide neither representative nor homogeneous data."

    I have rarely encountered situations as bad as with Amareleja. It can be even worse probably (wel VLissingen KNMI is awfull too). But this is about as bad as it gets. In my opinion, the records of Amareleja can be thrown in the garbage bin. If the wind is probably affected at 10 meters height, as you say, it is an easy guess how the influence is at sensorscreen height....and this is one of a series of influences on the thermometer.

    Are there warmer places...Did you take into account that more to the south, near Sobral da Adiça, you are at the same height but more sheltered by high hills to the south? NEar MOura, you are at 100 lower, may be this area is warmer. Considering the prevailing NW winds in summer, in the valley of the Chança near VIla Verde de Ficalho, you are again at 160 m height but now clearly more sheltered from the NW wind by the 520 meter high hill/moutain. Could also be warmer. And 1 K above an average of about 34 tot 34,5 C leads to 35,0 or 35,5 C...That is about as hot as it gets in Europe. So very hot!

    I have more confidence in the station of Herdade dos Lameiroes. And this station actually seems to be measuring the same or even somewhat higher temperatures than Amareleja. Nice to compare those two in the future!!
     

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