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Penman Analysis 17th June 2008 Kate Brown Becky Hewins Danny Hollinworth Andy Ratcliffe

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Results VariableValueErrorComment H (Wm -2 )148± 45largest error is from the canopy resistance, rc λE (Wm -2 )327± 45 B0.40± 0.19 E (mmh - ¹)0.48± 0.07 H/Rn0.29± 0.09 u * (ms - ¹)0.22± 0.02largest error from the roughness length, z 0 R n (Wm -2 )511± 15not affected by z 0 and rc, error arising from the equipment used (± 3%) G (Wm -2 )36.0± 1.1

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Sources of Error The experiment was only conducted over a short period of time and so fluctuations in the measurements appear heightened and are included in calculations. Sampling Error Accuracy of Instruments The Penman mast had a error of ± 3%. This introduces small errors to calculated values.

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Sources of Error The area the instruments measure is very small, but the net radiation is calculated over a large area The temperature is measured on a fixed point in the ground and the ground heat flux plate only has an area of a couple of centimetres squared. The solarimeters and the radiometers measure a couple of metres squared. Representativity Error

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Interpretation of Data H = 148 Wm -2 - this is fairly large and is due to the heating of the ground from the incoming short wave and causing a large temperature gradient between the air and the ground. λE = 327 Wm -2 - the ground was slightly moist and this means that there will have been evaporation and hence the latent heat flux has a large value. Bowen ratio= 0.40 - this value shows that there was a larger latent heat flux than sensible heat flux again due to moisture evaporating from the ground. Evaporation= 0.48 mmh - ¹ - this shows that the air was quite dry and warm and there was a moderate amount of evaporation.

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Interpretation of Data H/Rn=0.29 - this shows that the net radiation was quite a bit larger than the sensible heat flux giving a low ratio. U* = 0.22ms -1 - this shows that the velocity of the wind close to the ground was fairly low. R n =511Wm -2 - this is a reasonably high net radiation due to the sun being out. G = 36 Wm -2 - this value is small and means that there was not very much radiation entering the ground.

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Assumptions and Likely Validity It was assumed that the conditions were neutral, in reality this is not normally true and conditions are usually stable or unstable. Small change need to be made for the calculations in each of these cases. The conditions did not change throughout the IOP. This isn't valid as the sun often went behind clouds which affected readings and the wind was changing variably.

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Assumptions and Likely Validity The surface was homogenous, i.e flat. This is valid if you consider the atmospheric observatory alone, however, this is not realistic. The area to the west of the atmospheric observatory is on a slope extending by approximately 100m down to some trees at the side of Whiteknights lake. On this slope on the surrounding west and south sides was tall grass around 80cm high affecting the roughness length.

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