Radiation

The potential radiation (blue curve, right axis) is a theoretical maximum value that is only obtained on clear days with average atmospheric composition of moisture and pollutants. The annual pattern of potential radiation has a bell shaped pattern with a maximum at mid-summer.

Because of cloudiness, global shortwave radiation (black curve) is about half the value of potential radiaiton. Its annual pattern is similar to that of potential radiation, except for a local peak in May. The sunnier conditions in May are a result of higher atmospheric pressure and low ocean temperatures at this time of year.

Net radiaiton (green curve) is the sum of incoming and outgoing radiation - both shortwave (visible) and longwave (thermal). It has a pattern similar to global shortwave, but its values are more negative because of losses by reflected shortwave and thermal long wave emmisions.

Reflected shortwave radiaiton (red curve) represents the amout of incoming shortwave radiation that bounces off the surface back to the sky. Although its pattern is similar to that of incoming shortwave, it lacks the localized peak in May. This difference in patterns is the result of changes in the forests canopy albeo when new shoots develop during May and June.

The increase in albedo at the beginning of summer is clearly apparent in the albedos of both short wave radiation (black) and photosynthetically active radiation (PPFD, red curve) shown in the bottom figure. The values of the short wave albedo curve are three times larger than those for the albedo of photosynthetically active radiation - indicating the greater amount of photosynthetically active radiation absorbed by the canopy in the process of photosynthesis.

Temperature

The temperature of an ecosystem is closely related to the amount of radiation received because radiation is the ecosystems primary energy source. However, the time of maximum and mimum temperature will lag behind the maximum and minimum radiaiton because of the time it takes to heat up the ecosystem - in the same way it takes time for a kettle to boil after you place it on the stove. To see the effect of this lag we can plot the monthly average temperature against the monthly average radition - sometimes refered to as a climagram. If the ecosystems temperature increased immediately in response to increases in radiation the plot would appear as a straight sloping line. The more the temperature lags behind radiation the more circular the pattern becomes.

The effect of the oceanic environment of Scotland can be seen in the climagram for Griffin (the red curve). The dashed green curve represents a theoretical climagram for a location at the same latitude as Griffin. However, because of the warming influence of the Atlantic ocean the mean annual temperature of the Griffin site is several degrees warmer than the theoretical curve.

By shifting the theoretical curve upwards we obtain a new theoretical curve (black curve) which accounts for the warming effect of the ocean.

We see however that this adjusted curve still does not compare with the measured climagram (red curve), showing limitations to both temperature and radiation. During winter temperature at Griffin reaches a lower limit (~3 degrees) because of the surrounding ocean. During the summer, it appears that limitations to radiation cause a reduction in temperature, and thus a shortened lobe on the summer end of climagram curve

Humidity

The annual pattern of humidity differs depending upon the measure of humidity being used. Monthly average vapour pressure (green curve) has a maximum in August reflecting the temperature dependence of the ability of the atmosphere to hold water vapour. In contrast, vapour pressure deficit (red curve) and relative humidity (blue curve) peak in May. These two variables are a measure of the apparent dryness of the atmosphere. It is interesting to note that the apparent dryness of the atmosphere is most closely related to radiation.

The monthly averaged - daily patterns of vapour pressure were normalized by their corresponding monthly average values to obtain the contour plot shown here. In this figure contours larger than 1.0 indicate higher than average atmospheric water vapour content. In winter there is little variation in the atmospheric water content while during the summer water content is highest early in the morning and late in the evening. These times represent periods when radiation is inducing evapotranspiration but the atmospheric boundary layer is poorly developed – so that the water vapour is not mixed with the drier air above.

Wind

Wind at Griffin is strongly affected by the sites topography. Because it is situated in a valley the wind direction distributions have a bimodal pattern with peaks at 130 degrees and 290 degrees, corresponding to down-slope and up-slope flows. This bimodal pattern exists at moderate wind speeds (2 to 4 m/s) during both day (red curves) and night (blue curves). At higher wind speeds (>4 m/s) the details of the sites topography begin to exert influences, producing a tri-modal pattern. At low wind speeds (< 2 m/s) the effect of katabatic drainage flow is observed, with a greater frequency of down-slope flow at night and up-slope flow during the day.

Precipitation

The annual pattern of precipitation indicates consistent levels from March through August followed by a rapid rise to values twice as large from September through January.

Looking at the bi-monthly average daily patterns we can see that the daytime precipitation is similar throughout the year. In contrast, nocturnal precipitation is highest during autumn (September through December). Therefore, the annual pattern of precipitation can be attributed to changes in the amount of nocturnal precipitation.

Carbon Dioxide

The relative variation of atmospheric carbon dioxide concentration above Griffin forest reflects the strong influence of radiation on the uptake of carbon dioxide. The bi-monthly averages of daily carbon dioxide variation from the mean show the large variations during the summer months and small variations during the winter months resulting from correspondingly large and small ecosystem carbon exchange.