Vertical profiles

In this section, vertical profiles of bihourly humidity values are presented. The bihourly value is the mean of the humidity measurements from two hours. The humidity mea-surements for each hour are the mean value of the last minute. Profiles for relative (RH) and absolute (ρ) humidity are plotted using data from 28/6 (figure 4.26) and 30/6 (figure 4.27).

28/6/2019 was a day with light wind (average wind speed was 2.17 m/s) and where the wind changed direction continuously all day. It was also a day with clear skies, where the average ratio of diffuse to global radiation was 0.28. The RH values at 2 m and 1.25 m increased from 00-01 until 04-05, then decreased until 14-15, and then increased until 22-23, as can be seen in figure 4.26a. This was the inverse of the development of temperature on that day, shown in figure 4.19a, which is in line with the theory that relative humidity is inversely proportional to temperature. Absolute humidity is independent of temperature. As expected, the vertical profile of absolute humidity in figure 4.26b did not follow the development of the temperature that day. Furthermore, the maximum and minimum values did not occur at the same times for the three heights, and the values did not consistently increase and then decrease throughout the day. An example of this is that the ρ values at all heights and both locations decreased from 12-13 until 14-15, then increased in value at 16-17, and then decrease again at 18-19.

(a)

(b)

Figure 4.26: The plots in this figure are vertical profiles of bihourly humidity values on 28/6, which was a cloud-free day day with light wind (<5 m/s) in all except for two hours. The plots show vertical profiles over uncut (dashed lines) and cut (solid lines) grass. (a) shows the development of relative humidity and (b) shows the development of relative humidity.

On 30/6/2019, the average wind speed was 3.68 m/s, where ten hours had an average wind speed larger than 5 m/s. The wind came from the south until 14:00. After this, the wind direction changed to southwest, west, and northwest. It was a cloudy day with an average diffuse to global radiation ratio of 0.54. In the plot of relative humidity in figure 4.27a, it can be seen that the values increased from 00-01 until 04-05, and then decreased until 14-15. The values then actually increased at 16-17, before they started decreasing again and reached a minimum at 20-21. This was the inverse of the same development of air temperature values on this day (figure 4.19b). The range of the absolute humidity values was larger on 30/6 (figure 4.27b) than on 28/6 (figure 4.26b). The variation from smallest to largest values at 2 m over cut grass on 30/6 was 5.52 g/m³, while it was 3.22 g/m³ on 28/6. Just like the plot for ρ on 28/6/2019, the absolute humidity values on 30/6/2019 did not consistently increase and then decrease throughout the day.

(a)

(b)

Figure 4.27: The plots in this figure are vertical profiles of bihourly humidity values on 30/6, which was an overcast day with strong wind (larger than 5 m/s) for ten hours out of the day and average wind speed of 3.68 m/s. The plots show vertical profiles over uncut (dashed lines) and cut (solid lines) grass. (a) shows the development of relative humidity and (b) shows the development of relative humidity.

Chapter 5

Discussions

This study aims to find out how air temperature measurements are affected by vegeta-tion. The SC estimates the additional estimated uncertainty to be 2^◦C for temperature measurements at meteorological stations with vegetation taller than 25 cm under the measuring instrument. The class 4 limit in the vegetation height category is 25 cm, and the additional estimated uncertainty of 2^◦C is given to all class 4 temperature measuring stations.

The previous chapter presented results from analysis of the observational data from the experiment behind this thesis. In this chapter, these results will be analyzed and their significance will be discussed. The goal is to determine whether SC’s claim of an additional estimated uncertainty of 2^◦C can be confirmed or not. Additionally, the humidity data from the experiment will be discussed.

5.1 Overview of the data

On the time scale of the experiment’s duration in 2019, the differences between tem-peratures measured over uncut (U) and cut (C) grass at all three of the measuring instruments’ heights (2 m, 1.25 m, and 0.55 m), were small. The mean value of the dif-ference in temperature between the instruments was 0.0^◦C for all three heights, as could be seen in table 4.1. The mean value does not give insight into the spread of the data, which is expected to be higher for instruments mounted at lower heights. As table 4.1 shows, the standard deviation was 0.1^◦C for the difference between temperature at U and C at 2 m. This is 0.1^◦C smaller than the standard deviation equal to 0.2^◦C for this difference at 1.25 m and 0.55 m. This tells us that by evaluating the data from the ex-periment’s duration in 2019 (23/5-30/10), the increased grass height at U does not seem to have had a notable impact on the temperature. A more comprehensive evaluation of the temperature data that were presented in chapter 4 will follow in this chapter.

The increased vegetation height at U had no influence on the values of the difference between temperature measured at U and C at 2 m in the monthly time scale. The monthly mean and standard deviation values of (T_U−T_C) at 2 m is presented in table 4.2.

No development or pattern of these values throughout the experiment’s duration in 2019 can be seen. Increased grass density leads to a subdued variation of diurnal temperature.

If the grass at U had a large effect on the measured temperature, one would expect the difference between temperature values measured at U and C to steadily increase. This would result in an increase in the monthly standard deviation of (T_U−T_C). The standard deviation is 0.1^◦C in all five months, meaning this expected pattern cannot be seen.