Mean and extreme daily air temperature trends (Diekirch, Luxemburg): 1998 to 2003

file: dailymin_max_1998_2003.html

Francis Massen

Abstract:

Both daily mean and daily maximum air temperatures at Diekirch show a positive trend from 1998 to 2003, whereas the mean daily minimum temperature is decreasing, and as a consequence the difference between minimum and maximum temperature has a positive tend. This is contrary to the findings of the IPCC for the years 1950 to1993.
This report on a much too short time-span should  not be taken as a proof of an ongoing global warming trend!


1. Linear trends from 1998 to 2003 (6 years)

Temperature trends for the Northern Hemisphere landbased stations usually show a mean daily temperature increase with the following pattern 
[1] [3]:

- the daily mean temperatures have a positive trend
- the positive trend of the daily minimum temperatures is about ( or at least) 2 times greater than the positive trend of the daily maximum temperatures
- as a consequence the difference between maximal and  minimal daily temperatures (DTR) is decreasing

This short report checks the meteoLCD data of the last 6 years (http://meteo.lcd.lu/data/2003_OKK_fm.zip) for similar trends, with a partial comparison to the data of the Findel airport station (www.airport.lu); the measuring conditions have been practically been unchanged at meteoLCD (same sensors, relocation distance less than 50m); the analysis is restricted to the yearly means computed from the set of daily means.

parameter mean values 
meteoLCD
yearly trend [C/year]
yearly trend for Findel [C/year]
 [Drogue, ref.2]
meteoLCD
1998..2003
Findel
1971-2000
yearly 
mean temperature
10.3 8.8 + 0.11 1977..1998: 0.07
mean of daily minimum temperature 6.1 5.1 - 0.04 1949..1998: + 0.03
mean of daily maximum temperature 14.5 12.7 + 0.28 1949..1998: + 0.07
mean of daily delta temperature
DTR
8.4 7.6 + 0.33 1949..1998: + 0.04

The graph shows that the minima temperatures have a slight negative trend, the maxima a visible positive trend of 0.28C/year; the latter is mostly caused by the extreme hot 2003 summer; without the year 2003 the trend of the mean daily maxima is only 0.19C/year..
The trend of the mean difference of maxima and minima temperatures DTR is +0.33C/year.

All these findings are opposite to the [1950-1993] trends given by the IPCC and Karl, except the positive trend in mean and maxima temperatures.
Drogue [2] finds in his analysis on the 1949 to 1998 Findel airport data both trends being positive, the maxima increasing at a double rate to the minima (again opposite to IPCC).
The magnitudes of the meteoLCD maxima trends are (not unexpectably) much higher than those usually given for larger time-spans. 


2. Trend in daily extremes [min(min) and max(max)]

Is there an evolution to more extreme temperatures?
Checking for the yearly lowest daily minimum  ( = min. of min, as opposed to mean of min. used in the previous chapter) and highest daily maximum temperature gives the following trends (statistically significant at 95%):

parameter yearly trend [C/year]
min of daily Tmin - 0.49
max of daily Tmax + 0.60

Both trends are significative, and show (surprisingly!) extreme colds becoming colder and (as expected) maxima temperatures rising.

The percentage of days where the temperatures are lower or higher than the means are close to 50% (mean and median are close, as confirmed by calculation) and do not show a visible trend: even if the extreme temperatures seem increasing, one can not affirm that the number of the "extreme" days augments. 

 
year number of days where temperature is lower 
than mean
number of days where temperature is higher 
than mean
1998 159 = 44% 175 = 49%
1999 171 = 51% 163 = 49%
2000 163 = 47% 159 = 49%
2001 162 = 45% 178 = 48%
2002 177 = 48% 180 = 49%
2003 187 = 51% 169 = 46%


3. Conclusion

Yearly mean and mean daily maxima temperatures are increasing during the last 6 years, whereas the daily minima are slightly decreasing.
The rate of the positive trends is very high, compared to the findings made on much larger data sets. Extreme temperatures seem to be drifting in opposite directions.
Temperature trends should never be accepted on a time-span less than 30 years, so this short report should under no circumstances be taken as a proof for a definitive ongoing  warming, but only as a quick glance on the last 6 years pattern.


4. References

1 IPCC, 2001 : Third Assessment report. www.ipcc.ch
2 Drogue G. et al : Recent warming in a small region with semi-oceanic climate, 1949-1998: what is the ground truth?
drogue@crpgl.lu. Submitted to Theoretical and Applied Climatology.
3 Karl T. et al : Asymetric Trends of Daily Maximum and Minimum Temperature. Bulletin of the American Meteorological Society, Vol.74, No.6, June 1993, p. 1007-1023