There are different ways
to calculate daily mean temperature (DMT) at a station from data
collected at different times of the day. In many countries,
including the US, the approach is to average the minimum and
maximum temperature observed, although this may be the minimum and
maximum hourly readings or the actual minimum and maximum obtained
from minimum and maximum thermometers or other devices (see WMO,
2008, for details about temperature measurements). In other
countries a linear combination of measurements taken at different
times of the day is used, sometimes including the minimum and maximum as well. For
example, the Nordic countries each have a different linear
combination of data, depending on the frequency of recorded
observations (Nordli
et al., 1996, Appendix II), while Germany employs yet
another linear combination of data. When using temperature data
for climatological purposes, such as calculating the uncertainty
of estimates of global mean temperature, it is important to take
into account the bias and variability of the method used to
calculate DMT.
In order to assess the bias and variability of various approaches to
estimating DMT, we can look at data collected at different
frequencies. The finest resolution available is minute data, which
we have available for a station in Sweden (although not collected at
the standard 2m altitude above ground) and for several North
American stations. Data sets will be made available at the course
page.
Questions of interest include
How well does the average of daily min and daily max
approximate the daily mean temperature?
What is the effect on bias and variability of different
observational schemes (such as hourly, 3-hourly, 6-hourly etc.)
Does daily min and daily max add value to measures of daily
mean temperature, taken several times a day (such as the Swedish
Ekholm-Modén formula as described in Nordli et al, 1996,
Appendix II)
Does it matter much when using the average of min and max
whether you use actual min and max or min and max of hourly
readings?
If daily temperatures were described by a sine curve, the
average daily temperature would indeed be the average of min and
max. How well is daily temperature described by a sine curve?
The groups need to decide on one or two questions to answer, develop
a strategy for answering them, and give a 30 minute oral
presentation of their results to the class. The 30 minutes include
questions, so you need to leave enough time for that. You can use
any presentation software you like (including the white boards). The
due date depends to some extent on class progress, but I am aiming
at finishing this project no later than 23 September.
Visby minute data for January (column
14 is surface temperature)
Visby minute data for July
Minute data from Red Oak, TN