Posts Tagged ‘troposphere’

Klotzbach Revisited

March 1, 2013

Guest blog by Jos Hagelaars. Dutch version here.

The average surface temperature of the earth, measured by ‘thermometers’, are released by a number of institutes, the most well-known of these datasets are GISTEMP, HadCRUT and NCDC. Since 1979 temperature data for the lower troposphere are released by the University of Alabama in Huntsville (UAH) and Remote Sensing Systems (RSS), which are measured by satellites.
The temperatures of these two methods of measurement show differences, for instance: the NCDC data indicate a trend over land of 0.27 °C/decade for the period 1979 up to and including 2012, while over the same period, the trend based upon the satellite data by UAH over land is significantly lower at 0.18 °C/decade. In contrast, the trends for global temperatures indicate much smaller differences, for NCDC and UAH these are respectively 0.15 °C/decade and 0.14 °C/decade for the same period.

Big deal? Almost everything related to climate is a ‘big deal’, so it is of no surprise that the same applies to these trend differences. In a warming world it is expected that the temperatures of the upper troposphere increase at a higher rate than at the surface, regardless of the cause of the warming. The satellite data (UAH and RSS) do not reflect this. Why is the upper troposphere expected to warm at a higher rate and what is the cause of these trend differences between the surface  and satellite temperatures?

The temperature gradient in the troposphere / the ‘lapse rate’

When you go up in the troposphere it gets colder. This is caused by the fact that rising air will cool down with increasing altitude due to a decrease in pressure with altitude, by means of so-called adiabatic processes. This temperature gradient is called the lapse rate, a concept one will frequently encounter in papers regarding the atmosphere in relation to climate. When the air is dry, this temperature drop is about 10 °C per km. When the air contains water vapor, this vapor will condense to water upon cooling as a result of the rising of the air, which releases heat of condensation. So in this way, heat is transported to higher altitudes and the temperature drop with height will decrease. For air saturated with water vapor, this vertical temperature drop is approximately 6 °C per km.

When the earth gets warmer, air can contain more water vapor. This also has an impact on the lapse rate, since more water vapor means more heat transfer to higher altitudes. This effect on the lapse rate is called the lapse rate feedback. More heat at higher altitudes implies that there will be more emission of infrared light, a negative feedback. This effect is particularly important in the tropics. At higher latitudes, the increase in temperature at the surface is dominant, therefore the change in the lapse rate will turn into a positive feedback. See figure 1 (adapted from the climate dynamics webpage of the University of Leuven).

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