NO2 Diurnal Variations

The UARS satellite describes a non-sun-synchronous orbit. If the orbit were sun-synchronous, then the observation of each latitude in each orbit would correspond to two fixed local times (one for ascending and one for descending orbits). This is not the case and each day corresponds to a drift in local time of some 20 minutes.

In some 30 days, then, ascending and descending node observations will cover all local times. In practice, the geometry is not that simple, but most local times will be sampled at most latitudes. If all variation in the signal were diurnal, these monthly observations would directly show the local time dependence of the NO2 signal. In practice, this local time variation is combined with other time variation of the signal and is fuzzed out some.

If we take the results of a Grose model run and evaluate the field at longitude zero over the course of a month or so, we see the basic results:

GROSE Model NO2

We can fly the UARS satellite through this data and simulate a satellite dataset with its coverage reduced due to the realistic orbit track. We can then Kalman filter the ascending and descending node profiles separately as well as local time. Then each day's mapping at a given latitude corresponds to a given local time. We can test the ability of the sampled asynoptic data to show local time variation just as we can with the true synoptic data above:

GROSE Model NO2 Mapping

So, we see that at least in the Grose model case, the diurnal variation of NO2 is easily recovered from asynoptic data such as we can obtain from a satellite. We can then apply this to the CLAES instrument aboard the UARS satellite:

CLAES NO2 Mapping

We see qualitatively that the actual and model data agree well in their diurnal signal.