In accordance with thedecision to reconfigure the A-Train formation, the rephasing of the Aura satellite is progressing as planned, and the satellite continues to drift "forward", closer to CALIPSO, CloudSat, and Aqua. The Aura drfit will continue until the MLS retrieved column collocates with CALIPSO (and hence CloudSat), which is expected to be accomplished by the summer of 2008. One of the immediate substantial effectsof this maneuver is that the footprint of the MLS retrieved column drifts Eastwards, away from the MODIS/Aqua nadir.
Beforethe rephasing started, and since the Aura launch, the MLS retrieved column and the MODIS nadir field of view (FOV) used to be on average within 6 kilometers of each other in terms of lateral separation, Fig.1 . As of the end of 2007 however, the lateral arc distance at the Equator between the MLS retrieved column and the MODIS nadir was already on the order of 30 km. Furthermore, the lateral arc distance of their separation now varies substantially with the latitude.
Consequtively, the MLS and MODIS collocation cannot be executed anymore by the simple means of extracting the nadir (median) frame from each MODIS scan line. Thus A-Train Data Depot discontinued the production of the MODIS MLS-collocated subset for the time being, but can resume it with a more sophisticated algorithm, like the one we use for the MODIS CloudSat-collocated subset, if there is a demand for such a product.
Figure 1. The longitudinal (lateral) separation between the MLS retrieved column and the MODIS FOV nadir measurements, in terms of arc distance (km), and also the time difference (sec), for each equatorial crossing event (ascending orbit). FOV geolocation data from eachinstrument were used to calculate a precise estimate of the times and longitudes for each instrument's equatorial crossing event. A positive time difference implies MLS lags behind the MODIS measurement time and a positive arc difference implies that the MLS measurement is to the east of the MODIS nadir FOV measurement.