Disaggregating L2 SM product during AACES’10

Category : Cal/Val, Data, L2

Validating SMOS data is complicated by the large observation scale of spaceborne L-band radiometers. To circumvent the direct comparison between 40 km resolution SMOS data and point-scale in situ measurements one may either upscale ground measurements with aggregation rules, or downscale (disaggregate) remote sensing data at the representativeness scale of ground measurements. Here, we are investigating a combination of both bottom-up and top-down approaches to make ground truth and remote sensing observations match at an intermediate spatial scale.

A disaggregation methodology based on 1 km resolution MODIS (MODerate resolution Imaging Spectroradiometer) data is implemented over the Murrumbidgee catchment during the AACES’10 campaign. The SMOS L2 SM product extracted from UDP files is disaggregated at 4 km resolution on a projected grid (UTM 55S). Only the disaggregation part is presented. As a follow-up, the disaggregated SM product could be compared against the average of the ground measurements made within 4 km pixels.

Overview of the AACES’10 flight lines and ground sampling areas. The study area is 600 km (W-E) by 240 km (S-N).

Overview of the AACES’10 flight lines and ground sampling areas. The study area is 600 km (W-E) by 240 km (S-N).

Images of the disaggregated SM product on three dates are presented below. They correspond to days with one or two SMOS overpasses (at 6 am or 6 pm) and two clear sky MODIS overpasses (10 am/Terra and 1 pm/Aqua). Note that the L2 product is provided on a grid (DGG) whose resolution (about 15 km) is finer than the resolution of SMOS radiometer (about 40 km). In fact, SMOS oversamples the observation scene by a factor (40/15)x(40/15)=7. This allows running the disaggregation algorithm at each point of the DGG and applying additional constraints on the disaggregated SM in the overlapping edges of adjacent 40 km pixels. Moreover, random errors in the disaggregated SM product can be estimated as the standard deviation of the different disaggregated SM values obtained using MODIS data collected at both 10 am and 1 pm and using the oversampling of SMOS data.

L2 SM product is disaggregated at 4 km resolution over the AACES'10 study area.

L2 SM product is disaggregated at 4 km resolution over the AACES'10 study area.

Gaps in the disaggregated SM product are partly due to cloud cover (availability of MODIS data), but mainly to gaps in the L2 SM product. Currently, many SM values retrieved by SML2PP are negative, and negative values are systematically set to -999 (inversion failed) in the UDP files provided by the DPGS. Note that the commisioning phase only takes care of brightness temperatures, not retrieved parameters.

In the images above, a « boxy artifact » is still apparent at the low (DGG) resolution. This artifact could be explained both by the parameterization of the disaggregation algorithm, which is done at low resolution from SMOS and aggregated MODIS data, and by errors in L2 product. Analyzing the time series of the disaggregated product and more specifically the persistency of disaggregation parameters may help quantify both effects and hence evaluate SMOS data at the intermediate resolution of 4 km.

Back from Kangaroos land

Category : Cal/Val

After 5 weeks spent in Australia for the AACES campaign, Arnaud Mialon, Claire Gruhier and Delphine Leroux are back in Europe at Cesbio. During these 5 weeks, we have met different soil types : very dry and flat with low vegetation in the western part and even smooth green grass with trees in the eastern part (as mentioned before in Ahmad and Arnaud’s articles, several rain events occured after the 5th of February).

Patch 1 (dry, flat, ...)

Patch 1 (dry, flat, ...)

Patch 9 (green grass, trees, ...)

Patch 9 (green grass, trees, ...)

For each focus farm, we walked a 5 km transect and every 50 meters we did 3 soil moisture measurements with the hydra probe. Moreover, we took some vegetation, gravimetric and dew samples, LAI, surface reflectance (ASD) and roughness measurements. 1 TIR and 1 monitoring stations were installed as well (temperature sensors, theta probes, TIR, rain gauge, leaf wetness).

Patch example : 6 sampling lines + 2 stations

Patch example : 6 sampling lines + 2 stations

Hydraprobe Data Acquisition System

Hydraprobe Data Acquisition System

Installation of a monitoring station

Installation of a monitoring station

TIR station all set

TIR station all set

This was also the occasion to see many beautiful sunrises and sunsets.

Patch 10 - sunset

Patch 10 - sunrise

Patch 10 - sunset

Patch 10 - sunset

In a nutshell, this was an amazing experience for all of us ! Many thanks to the University of Melbourne and especially to Jeff, Chris and Sandy for the organization of this campaign !

AACES participants

AACES participants

Arnaud, Claire & Delphine

It was raining on Australia ? …. Did SMOS see it !

Category : Cal/Val, L2

During the AACES campaign in Australia several rain events occurred as Arnaud and Chris mentioned in earlier blogs. Everyone was happy about it (may be not the guys on the field working in the mud) and eager to see SMOS data.

Well, figure 1-a shows the soil moisture over four controls points near Charleville Queensland (see figure 2). The control points shown here are not exactly inside the campaign area, but rain occurred at these locations at least this is what ECMWF forecast predicts (figure 1-c) and what was observed by a nearby weather station in Charleville Queensland (figure 1-b).

rain_australia

In the plot above, Ascending ( circle symbol) and descending (diamond symbol) orbits are considered from 14/01/2010 till 12/02/2010. Only pixels where the inversion was successful are plotted. The points have nominal surfaces with nearly no vegetation (so no optical thickness is inversed). The Soil moisture increases after the rain event and it follow an exponential decay as expected. The event of 30 January 2010 is well depicted.

From the curves it is clear that SMOS Level 2 processor can already deliver valuable data over nominal surfaces. The Level 1 data used here are not optimal. Several corrections has been omitted and the L1 processor version is not the final. So we can expect even better data in the nearby futur.

I wan’t be long in this blog, many comments can be made but we will leave it for later.

 

SM_OPER_MIR_SMUDP2_20100114T202357_20100114T211758_305_001_1_Soil_Moisture

 

Cheers

Ahmad

 

Australian Airborne Cal/Val Experiment for SMOS

Category : Cal/Val

Australian Airborne Cal/Val Experiment for SMOS
1st Field Experiment currently under way

A very warm ‘G’day mate’ from Australia!

The first Australian Airborne Cal/Val Experiment for SMOS (AACES) is under way since 18 January 2010 in the Murrumbidgee catchment in southern New South Wales, Australia. Given the recent launch of SMOS, AACES represents the first extensive and probably most comprehensive of such experiments undertaken during the commissioning and later operational phase of SMOS. The project is led by Jeff Walker and Chris Rudiger from the University of Melbourne and supported by the invaluable help of their PhD students. Moreover, thanks to the generosity of various research institutes from France, Germany, Denmark, Poland, and the Netherlands in addition to the flights covered by ESA, a total of 14 people have joined this campaign to boost the manpower of the sampling teams.
The Murrumbidgee catchment is unique as it comprises a distinct variety of topographic, climatic and land cover characteristics, and therefore represents an excellent validation site for the land component of this satellite mission. A total of 10 patches of 100km x 50km (aligned with the synthetic SMOS grid, and therefore including two independent SMOS footprints) are covered by an aircraft carrying an L-band radiometer (www.plmr.unimelb.edu.au) and VIS/NIR/SWIR/TIR sensors. With a flight altitude, the ground pixel resolution is 1km, resulting in an almost complete coverage of the catchment (50,000 of 70,000km2). The aircraft flights are aligned with SMOS morning overpasses. At the same time, two ground teams are covering an area of 5km x 2km each, collecting soil moisture (50m spacing along the 5km transects), soil temperature, and soil salinity. A sub-group of each team is meanwhile collecting LAI, dew, destructive vegetation samples, and hypersepctral measurements of the surface conditions.
So far, eight patches have been covered (see below for some examples of the aircraft data). We started off in the west of the catchment, near the twon of Balranald, with temperatures around 46C at 2pm (measured in the shade, if there were any…..) and soil moisture conditions, in which 8% was considered an exceptional spike. Though, we have also just spent a week-end observing a rainfall event of 100mm over two days in the area of the Australian capital of Canberra, which, in a region that usually has 600mm as an annual average, is a fairly good quantity. Creeks that haven’t seen any water since 1998 have overflown, big trees have been uprooted…. We sampled today and values easily reached 40% soil moisture. The aircraft data promises quite some contrasting responses in the SMOS observation. We are certainly looking forward to those data.
The earnest post-processing of all the data will begin, when we will be back in Melbourne in 10 days time. If interested you can contact Jeff (j.walker@unimelb.edu.au) or Chris (crudiger@unimelb.edu.au) directly for more information. I will try to put some more infos on this blog in the next few days…..

Until then.
Cheers, mates.
Chris

Post scriptum: Despite the fact that Australia habours the most deadly snakes, spiders, jellyfish, krokodiles, sharks, and a few less mortal but not less vicious animals, in addition to an environment in which drinking 6l of water still means that you are thirsty, no participant has been harmed so far…. ;-)

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