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AN INITIAL LOOK AT THE IMPACT OF THE NEW ANTENNA LOSS MODEL Joe Tenerelli SMOS QUALITY WORKING GROUP #4 7-9 March 2011. OVERVIEW.

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AN INITIAL LOOK AT THE IMPACT OF THE NEW ANTENNA LOSS MODEL

Joe Tenerelli

SMOS QUALITY WORKING GROUP #4

7-9 March 2011

slide2

OVERVIEW

  • The Level 1 Team has developed a new antenna loss model that should help to correct for the impact of variations of the antenna physical temperatures on the reconstructed brightness temperatures.
  • In the following slides we will:
  • Review the bias problems in the SMOS brightness temperatures over the ocean;
  • Show that the direct sun correction algorithm may induce significant latitudinal variations in the bias between model and data over the open ocean;
  • Show that the direct sun correction may not behave as expected when the sun is in the back half-space of the array (Jerome’s presentation).
  • Look at the impact of the new loss model on a small set of Pacific Ocean orbits reprocessed by Roger Oliva (drift with latitude and temporal evolution).
  • Caveat: Given potential impact of the direct sun correction on the bias variations, we must use caution in interpreting the results.
slide3

10-DAY MEAN SSS BIAS: DESCENDING PASSES

Problem 1: The differences between SMOS reconstructed brightness temperatures produced by DPGS and those predicted by our ‘best’ forward scene model drift with time.

Certainly, a portion of this drift may be related to problems in the model (sun glint, galactic noise reflection). Another portion may originate with the instrument and Level 1 processing.

slide4

10-DAY MEAN SSS BIAS: DESCENDING PASSES

Problem 1: The differences between SMOS reconstructed brightness temperatures produced by DPGS and those predicted by our ‘best’ forward scene model drift with time.

Certainly, a portion of this drift may be related to problems in the model (sun glint, galactic noise reflection). Another portion may originate with the instrument and Level 1 processing.

+0.5 K (Tx+Ty)/2 -1 psu

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Sensitivity of the first Stokes parameter to a 1 psuincrease in SSS:

Roughly 1 K/psu in (Tx+Ty) in warm water

Roughly 0.5 K/psu in (Tx+Ty) in cold water

Best case (warm water): +.5 K (Tx+Ty)/2 -1 psu

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10-DAY MEAN SSS BIAS: ASCENDING MINUS DESCENDING PASSES

MID-MAY

MID-AUGUST

Differences in SSS retrieved from descending and ascending passes can reach more than 1 psu and change (even in sign) with time.

END AUGUST

MID-NOVEMBER

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BIAS TRENDS OVERVIEW

Several months ago it was noticed that, in terms of descending-ascending pass differences, there is a qualitative correlation between Tp7 and SMOS-model bias variations as a function of latitude and time:

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BIAS TRENDS

We have also seen discontinuities in the bias trends that seem to correlate with the sun passing between the front and back half-spaces. More on this later…

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BIAS TRENDS

The discontinuities in the bias as well as the correlation between Tp7 and the bias trends remain in the reprocessed data:

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BIAS TRENDS

However, the correspondence between Tp7 and the bias is less clear when we apply the simplified reconstruction, with no direct sun correction, to the reprocessed Level 1A files:

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TP7 VARIATIONS AND SUN GEOMETRY

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TP7 VARIATIONS AND SUN GEOMETRY

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TP7 VARIATIONS AND SUN GEOMETRY

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LATITUDINAL BIAS DRIFT

We have also seen bias drifts with latitude, especially in November 2010

SSS bias decrease with latitude for descending passes is associated with (Tx+Ty)/2 increase by about 1 K (red curve).

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BIAS TRENDS IN TERMS OF SSS BIAS

For the November descending passes, the DPGS SSS bias drift with latitude is consistent that the drift in AF-FOV bias in (Tx+Ty)/2:

DPGS DESC-ASC

JRECON DESC-ASC

DPGS (Tx+Ty)/2 increase

With latitude

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BIAS TRENDS

DPGS – JRECON DESC-ASC FOR NOVEMBER 2010: POSSIBLE IMPACT OF DIRECT SUN CORRECTION

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IMPACT OF NEW LOSS MODEL

Roger Oliva has reprocessed several orbits using the new antenna loss model implemented by DEIMOS. Results suggest that latitudinal drift in (Tx+Ty)/2 is reduced using the new loss model with the L1PP image reconstructions. Nevertheless, the drift is reduced further when we switch to the simplified reconstruction with no direct sun correction:

Original loss model

New loss model

Extension to LICEFs

November 9 Pacific Descending (red) vs Ascending (blue) Passes

Fixed-Cal Reprocessing solutions (includes NIR AB)

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IMPACT OF NEW LOSS MODEL

A similar result on November 10:

Original loss model

New loss model

Extension to LICEFs

November 10 Pacific Descending (red) vs Ascending (blue) Passes

Fixed-Cal Reprocessing solutions (includes NIR AB)

slide20

IMPACT OF NEW LOSS MODEL

In June, when the direct sun correction has less impact than in November, drift with latitude is not a large problem in any solution. Actually, it seems the direct sun correction may introduce an offset that is largely independent of latitude (Needs to be confirmed using L1PP). In any case, the new loss model does improve agreement between ascending and descending passes.

Original loss model

New loss model

Extension to LICEFs

June 13 and 14 Pacific Descending (red) vs Ascending (blue) Passes

Fixed-Cal Reprocessing solutions (includes NIR AB)

slide21

BIAS TRENDS OVER TIME

With the old loss model, differences between descending and ascending pass SMOS-model biases are significant and drift over time, as we have observed for a while:

L1PP Level 1B

JRECON

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BIAS TRENDS OVER TIME

With the fixed calibration used to obtain the reprocessing solutions, ascending/descending differences remain significant but the drift over time of these differences seems to be reduced:

L1PP Level 1B

JRECON

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BIAS TRENDS OVER TIME

With the new antenna loss model (NAM, here applied only to the NIRs), differences between descending and ascending pass solutions are much reduced, but the overall drift over time reaches about 0.8 K:

L1PP Level 1B

JRECON

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BIAS TRENDS OVER TIME

Extension of the loss model to the LICEFS makes only a small impact in the bias trends over time:

L1PP Level 1B

JRECON

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CONCLUSIONS

Relative to our best available ocean scene model, significant biases exist in all SMOS-derived brightness temperatures over the alias-free field of view. The biases seem to have an impact on retrieved SSS that is roughly independent of longitude (see the SSS bias maps).

These biases drift with latitude (short time scale) and slowly over time (weekly to monthly time scales).

The latitudinal bias drift appears to involve both bias in the calibrated visibilities and bias introduced by the direct sun correction algorithm (see also Jerome Gourrion’s presentation).

The new antenna loss model reduces discrepancies between descending and ascending pass biases, but also increases the bias trend with time.

slide27

LAND CONTAMINATION AND BIAS

Maps of bias between SSS derived using DPGS L1B data and SSS derived using L1A and the JRECON breadboard are shown below.

The strong land contamination halos in the DPGS solutions are not as evident in the JRECON solutions.

For the descending passes, the linear bias gradient with latitude in the DPGS solutions (lower left) is not seen in the JRECON solutions (lower right). For all maps the sets of fixed OTTs shown earlier were used for both DPGS and JRECON solutions.

DPGS ASC

JRECON ASC

DPGS DESC

JRECON DESC

slide33

The difference between the two preceding maps gives an indication of how the model is in error. Here we have taken just the SMOS sky map produced from data corresponding to ECMWF 10-m wind between 3 and 6 m/s and we subtracted the 3 m/s model solution. The difference map shows how the model underestimates the data in the vicinity of the galactic equator by over 1 K.

Next we will compare the data and model in more detail along the cross section shown by the magenta line segment.

SMOS-MODEL

slide34

This is a cross section through the dwell lines near the galactic equator along the magenta line segment in the previous slide. The solid black curve is the ideal flat surface solution, the dashed black curve is the 3 m/s solution, and the colored curves are the actual SMOS data in various wind speed ranges with all geophysical sources except galactic noise removed. These curves are averages over celestial dwell lines and thus do not correspond to a specific viewing geometry.

It is obvious from this plot that the actual SMOS data have peaks that are between the flat surface solution and the 3 m/s solution. However, away from the galactic plane the flat and rough surface solutions are very similar so that the actual SMOS residual is NOT A SIMPLE LINEAR COMBINATION of the flat and 3 m/s model solutions.

Colored SMOS residualsky noise lies between the flat and rough surface model solutions (black curves) here.

Colored SMOS residualsky noise curvesdo not lie between the flat and rough surface model solutions here.

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TP7 VARIATIONS

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TP7 VARIATIONS: SUN GEOMETRY

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TP7 VARIATIONS: SUN GEOMETRY

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TP7 VARIATIONS: SUN GEOMETRY

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TP7 VARIATIONS: SUN GEOMETRY

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