MEETING REPORT GPM Intersatellite Calibration Working Group (X-CAL) University of Michigan July 11-12, 2012

Transcription

1 MEETING REPORT GPM Intersatellite Calibration Working Group (X-CAL) University of Michigan July 11-12, 2012 ]On July 11, 2012 at 9AM, a meeting of the GPM Intersatellite Calibration Working Group (X- CAL) was convened at the Space Research Building of the University of Michigan, graciously hosted by Darren McKague. After a welcome and logistical information from Darren McKague (U.Michigan), Tom Wilheit (Texas A&M Univ.) covered several small issues and presented his goals for the meeting. The participants indicated that 6 papers had been submitted and accepted for the Special Issue of IEEE Transactions on Geosciences and Remote Sensing. All X-CALers were encouraged to submit abstracts for the upcoming Fall AGU meeting. It was also pointed out that there is a TRMM/GPM science meeting planned for November of 2012 in Tokyo. Megha-Tropiques data were discussed and it was concluded that the working group needs to have at least two months of data from both Saphir and Madras in hand at least 3 months before any planned meeting. Wilheit suggested that since we don t have any real standard for radiances in the GHz region, the working group should take the TMI 85.5 GHz channels as the standard at least up to the GMI launch. The working group agreed. [This suggestion was ratified the following week at the Radar Enhanced Radiometer Algorithm meeting.] Keiji Imaoka (JAXA) could not make it to the meeting but he ed a presentation which was shown by Tom Wilheit. The GCOM-W spacecraft with the AMSR-2 instrument was successfully launched on May 18. Data collection began on July 2 and the presentation included several first light images. Public release of the Level 1 data is planned for 8 months after launch (i.e. mid January 2013). JAXA plans a cross calibration of the AMSR using X-CAL methods. We were left with some confusion about the timing of data availability for the working group. The University of Central Florida team has obtained Windsat data covering the X-CAL year (July July 2006), 2011, and more recent data. The data are calibrated in a consistent manner and the calibration method has been documented. Thus, they provide an excellent test of the hypothesis, advanced by the CSU, team, that the TMI instrument has a secular calibration drift. UCF produced matchup data sets for both the X-CAL year and Spencer Farrar (UCF) presented an extensive analysis (done by Shadi Aslebagh) of the matchup data set. They showed an upper limit to the drift much smaller than that hypothesized by CSU, at least at 10 and 37 GHz. The results nearer the 22 GHz water vapor line were murkier. Tom Wilheit (TAMU) presented a quick analysis of the same data set than corroborated the UCF results. The UMI and CSU teams also plan to compare TMI with the new Windsat data set. Linwood Jones will take charge of pulling all the elements into a paper.

2 Wes Berg (CSU) gave an update on the SSM/I and SSMIS Fundamental Climate Data Record (FCDR) processing. He pointed out roll-offs at the edge of the scan in several channels of SSMIS. These will be documented carefully, but different users will want to use (or not use) the affected data differently. There were also a number of features in the legacy code that didn t quite make sense; these are being re-evaluated. These data sets are still a work in progress and the feedback from the X-CAL team is very useful in detecting and correcting problems. Rachael Kroodsma (UMI) presented the UMI analyses of SSM/I and SSMIS data. She showed some small scan dependence issues (within acceptable error bars) except for a few beam positions at each end of the SSMIS scans. The issues at the end of the scans were much like those shown by Wes Berg (above). She also presented deltas relative to CC_1.1 for the SSM/Is on F13, F14 and F15 and the SSMISs on F16, F17 and F18. As a bonus she presented updated values for the AMSRE-TMI (CC_1.1) deltas and an initial look at the Windsat-TMI differences based on the new Windsat data set. Spencer Farrar (UCF) presented the UCF analyses of SSM/I and SSMIS data. He showed scan angle dependences similar to those shown by Berg and Kroodsma. (i.e. within acceptable limits) but also showed disturbing Ascending/Descending differences. These results must be considered preliminary since 1) the input FCDRs themselves are not static and 2) they haven t implemented sun glint filters. Tom Wilheit (TAMU) presented analyses of SSM/I and SSMIS. He showed preliminary calculation of the deltas of the F13,F14 and F15 SSM/Is with respect to TMI CC_1.1. Superficially they look much like the deltas given by the UCF and UMI group but detailed analysis is yet to be done. He also showed scatter plots of several channels of SSMIS data. Several channels, particularly the 91 GHz channels of F16 and F17, showed unreasonable numbers of unphysical wild points; Wes Berg will follow up at CSU. Kyle Hilburn presented the Remote Sensing Systems (RSS) work on the full complement of SSM/Is and SSMISs. This effort is independent of X-CAL but, nevertheless, of interest. The approach is to adjust the instrument calibrations, including a non-linear correction, the atmospheric absorption model, and the ocean surface emissivity model to optimize the fit between the retrievals and in situ observations of Sea Surface Temperature, Sea Surface Wind and precipitable water. Hilburn announced that their atmosphere absorption model is now available to the community. In discussion, it was concluded that we are not yet in a position to wrap up the SSM/I, SSMIS work. We found enough problems to keep our inputs in a state of flux. We are still in the mode of providing feedback to CSU to help clean up the FCDRs that we use as inputs. Bill Blackwell of Lincoln Laboratories gave us an update on the Advanced Technology Microwave Sounder(ATMS) on the Suomi Natianal Polar-orbiting Partnership (NPP). AMTS has 22 channels ranging from 23 to 183 GHz and is an improved version of the AMSU-A and

3 AMSU-B (or MHS) combination. He reported that checkout is going well and that preliminary data should be available around the first of September. Fuzhong Weng of NOAA expanded on Blackwell s presentation. He primarily concentrated on the instrument calibration and showed that the absolute calibration appears to be good to the sub Kelvin level for nearly all channels. He and Xiaolei Zou (Florida State University) also presented a EOF based method for detecting RFI. It assumes that the first 3 EOFs represent the geophysical signal and the remaining EOFs are man-made signals. It appears to work well in areas where RFI is a reasonable expectation. Greenland serves as a counter-example. Linwood Jones presented an update on the MicroWave Radiometer (MWR) on the SAC-D spacecraft. The spacecraft and the MWR were contributed by the Argentine space agency and the Aquarius instrument by NASA. The unusual dual push-broom design of the MWR has resulted in many issues, but they seem to be adequately resolved. There are now 10 months of data in hand and the calibration appears to be stable when compared with Windsat. Tiger Yang (U. MD) presented an extensive analysis of the calibration of the FY3-B MicroWave Radiometer Imager(MWRI). In terms of frequencies and spatial resolution, MWRI is quite similar to TMI. He showed double differences with the F18 SSMIS as the baseline and also showed equivalent double differences for TMI and AMSR-E. With the TMI double differences he gave, it would be straightforward to compute adjustments for FY3-B to CC_1.1. The bottom line is that FY3-B MWRI data appear to be quite good. Wes Berg (CSU) also presented a succinct analysis of a sample of FY3-B MWRI data. He clearly showed that the FY3-B MWRI data are stable enough to be useful. In discussion, the Working Group was unanimous that these would be good data to add to the constellation. However, we are currently prevented from doing so by legal obstacles. The team requests that NASA management seek to resolve the legal issues. Bill Blackwell of Lincoln Labs (and a mere 35 co-authors) presented the MicroMAS program. It is a atmospheric temperature sounder with 9 channels straddling the 118GHz O2 line. While, strictly speaking, not part of X-CAL it is clearly of interest to most of the working group members. The entire satellite weighs only 4.5Kg (i.e. a little more than a gallon of milk) and uses only 12 Watts. NASA will launch the satellite into a low inclination orbit in 2013 or At this point, Vivienne Payne joined us by telephone for the sounder discussion. Jim Wang (SSAI) kicked off the sounder discussion with an examination of the MHS instruments on NOAA 18 & 19 and METOP. He made the comparisons two ways: using high latitude crossovers and by using radiosondes from ARM sites as transfer standards. One can see a clear annual signal in the results but yearly averages are stable to a few tenths of a Kelvin.

4 Saswati Datta (DIPCON) is new to our program. She has extended Jim Wang s work on the high latitude sounder matchups to more recent data as well as verifying his earlier work. In the overlap period, their results were identical to much less than 0.1K. Wenze Yang (NOAA) gave a presentation on his project to generate Fundamental Climate Data Records for AMSU (including AMSU-B and MHS). Several of the channels are of interest for precipitation retrievals. Although the project is in its early stages, we will eventually need to ingest the product into X-CAL. He has found that the scan bias asymmetries reported by several investigators differ among satellites but are stable in time. The Chinese FY-3 satellite also contains a water vapor sounder, the MWHS, which is much like AMSU-B and MHS. Xiaolei Zou of Florida State University presented an analysis of the biases in the radiances of the MWHS and compared them with the present operational set of MHSs and AMSU-Bs. She found similar biases suggesting that the calibration of the MWHS is consistent with the other water vapor sounders. She also found a high frequency scan position dependent bias with a period of about 2.5 beam positions. Rachael Kroodsma (UMI) discussed plans for sensitivity studies using the TMI 3A11 algorithm in collaboration with Long Chiu of George Mason University. 3A11 uses the 19V and 21V channels to infer monthly average rain rates over the 5 x5 oceanic boxes. The algorithm teams have never articulated requirements for the cross calibration. Sensitivity studies such as this will enable us to approximate the requirements on our intercalibration. Tom Wilheit (TAMU) also presented some simulation results. He uses the GPM_2014 algorithm which is an early version of the algorithm planned for use at PPS on all the constellation radiometers. In addition to being able to see the impact of small changes in the brightness temperatures over all backgrounds, the analysis provides a tool to test the stability of the algorithm. The algorithm being in a preliminary stage, it is too soon to draw any conclusions. This is merely a demonstration of a tool. The attendees agreed that, in the short term, wrapping up the TMI drift issue would be the highest priority and that those working on sounders should be prepared to move out rapidly when SAPHIR data become available. The uncertainties surrounding Megha-Tropiques data make planning the next meeting problematic but a winter meeting at UCF seems to make sense. Linwood Jones said he would be willing to host.

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