Intersatellite Calibrated HIRS Upper Tropospheric Water Vapor

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1 Intersatellite Calibrated HIRS Upper Tropospheric Water Vapor Lei Shi and John J. Bates NOAA National Climatic Data Center Asheville, North Carolina, U.S.A. 1 NOAA National Climatic Data Center ITSC-XVI

2 Outline Motivation Time series discontinuity from satellite to satellite, particularly from TOVS to ATOVS Upper tropospheric water vapor (UTWV) is an important fundamental climate data record (CDR) UTWV is key to water vapor feedback Approach New and improved intersatellite calibration based on two datasets Overlaps of zonal means Simultaneous nadir overpass (SNO) Result Extended time series of the fundamental CDR to present 2 NOAA National Climatic Data Center ITSC-XVI

3 Motivation Uncorrected Intersatellite Differences of UTWV (Channel 12) 30S - 30N N06 N07 N08 N09 N10 N11 N12 N14 N15 N16 N17 Start of HIRS/ Due to the independence of individual HIRS instrument s calibration, biases exist from satellite to satellite. These intersatellite biases have become a common source of uncertainty faced by long-term studies. 3 NOAA National Climatic Data Center ITSC-XVI Year

4 Spectral Filter Functions 1 HIRS/2 HIRS/3 N06 N N08 N N10 N11 N12 N14 N N Wave Number (per cm) Differences between HIRS/2 and HIRS/3 are expected due to different filter functions. In-orbit performance still has biases unexplained by filter functions. Thus empirical approach is considered. 4 NOAA National Climatic Data Center ITSC-XVI N17 Tran

5 Approach Two Datasets for Intersatellite Calibration to Cover Diverse Atmospheres Overlaps of zonal means for tropical and midlatitude atmosphere cases Zonal means of channel 12 (UTWV) are computed for every 10-degree latitude belt. Differences are derived from overlapping satellites. Simultaneous nadir overpass (SNO) observations for polar cases SNOs occur when two satellites cross each other Data taken at the same location at the satellite nadir within a few seconds In the regions 70-80N and 70-80S 5 NOAA National Climatic Data Center ITSC-XVI

6 Bias (K) Temperature-dependent Intersatellite Differences from Zonal Mean Approach Tropical and Mid-latitude Cases T_N N06 N06 N07 N07 N08 N09 N10 N10 N11 N11 N12 N12 N14 N14 N15 N15 N16 N16 N17 N17 M02 Bias (K) T (K) T (K) More than half of satellites have bias variations larger 6 NOAA than National 0.5 K. Climatic Data Center ITSC-XVI

7 Temperature-dependent Intersatellite Differences from SNO Approach Polar Cases N6-N7 N7-N8 N9-N10 N10-N11 N11-N12 N12-N14 N14-N15 N15-N16 N16-N N6-N7 N7-N8 N9-N10 N10-N11 N11-N12 N12-N14 N14-N15 N15-N16 N16-N CH T(K) CH Biases are also temperature dependent. 7 NOAA National Climatic Data Center ITSC-XVI T(K)

8 Extension of Time Series Orbital, daily, and monthly, 1979-present At both pixel resolution and 2.5x2.5 lat/lon grids Intersatellitecalibrated S - 30N V1 V2 Year * V1 is derived based on methods discussed in Bates, J. J., X. Wu, and D. L. Jackson, (1996) Interannual variability of upper-troposphere water vapor band brightness temperatures. J. Climate, 9, NOAA National Climatic Data Center ITSC-XVI

9 Intersatellite Calibrated to N-12 (showing 30S 30N) Biases minimized. Temperature dependent biases accounted for. Similar overall variances between HIRS/2 and HIRS/3. Time series can be extended as variance preserved Year 9 NOAA National Climatic Data Center ITSC-XVI N06 N07 N08 N09 N10 N11 N12 N14 N15 N N17

10 Climatologic Mean for Jan, Apr, Jul, and Oct 10 NOAA National Climatic Data Center ITSC-XVI

11 Seasonal Variation Within tropics, drier atmosphere produces a higher value of UTWV (channel 12 sees lower atmosphere). Minima in the tropics indicate moist upper troposphere caused by convection. Maxima in northern tropics in January move to southern tropics in July, indicating the move of dry regions in upper troposphere from north to south. Zones of low values indicate positions of ITCZ. 11 NOAA National Climatic Data Center ITSC-XVI

Difference of UTWV between 2002-2003 El Niño and

12 Difference of UTWV between El Niño and La Niña 12 NOAA National Climatic Data Center ITSC-XVI

13 UTWV Time Series (showing 30S 30N) Year UTWV anomaly 13 NOAA National Climatic Data Center ITSC-XVI

14 Long-term UTWV Anomaly Dataset 7.5S-7.5N 14 NOAA National Climatic Data Center ITSC-XVI

15 UTWV Has Good Coverage for Tropical Convections OLR HIRS Upper Tropospheric Water Vapor Obtained from 15 NOAA National Climatic Data Center ITSC-XVI

16 Conclusions Improved intersatellite calibration. Temperature dependent inter-calibration Extension of time series to current. HIRS/2 and HIRS/3 series connected The HIRS UTWV anomaly data are useful in monitoring Madden-Julian oscillation and various equatorial waves. 16 NOAA National Climatic Data Center ITSC-XVI

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