Corrections for Temporal Discontinuities in the OPI

Abstract

The longest record of precipitation estimated from satellites is the outgoing longwave radiation (OLR) precipitation index (OPI), which is based on polar-orbiting infrared observations from the Advanced Very High Resolution Radiometer (AVHRR) instrument that has flown onboard successive NOAA satellites. A significant barrier to the use of these data in studies of the climate of tropical precipitation (among other things) is the large bias caused by orbital drift that is present in the OLR data. Because the AVHRR instruments are deployed on the polar-orbiting spacecraft, OLR observations are recorded at specific times for each earth location for each day. Discontinuities are caused by the use of multiple satellites with different observing times as well as the orbital drift that occurs throughout the lifetime of each satellite. A regressionbased correction is proposed based solely on the equator crossing time (ECT). The correction allows for separate means for each satellite as well as separate coefficients for each satellite ECT. The correction is calculated separately for each grid box but is applied only at locations where the correction is correlated with the OLR estimate. Thus, the correction is applied only where deemed necessary.The OPI is used to estimate precipitation from the OLR estimates based on the new corrected version of the OLR, the uncorrected OLR, and two earlier published corrected versions. One of the earlier corrections is derived by removing variations from AVHRR based on EOFs that are identified as containing spurious variations related to the ECT bias, whereas the other is based on OLR estimates from the High Resolution Infrared Radiation Sounder (HIRS) that have been corrected using diurnal models for each grid box. The new corrected version is shown to be free of nearly all of the ECT bias and has the lowest root mean square difference when compared to gauges and passive microwave estimates of precipitation. The EOF-based correction fails to remove all of the variations related to the ECT bias, whereas the correction based on HIRS removes much of the bias but appears to introduce erroneous trends caused by the water vapor signal to which these data are sensitive. The new correction for AVHRR OLR works well in the tropics where the OPI has the most skill, but users should be careful when interpreting trends outside this region.

The expanding tropics: a critical assessment of the observational and modeling studies

“…These values are similar to those previously reported using the same dataset and analysis protocol. 66 Using a dataset that has a statistically based ECT correction 107 Volume 5, January/February 2014 applied effectively removes any expansionary trends in the data. While it cannot be absolutely stated that the statistical correction algorithm has not removed a valid long-term climate signal, these results raise questions about the true magnitude of OLR-based estimates of tropical expansion.…”

“…104 This alters the time of day when the satellite views a scene and has been shown to produce significant biases, particularly over land. [105][106][107] Figure 4 explores the possible impact on tropical expansion calculations. The left panel shows the uncorrected NOAA OLR data 108 ; expansionary trends of the 250 W m −2 contours of 0.82 and −0.32 • decade −1 are seen in the NH and SH, respectively.…”

“…Time‐latitude profiles of annually averaged NOAA AVHRR OLR for uncorrected (left) and the equatorial crossing time ( ECT )‐corrected data (Ref ) for the period 1979–2008. Vertical dashed lines indicate the changes in the satellite.…”

“…One potential factor responsible for the trend—overlooked in tropical expansion studies to date—is a change in the equatorial crossing time (ECT) of the polar‐orbiting satellites . This alters the time of day when the satellite views a scene and has been shown to produce significant biases, particularly over land . Figure explores the possible impact on tropical expansion calculations.…”