Inter-Calibrating Observations from Microwave Humidity Sounders Onboard NOAA and Metop Satellites
Abstract:Satellite data from microwave humidity sounders onboard NOAA and MetOp satellites play an important rule in weather forecasting as well as climate monitoring and assessment. These data are widely used to derive various hydrological products, including precipitation, precipitable water vapor, snow cover, tropospheric humidity, cloud liquid water, deep convective clouds, and land surface emissivity. MW data are subject to several radiometric and geometric errors and need to be properly bias corrected and inter-calibrated before being used for any climate assessment.
Inter-calibration theoretically requires that two satellites should observe the same target at the same time with the same geometry. One specific case is when two satellites observe the same target at the same time at the nadir position and is called Simultaneous Nadir Observation (SNO). However, in terms of polar orbiting satellites, SNO's only occur in polar regions but the inter-satellite differences are scene dependent. The requirement that two satellites should observe the target at the same time can be neglected if the diurnal variation of brightness temperatures is negligible. There are several places where the diurnal variation can be neglected including tropical region as well as polar regions during polar nights. Therefore, we use observations averaged over tropical region as the warm end and averages over polar regions for the cold end of the brightness temperatures for inter-calibrating data from similar instruments. Then, the calibration coefficients are calculated using regression analysis for monthly data and interpolated to derive the daily values. We present the results of inter-calibrating AMSU-B and MHS observations for the period 2000 – 2010. The time series of the observations show significant improvement and consistency between multiple satellites after inter-calibration and bias correction.