Microwave Vicarious Calibration (VC) Method

VC Method Summary

Vicarious calibration (VC) is a useful tool for monitoring long-term trends of microwave radiometers as well as inter-calibration between two instruments. This method does not use coincidental cross-over locations between two instruments; rather, VC finds an Earth target that has been shown to be relatively stable in brightness temperature (TB) over time. This method was first developed using the stable Earth target of a cold ocean (minimal atmospheric water vapor and cloud liquid water and low surface wind speed) for a nadir viewing radiometer at frequencies 19-37 GHz (Ruf 2000). This method was expanded to include conically scanning radiometers at frequencies 6-89 GHz (Kroodsma et al. 2017) and discovered to be useful for inter-calibration (Kroodsma et al. 2012). Another Earth target used for vicarious calibration is very dense forest canopies which give a stable warm TB reference. The Amazon rainforest was first used as a vicarious warm reference (Brown and Ruf 2005), and the algorithm was later modified to include other dense forest locations (Yang et al. 2016).

VC Method Optimization

Many current and past spaceborne microwave radiometer observations have been used to optimize the vicarious calibration algorithms and ensure that the algorithms can be applied to a variety of instruments. Vicarious cold calibration (VCC) was first developed for a nadir viewing radiometer but is now used on both conical and cross-track scanning radiometers and as an intercalibration method. Vicarious warm calibration (VWC) was developed using only frequencies from 19 to 37 GHz over a small region in the Amazon rainforest but has since been optimized for frequencies from 10 to 89 GHz and over a wide range of latitude regions. Optimization considers how to minimize the vicarious calibration dependence on latitude/region, geophysical variables/simulation model used, and number of observations.

Model Usage

VCC and VWC both utilize a radiative transfer model (RTM) to simulate the observed TBs. The simulated TBs allow these methods to be used for intercalibration through the double difference method. The single difference (observed minus simulated TB) for an individual radiometer can also be used as a monitoring tool for observing scan biases or calibration drifts with time. However, the simulations can introduce some error to the calibration or result in biases that are not due to instrument calibration but instead are a feature of the RTM. Frequencies most sensitive to atmospheric water vapor have a higher uncertainty in vicarious calibration.

Latitude Regions

VCC utilizes over-ocean regions with minimal atmospheric water vapor and cloud liquid water and low surface wind speed. These regions produce the coldest over-ocean brightness temperatures. Figure 1 shows an example of where these regions occur using AMSR2 data for 10.65 GHz and 23.8 GHz horizontally polarized TBs (Kroodsma et al. 2017). Since the frequencies have different dependence on atmospheric water vapor, the location of the coldest TBs changes slightly with frequency, but the coldest observations are generally concentrated toward the poles. It is important to note that if applying VCC to a radiometer onboard a spacecraft that does not observe all latitudes (e.g. TMI on TRMM or SAPHIR on Megha-Tropiques), the regions of the coldest TBs will change and will be at the highest latitudes that are observed. If using VCC for inter-calibration between sensors that observe different latitude ranges, the observations of both sensors must be filtered to only include similar latitudes due to potential errors in the simulations (Kroodsma et al. 2012).

Figure 1: Locations of the coldest 10.65H TBs (top) and 23.8H TBs (bottom), taken from Kroodsma et al. 2017.

VWC utilizes regions of dense vegetation since these regions create a homogenous TB scene that is stable over time at microwave frequencies. Figure 2 shows the regions considered for VWC (Yang et al. 2016). Including boreal, temperate, coastal, and island sites alongside the original tropical sites significantly increases the number of observations that can be used for VWC, resulting in a more statistically robust algorithm. These sites also show that there is some latitude dependence in VWC for frequencies sensitive to atmospheric water vapor. Using a global dataset of warm calibration sites helps to mitigate this dependence.

Figure 2: Dense vegetated regions considered for vicarious warm calibration, taken from Yang et al. 2016.

Number of Observations

A year of radiometer observations is typically required to obtain a robust vicarious calibration statistic. Shorter time periods may be used to monitor calibration drifts; however, this increases the chance of seasonal variability having an impact on the calibration. While the RTM significantly reduces seasonal variability, the simulations are not perfect. Channels most sensitive to atmospheric water vapor will still have some seasonal variability in vicarious calibration, so averaging over a year of observations will minimize this impact.

Kroodsma, Rachael, University of Maryland, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771; rachael.a.kroodsma@nasa.gov

Key Scientific Papers

1. Kroodsma, R. A., D. S. McKague, and C. S. Ruf, “Vicarious cold calibration for conical scanning microwave imagers,” IEEE Trans. Geosci. Rem. Sens., 55(2), pp. 816-827, Feb. 2017.
2. Yang, J. X., D. S. McKague, and C. S. Ruf, “Boreal, temperate, and tropical forests as vicarious calibration sites for spaceborne microwave radiometry,” IEEE Trans. Geosci. Rem. Sens., 54(2), pp. 1035-1051, Feb. 2016.
3. Kroodsma, R. A., D. S. McKague, and C. S. Ruf, “Inter-calibration of microwave radiometers using the vicarious cold calibration double difference method,” J. Selected Topics Remote Sensing, 5(3), pp. 1006-1013, Jun. 2012.
4. Brown, S. T. and C. S. Ruf, “Determination of an Amazon hot reference target for the on-orbit calibration of microwave radiometers,” J. Atmos. Oceanic Tech., 22, pp. 1340-1352, 2005.
5. Ruf, C. S., “Detection of calibration drifts in spaceborne microwave radiometers using a vicarious cold reference,” IEEE Trans. Geosci. Rem. Sens., 38(1), pp. 44-52, Jan. 2000.

-- RobbiIacovazzi - 24 Nov 2020