Analysis of Climate-Relevant Gas Absorption Properties from ARM Spectral Measurements

Description

The use of ARM spectral radiation measurements to derive important spectroscopic properties of water vapor has a long and successful history in the ARM and ASR program. These studies require an accurate specification of the water vapor field above a radiometric instrument (e.g. the AERI). Prior research has determined that water vapor profile accuracy can be improved by scaling a radiosonde water vapor profile so that its associated water vapor column amount (PWV) is consistent with a collocated microwave radiometer (MWR) observation. The ARM MWRRET product, which was developed for this purpose, is a physical retrieval of PWV and cloud liquid water path based on measurements from the ARM two-channel (23.8 and 31.4 GHz) microwave radiometer. The foundation for this retrieval is forward model calculations from AER’s MonoRTM radiative transfer model. The key spectroscopic parameters for the forward model calculations in MWRRET are the line parameters for the 22 GHz water vapor line in MonoRTM and microwave water vapor continuum coefficients in the MT_CKD model, which are based upon the analyses in Payne et al. (2008) and Payne et al. (2011), respectively. This poster will present the results from the first objective of our ASR project, a comprehensive reassessment of the accuracy of these spectroscopic parameters. Reapplying the methodology of Payne et al. (2011) to a large dataset of recent SGP and GoAmazon MWR measurements has demonstrated that the previous microwave foreign and self continuum were not accurate. Of critical importance in this analysis were the higher PWV measurements in the GoAmazon dataset, although it also is clear that there are systematic differences of unknown origin between recent SGP MWR measurements and the ones used in Payne et al. (2011). Our analysis has determined that the microwave foreign continuum in MT_CKD needs to be reduced by ~20% and the self continuum needs to be increased by ~45%. These modifications to the water vapor continuum will result in a decrease in PWV retrieved from the MWR by 1% in very moist cases, while reducing the PWV in the driest cases. The improvements in PWV accuracy resulting from these continuum modifications will allow us to more accurately accomplish the second objective of our project, the derivation of improved water vapor line parameters and continuum absorption coefficients in the infrared, most notably in the critically important atmospheric window region.