Influences of 1DVAR Background Covariances and Observation Operators on Retrieving Tropical Cyclone Thermal Structures

Hao Hu,Fuzhong Weng

doi:10.3390/rs14051078

Abstract

Spaceborne passive microwave sounding instruments are important for monitoring tropical cyclones (TCs) over oceans. However, previous studies have found large retrieval errors at TCs’ inner region at the lower troposphere where heavy precipitation occurs. In this study, the background error covariance matrix used in the variational retrieval algorithm is designed to vary with atmospheric conditions. It is found that the errors of retrieved temperature and humidity profiles are significantly reduced under the TC conditions, when they are compared with those from using a static covariance matrix. The retrieval errors of temperature and humidity are about 1.5 K and 10–20%, respectively, in the troposphere. Moreover, the influence of different observation operators on the retrievals are also investigated. It is shown that ARMS (Advanced Radiative Transfer Modeling System) used as an observation operator can produce a higher retrieval accuracy, compared to CRTM (Community Radiative Transfer Model). For the relative humidity profile, the error can be reduced by up to 5% from ARMS. The reason may be attributed to the more comprehensive handling of the scattering from various hydrometeors in ARMS, which results in a higher retrieval accuracy under cloudy conditions.

Highlights

Satellite observations are one of the most important methods for observing TropicalCyclones (TCs) over oceans
This study focused on the retrieval of hurricane thermal structures over the Atlantic (ATL) and East Pacific (EP) oceans, as the operational flight dropsonde observations are available for assessment in these regions
Considering that this study focuses on the retrieval under tropical cyclones conditions, the covariances of T and Q are calculated based on the ERA5 reanalysis dataset at low latitudes (30◦ S–30◦ N), while the covariances of hydrometeor variables are calculated based on the WRF simulation results of typhoon LEKIMA between

Summary

Introduction

Satellite observations are one of the most important methods for observing TropicalCyclones (TCs) over oceans. It is difficult to rely on visible or infrared data to derive the TCs’ vertical thermal and dynamic structure within clouds [3]. Many observational studies on the vertical structure of TC are based on aircraft dropsonde data [4,5]. While aircraft observations over Eastern Pacific (EP) oceans and the Atlantic (ATL) oceans are made operational for TCs, the in situ observations of TCs’ vertical structure are very limited over other oceanic areas such as the Western North Pacific where typhoons frequently happen. The passive MW sounding instruments, such as the AMSU-A (Advanced Microwave Sounding Unit-A [6]) and the ATMS (Advanced Technology Microwave Sounder [7]), can detect temperature and water vapor information in different atmospheric layers with its oxygen and water vapor absorption bands, which have been used in retrieving TCs’ thermal structures in many previous studies.

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