# Passive Microwave Imagers, Their Applications, and Benefits: A Review
## 2 PMWI Applications and Impact Areas
### Applications and Benefits of PMWIs
#### 2.1.5 Cryosphere  
PMWIs are essential tools for monitoring cryospheric conditions, including snow cover, sea ice concentration, and ice sheet dynamics. Their ability to penetrate clouds and operate at night enables long-term and consistent observations in polar regions. These measurements are essential for tracking seasonal and interannual changes in ice and snow, supporting hydrological modeling, climate monitoring, and understanding of polar processes [20].  
The cryosphere is a vital source of freshwater for the world's population. It also plays a crucial role in Earth's climate due to its high albedo. However, most of the cryosphere exists in places where obtaining on-site measurements is difficult, which emphasizes the need to use spaceborne instruments for its detection. Due to PMW instruments' continuous presence over polar regions, a long-term dataset is available, making them ideal for cryosphere-related climate studies at the poles [20; 74]. For example, NASA's NSIDC Distributed Active Archive Center (NSIDC DAAC) maintains the AMSR Unified collection, which includes data from PMWIs such as AMSR-E sensors aboard NASA's Aqua satellite (launched on 2 May 2002, and ceased operations on 4 December 2011) and AMSR-2 sensors aboard JAXA's GCOM-W1 satellite. These PMW data products provide valuable information on snow, sea ice, and soil moisture in polar regions. The dataset includes horizontally (H) and vertically (V) polarized Tbs at 18.7 GHz, 23.8 GHz, 36.5 GHz, and 89.0 GHz, along with key cryospheric variables such as sea ice concentration, sea ice concentration difference, and snow depth. As an example, Figure 3 illustrates the daily average ice concentration with a nominal spatial resolution of 12.5 km over the Northern and SouthernAdditionally, the polarization of these instruments is useful in determining sea ice edges, as liquid water is more polarized and less emissive than ice [21; 74]. PMWIs have been used extensively to study the cryosphere, including sea ice extent and thickness and glacier dynamics [75]. Much of what is currently known about the large-scale variability of global sea ice cover has been derived from data provided by satellite microwave sensors [76]. PMWI, like ESMR operating at 19.35 GHz aboard Nimbus-5 in 1972, was among the first to observe global sea ice, with the spatial resolution providing unprecedented daily coverage in polar regions without gaps [76; 77]. Furthermore, more recent PMWIs, such as the Copernicus Imaging Microwave Radiometer (CIMR), AMSR-E, AMSR-2, SSM/I, SSMIS, and SMMR, have been extensively utilized to derive critical sea ice parameters (e.g., Arctic sea ice extent and thickness) [76; 77]. For instance, EUMETSAT's Ocean and Sea Ice Satellite Application Facility (OSI SAF) Global Low Resolution Sea Ice Drift (GBL LR SIDR) and Global Sea Ice Concentration (GBL SICO) product uses AMSR2 and SSMIS data to differentiate between ice and water based on their emissivity signatures [78]. The OSI SAF GBL LR SIDR also uses Tbs from AMSR-E and AMSR2 from 37 GHz channels, SSM/I from 85 GHz channels, and SSMIS from 91 GHz channels as part of its input [79], which can be used in NWPs and ocean ice models. Moreover, PMW instruments are capable of distinguishing between multi-year ice and first-year ice; this is because multi-year ice has lower emissivity that declines at higher frequencies, because of the air pockets formed during summer melts [21; 74]. Finally, another area of PMWI application in the cryosphere is sea ice motion, which started with the use of SSMI data to monitor large-scale motion patterns in the late 1990s [79].  
Figure 3: The daily average ice concentration over (**a**) the Northern and (**b**) Southern Hemispheres on 1 January 2025, obtained from the Land Atmosphere Near Real-Time Capability for EOS (LANCE) AMSR2 sensors (source: figure created by the authors).