During the summer season tropical cyclones (TC) tend to cause flood disasters over coastal areas. In ancient times fishermen along the coast of China predicted the coming of tropical cyclones by observing weather phenomena. They observed the shape of clouds and the sunset glow to anticipate them. Tropical cyclones occur in various places where they are named differently as well. In the North Atlantic, central North Pacific, and eastern North Pacific the term hurricane is used. The same type of disturbance in the Northwest Pacific is called a typhoon.

In recent decades, tropical cyclones have caused great damages in United States of America, China, and other Asian countries. Therefore, the development of space-based technology for monitoring tropical cyclones, predicting their path, and early warning is crucial for people’s safety, fishery, as well as operations at sea.

The National Aeronautics and Space Administration (NASA) and the Japan Aerospace Exploration Agency (JAXA) have developed a space-based radar, the Dual-frequency Precipitation Radar (DPR), on the Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Measurement (GPM) mission (https://gpm.nasa.gov/missions/GPM/DPR) for tropical cyclones monitoring and research (NASA 2018).

Dual-frequency Precipitation Radar advances our understanding of the precipitation microphysics in tropical cyclones

The GPM DPR is more sensitive than its TRMM predecessor, especially in the measurement of light rainfall and snowfall in mid-latitude regions (NASA 2018).

“The instrument will provide 3-D measurements of the shapes and sizes of raindrops and snowflakes, and other physical characteristics that will allow scientists to better understand the physical properties of storms.” - Candace Carlisle, GPM Deputy Project Manager.

The DPR consists of the Ku-band precipitation radar and the Ka-band precipitation radar, abbreviated as KuPR and KaPR, respectively. They provide rain sensing over both land and ocean. Table 1 shows the KuPR and KaPR design specifications. The KuPR and KaPR are co-aligned on the GPM spacecraft bus. The precision of the instrument allows that data collected from the KuPR and KaPR will provide an accurate estimation of rainfall rate and will also provide the 3-dimensional observation of TCs.

Figure 1 illustrates the dimensions covered by the GPM Core Observatory's DPR and GPM Microwave Imager (GMI) instruments. Simultaneous measurements by the overlapping of Ka/Ku-bands of DPR can provide new information on particle drop size distributions over moderate precipitation intensities.

 
The GPM Core Observatory’s GMI and DPR instruments allow scientists to study the internal structure of storms throughout their life cycle (Brauer et al. 2024). Figure 2 shows the views of hurricane Lane’s precipitation, with intense storms near the hurricane’s eye. Specifically, the GMI measures the amount, size, intensity from heavy to moderate rain to light rain and snowfall. The DPR returns 3-dimensional profiles and values for the intensities of liquid and solid precipitation, revealing the internal structure of storms. Scientists use these instruments to track TCs and forecast their progression and to verify their tropical cyclone computer models. They also use instrument data to understand the distribution and movement of latent heat throughout the storm, particularly in the development of hot towers in the wall of clouds around the eye, which have been linked to rapid intensification (NASA 2018).
 

3-D structure and vertical profiling of tropical cyclones revealed by GPM-DPR

DPR provides three-dimensional measurements of precipitation structure and characteristics. For example, Figure 3 displays the three-dimensional structure of typhoon Lekima rainstorms revealed by GPM-DPR (Qi, Yong, and Gourley 2021). Figure 3a shows the scanning orbit when typhoon Lekima was passing the territory of Shandong Province, China. Figure 3b clearly shows the vertical profile of typhoon rainfall with different precipitation intensities. Combined with the horizontal distribution in Figure 3c, the 3-D precipitation structure of storm centers located within the scanning orbit of GPM-DPR. The columnar structure of typhoon storms with a relatively thin top and thick bottom can be clearly exposed by GPM-DPR in three dimensions.

Latent heat released, forming various microphysical processes (i.e. warm-cloud process, cold-cloud process), can affect the structure of typhoon precipitation. Figure 4 presents the latent heat estimates of typhoon Lekima. Two typical height-levels below (3 km) and above (7 km) freezing level (~5 km) are selected to study the thermal and microphysical structure of typhoon precipitation. The latent heat released at 3 (7) km lives up to 37.84 (44.33) K h−1. The heating area (shown in red color) reflects specific microphysical processes including accretion of cloud droplet, riming of ice crystals and supercooled water, and deposition process of water vapor, releasing a large amount of latent heat. Cooling area (shown in blue color) reflects the melting process of ice particles, evaporation of raindrops, absorbing a lot of latent heat. Heating at both levels denotes convective regimes, the heating rates at 3 km and 7 km reveal the central eyewall within Lekima to be convective and a small amount of isolated and banded convective precipitation. Concurrently, the mean heating rates below freezing level (9.22 K h−1) are higher than that above freezing level (6.81 K h−1), which indicates that the latent heat release of warm-cloud process (e.g. condensation, coalescence) in the lower layer is stronger than that of cold-cloud process (e.g. riming, aggregation, and Bergeron process) in the upper layer, and the warm-cloud process plays a major role (Wu et al. 2021).

Applications and challenges of GPM-DPR

GPM-DPR, as a space-borne radar, is considered a useful tool for TCs monitoring and forecasting, compared to ground radars. As TCs spend a large portion of their life over the open ocean, scientists utilize the GPM-DPR to study TCs over the world. Its application can be extended to cross-cutting areas (Tiberia et al. 2021), including weather forecasting, disaster reduction, water security and flood control.

GPM-DPR performs better than its previous version TRMM, since GPM-DPR improves precipitation type classification remarkably, reducing the misclassification of clouds and noise signals as precipitation type “other” from 10.14% of TRMM PR to 0.5% (Gao, Tang, and Hong 2017). This is a great advantage compared to currently used spaceborne radars, which have limitations with respect to observations of low-level clouds, midlatitude/high-latitude precipitation, and convection (Battaglia et al. 2020).

Conclusions

Global warming results in the formation of increased amounts of water vapor, leading to the formation of more tropical cyclones. As space-based radars develop, tropical cyclones monitoring, forecasting and early warning has become more reliable. The instruments, GMI and DPR, improve tropical cyclone tracking and forecasts, which can help decision makers save lives, particularly over coastal areas. Additionally, GPM-DPR reveals the three-dimensional structure of TC allowing scientists to further understand the underlying microphysical processes in TC to improve forecasts and warning operations.