Evaporation ducts have a significant impact on electromagnetic (EM) wave propagation. The accurate prediction of EM propagation loss (PL) is important for the practical application of EM systems. However, the random weather processes and interaction of air–sea variables will cause complex spatial and temporal fluctuations in PL, especially for an over-the-horizon link. The physics prediction models show some limitations in practice. This paper proposes a data-driven PL prediction method based on the Gated Recurrent Unit network (GRU-PL). An X-band EM propagation system was developed and deployed for one month to collect PL of a 53 km link in the South China Sea for model training and testing. The GRU-PL model uses the air–sea variables as inputs and output the predicted PL. The root-mean-square error (RMSE) of the GRU-PL model is 7.04 dB, decreasing by 57.2% compared with the classical method. Furthermore, a method which uses the inversion evaporation duct heights (EDHs) as the output to train the GRU network, and then uses the parabolic equation model to predict the PL based on the GRU predicted EDHs, is also discussed. The RMSE is 7.3 dB. This method shows good potential in PL prediction of dynamically changing EM propagation links.

Perennial grains have potential to contribute to ecological intensification of food production by enabling the direct harvest of human-edible crops without requiring annual cycles of disturbance and replanting. Studies of prototype perennial grains and other herbaceous perennials point to the ability of agroecosystems including these crops to protect water quality, enhance wildlife habitat, build soil quality, and sequester soil carbon. However, genetic improvement of perennial grain candidates has been hindered by limited investment due to uncertainty about whether the approach is viable. As efforts to develop perennial grain crops have expanded in past decades, critiques of the approach have arisen. With a recent report of perennial rice producing yields equivalent to those of annual rice over eight consecutive harvests, many theoretical concerns have been alleviated. Some valid questions remain over the timeline for new crop development, but we argue these may be mitigated by implementation of recent technological advances in crop breeding and genetics such as low-cost genotyping, genomic selection, and genome editing. With aggressive research investment in the development of new perennial grain crops, they can be developed and deployed to provide atmospheric greenhouse gas reductions.

This article presents the measurement results of maritime wireless propagation channel at 8GHz in the South China Sea region. Large-scale path loss and small-scale fading of channels between a marine observation platform and a moving vessel are investigated. For large-scale path loss, comparisons are conducted between the free space model, two-ray model, parabolic equation model and log-distance model with the measured path loss data. The results show that the log-distance model agrees well with the general trend of the measured data, whereas the parabolic equation model has a better ability to capture the variations of the interference nulls. In addition, the free space model and two-ray model have larger deviation from the empirical data, implying that they are not suitable for the desired scenario. For small-scale fading, various distributions are considered to fit the fading. Based on the criteria of Kolmogorov–Smirnov statistics, the Rician distribution has the best goodness of fit, compared with the Rayleigh, Nakagami-m, Welbull, or Lognormal distributions. These research can provide good guidance for efficient designing and future deployments of distributed buoy networks in the South China Sea region.

Evaporation ducts, which form as a result of evaporation from the ocean, have a significant impact on electromagnetic (EM) propagation in coastal and marine environments. Theoretically, typhoons would substantially increase the mass, momentum, and heat exchanges at the air-sea interface and thus inevitably influence these EM propagation environments, including the duct conditions and propagation boundary conditions. However, the effects of typhoons on EM propagation have seldom been reported. In this study, anomalous propagation in the evaporation duct induced by Typhoon Kompasu (202118) was recorded by using an EM propagation system and automatic weather station. The length of the propagation link was 53 km, which was beyond the horizon. The evaporation duct's impact on EM propagation during Typhoon Kompasu was studied based on model simulation results. Propagation far beyond the horizon indicated that ducting was present during the typhoon passage. Furthermore, the approaching of the typhoon caused a decrease in path loss of about 20 dB. This was because the abnormal marine meteorological conditions induced by typhoons result in a high evaporation duct, which leads to a small path loss for EM propagation.

Evaporation duct is a layer above the ocean surface due to the inherent humidity inversion at the air-sea boundary layer. The formation of the duct depends on meteorological factors near the ocean surface and is affected by weather process above the ocean. The presence of this duct can have profound effects on over-water electromagnetic propagation at microwave bands. However, the information on evaporation duct features during the period of typhoon is limited. Here we show the impact of a typhoon on evaporation duct using the typhoon Parpiroon occurred in 2012 as an example. It is very interesting to find that there is an “evaporation duct eye” with very low evaporation duct height in the eye of typhoon. The trajectory of the typhoon center is almost consistent with that of the evaporation duct height minimum center during the typhoon Parpiroon. This is also confirmed by 19 more typhoons over northwest Pacific Ocean in 2018. Furthermore, we found that the low wind speed in the typhoon center is the primary cause of this interesting phenomenon. Our results demonstrate the characteristics of evaporation duct distribution during the typhoon process and illustrate the correlation between typhoon trajectory and evaporation duct height minimum center trajectory.

Atmospheric ducts are special super-refractive atmospheric structures that can cause over-the-horizon propagation of electromagnetic waves. Different types of atmospheric ducts have different influences on electromagnetic wave propagation. Owing to the complex marine atmospheric environment, different types of atmospheric ducts often occur together. When evaluating the performance of an electromagnetic system near the sea surface, the combined influence of various atmospheric ducts should be considered comprehensively. In this paper, the statistical distribution of atmospheric ducts over the northern South China Sea is analyzed using sounding data and reanalysis data. This paper uses the parabolic equation model to analyze the propagation characteristics of microwaves near the sea surface in the presence of both surface and evaporation ducts. It is found that compared with cases where only one type of atmospheric duct is considered, a hybrid atmospheric duct structure can capture more microwave energy at a lower receiving height. At an antenna height of 5 m, the path loss begins to fluctuate beyond a propagation distance of 50 km, with the maximum fluctuation reaching about 15 dB. Microwave propagation characteristics at different microwave frequencies and antenna heights are also simulated and analyzed.