Spatiotemporal Variations of Microwave Land Surface Emissivity (MLSE) over China Derived from Four-Year Recalibrated Fengyun 3B MWRI Data

Abstract

Microwave Land Surface Emissivity (MLSE) over China under both clear and cloudy sky conditions was retrieved using measurements of recalibrated microwave brightness temperatures (Tbs) from Fengyun-3B Microwave Radiation Imager (FY-3B MWRI), combined with cloud properties derived from Himawari-8 Advanced Himawari Imager (AHI) observations. The contributions from cloud particles and atmospheric gases to the upwelling Tbs at the top of atmosphere were calculated and removed in radiative transfer. The MLSEs at horizontal polarizations at 10.65, 18.7, and 36.5 GHz during 7 July 2015 to 30 June 2019 over China showed high values in the southeast vegetated area and low values in the northwest barren, or sparsely vegetated, area. The maximum values were found in the belt area of the Qinling-Taihang Mountains and the eastern edge of the Qinghai-Tibet Plateau, which is highly consistent with MLSEs derived from AMSR-E. It demonstrates that the measurements of FY-3B MWRI Tbs, including its calibration and validation, are reliable, and the retrieval algorithm developed in this study works well. Seasonal variations of MLSE in China are mainly driven by the combined effects of vegetation, rainfall, and snow cover. In tropical and southern forest regions, the seasonal variation of MLSE is small due to the enhancement from vegetation and the suppression from rainfall. In the boreal area, snow causes a significant decrease of MLSE at 36.5 GHz in winter. Meanwhile, the MLSE at lower frequencies experiences less suppression. In the desert region in Xinjiang, increases of MLSEs at all frequencies are observed with increasing snow cover.

摘 要

利用风云3B卫星搭载的微波辐射成像仪(MWRI)再定标微波亮度温度(Tbs)测量数据,结合基于葵花8(Hamawari-8)卫星搭载的可见光和红外成像仪(AHI)观测反演的云参数产品,本研究实现了在中国地区晴朗和非降水云条件下微波地面比辐射率(MLSE)的反演。其中,大气气体分子和云粒子对上行微波辐射的吸收和散射效应通过辐射传输模式计算得到,并加以消除。从反演得到的2015年7月7日至2019年6月30日的中国地区平均微波地面比辐射率空间分布来看,10.65、18.7和36.5 GHz水平极化微波地面比辐射率的高值出现在中国南部的植被茂密区,低值则出现在中国西北的植被稀疏区,最大值出现在秦岭-太行山脉和青藏高原东缘。以上分布格局与基于美国宇航局水卫星所搭载的先进微波扫描成像仪(AMSR-E)观测的微波地面比辐射率结果高度一致。这表明,风云3B 微波辐射成像仪MWRI一级亮温产品的定标和验证是可靠的,并且本研究使用的微波地面比辐射率MLSE反演算法是成功的。进一步研究发现,中国地区微波地面比辐射率的季节性受到了植被,降雨和积雪的综合控制。在热带和南部的森林区域,微波信号受到植被增强和降水抑制作用的平衡,微波地面比辐射率表现为较弱的季节性。而在中国北方区域,积雪对高频波的微波地面比辐射率,如36.5 GHz,存在明显抑制作用,对于低频微波地面比辐射率的影响则较弱。研究还发现,在新疆的沙漠地区,微波地面比辐射率表现出随积雪面积增加而升高的现象。