The Chinese HY-2, a satellite designed for ocean dynamic environment monitoring, was launched on August 16, 2011. The onboard scanning microwave radiometer (RM) is primarily designed for sea surface temperature and wind speed mapping. However, our objective of this investigation is to exploit the large amount of land observations of RM and to extend the mission scope to the retrieval of surface soil moisture, which is also an essential boundary condition for coupling with atmospheric dynamics. The single-channel algorithm (SCA) was implemented using only the RM observed brightness temperature to estimate the surface soil moisture. Ancillary data of a normalized difference vegetation index were processed and used as inputs for the SCA to calculate the vegetation water content, which is a required parameter for estimating the vegetation optical depth. The retrieved soil moisture results agree with the global climate pattern of wet and dry regions. Initial assessments were performed using soil moisture measurements by in situ underground sensors over two selected networks: REMEDHUS in Spain and CTP-SMTMN network over the Tibetan Plateau. Results showed a good performance of soil moisture estimation for these land surface conditions for the year 2012, with the lowest root mean square error of 0.047 m3/m3. This product will contribute to continuous soil moisture information on a global scale for global change studies.
The system structure and development scheme for the simulation of spaceborne
microwave scatterometer are presented based on systematically reviewing and
summarizing the related previous investigations and the wind measurement rationale
of scatterometer, in combination with the consideration of current application
requirement of China. Subsequently, as an example, a scatterometer simulation
system of HY-2 is developed by using the object-oriented programming method.
Taking the parameters for HY-2 as input, the simulation experiments of wind field
retrieval for conically scanning scatterometer with co-polarizations are conducted by
using the simulation platform designed in this paper.
Modern microwave satellite altimeters can measure the instantaneous sea surface height to a precision of approximately
4.1 cm in the open ocean. One limiting factor is that if land appears within the altimeter footprint the data is flagged as
useless due to land contamination of the altimeter return waveforms and inappropriate geophysical corrections. This
means that many valuable coastal altimetry data values are simply edited out. In order to make improved use of the
altimetry waveform data near the coastal area, we derived the altimeter ranges from one year (March, 2006 to February,
2007; cycle 155 to cycle 188) of Jason1 waveform off the China coast (14-45°N, 105-130°E) by using four specialized
retrackers: Ocean, Ice-2, OCOG (Offset Centre of Gravity), and Threshold retracking algorithms, which are employed by
Envisat RA2 altimeter standard processing. In order to compare the four retracking algorithms quantitatively, we
calculated the bias, root mean square, and standard deviation of the sea level anomaly difference between the ascending
and descending tracks at crossovers. In addition, we compared the sea surface height derived from the four retracking
algorithms and in-situ tide gauge station measurements. The comparisons showed that the OCOG algorithm provides
more accurate results than the other three in coastal waters.
Altimetry data under extreme weather events such as tropical cyclones, typhoons and hurricanes are valuable although
the measurements are affected by various factors associated with high sea states, such as big winds, high waves,
especially atmospheric rains and ocean surface foams. Both rains and foams can strongly affect the propagation and
reflection of microwave signal, but the effects of sea foam on altimeter measurements have not been adequately
addressed. Although the sea foam only appears when the wind speed is high and ocean wave is breaking, the attenuation
effect of foam in typhoons should not be neglected. The major challenge is the effects of rain and foam on altimeter
received backscatter are similar and are hard to be separated. In this paper, we proposed a iterative method to correct
both rain and sea foam effects using a simplified stratified foam model and the liquid water content measurements from
nadir viewing Jason1 Microwave Radiometer. Finally a case study to maintain accurate wave height, wind speed, and
rain rate measurements and to retrieve the additional results of foam coverage considering both the effects of rain and sea
foam in the typhoon Shanshan is presented.
By analyzing several collocated QuikSCAT measurements and Special Sensor Microwave Imager (SSM/I) rain rate in
typhoons, the evidence of rain influence to wind retrieval is presented. The results show that the present of heavy rain in
typhoons will cause large bias in both wind speed and wind direction for QuikSCAT wind retrieval. So that the
Holland's model is applied in the ambiguity clear process to correct the errors in wind direction induced by rain. Also, a
radiate transfer function (RTF) and collocated SSM/I rain rate is employed to correct the attenuation and scattering
effects of rain. The performance of RTF method is limited due to lack of information of the collocated rain rate and the
un-precise of the RTF. As an alternative way to RTF, an interpolation method which just uses the data from QuikSCAT
itself is introduced to correct the attenuation and scattering effects of rain. This method is applied to retrieve the wind
vector in typhoon Ioke, the result shows that the bias in the wind direction and wind speed induced by rain has both been
well corrected, indicating that our correction to the impacts of rain on QuikSCAT wind retrieval in typhoons is effective.
With the availability of scatterometer data, surface wind vectors can be estimated from the backscatter measurement over
oceans, guarantee global, long-term monitoring of the winds on the oceans, which make them very valuable for climate
studies and other applications. At moderate wind speeds, the wind speed derived by scatterometer is considered reliable.
But at higher wind speeds, scatterometers appear to underestimate the wind speed, especially in tropical cyclones,
because of deficiencies of the geophysical model function for high winds, attenuation caused by rain, influence of wind
gradient, and the saturation of the backscattering under high wind.
As a passive microwave sensor, radiometer does not show obvious saturation phenomena under high wind, therefore it is
an appropriate candidate to be used to retrieve high wind speed. In this paper, combined scatterometer and radiometer
data is used to retrieve wind field under high wind condition. Using in situ data and meteorological data as a criterion,
we compared the wind retrieval performances of scatterometer and radiometer. Results show that it is better to use
radiometer data as a replacement of scatterometer while observing high wind speed.
Data of wind field of satellite scatterometer, height abnormality of altimeter and sea surface temperature of radiometer were used to analyze the seasonal variations of monsoon, circulation and surface temperature in South China Sea as well as the interaction among monsoon, circulation and heat radiation. It is proved from long time series data that South China Sea is prevailed with northeast monsoon in winter and there are two stable or one merged cyclonic circulations. In summer, southwest monsoon prevails and the seasonal circulation is consisted of two stable anticyclonic circulations, while the contours of surface temperature is in northeast-southwest direction corresponding with the direction of monsoon either in winter or summer, in addition there is a cold water tongue in sea areas south to Indo-Chinese Peninsula. There is significant resonant relationship among the fields of sea surface height, temperature and the wind field of monsoon in South China Sea in periods of annual scale, it can be concluded that the stable and strong monsoon is one of the major driving forces for circulation and sea surface heat exchange in South China Sea.