Atmospheric Sensing Using GNSS RO - GNSS Remote Sensing: Theory, Methods and Applications - page 142

tropical cyclone (Anthes 2011 ). Such high-quality RO soundings could provide the
thermodynamic temperature and water vapor structure that are essential for cyclone
genesis studies. The current number RO LEO satellites only produce relatively
sparse RO soundings in a regional scale (e.g., 1 sounding per day per 4-degree
grid, given 3,000 soundings per day globally). However, occasionally, the RO
soundings provide critical information near the cyclone and can make significant
positive impact on forecast. For example, Huang et al. ( 2005 ) and Chen et al.
( 2009 ) showed that GPS RO had positive impacts on TC simulations by conducting
data assimilation experiments using 3D-Var. Liu et al. ( 2012 ) use the Weather
Research and Forecast (WRF) model to simulate the hurricane Ernesto in 2006.
After assimilating COSMIC soundings, the lower troposphere is moistening and
leads to hurricane genesis as seen in the observations. Studies also demonstrates
that assimilating COSMIC observations in the NCEP operational model improved
the temperature, water vapor, geopotential and wind fields (Cucurull and Derber
2008 ), which provides the key thermodynamic background for correctly simulating
the cyclone genesis. With multiple RO missions planned for launch in the near
future, it is reasonably to believe that such improvement in the dynamic field will
significantly improve the tropical cyclone prediction when RO soundings density
increases considerably near the cyclone.

6.3.5 Atmospheric Boundary Layer (ABL)

The shallow atmospheric boundary layer (1-2 km) and the frequent cloud and
aerosol presence make it extremely difficult to observe from the space. GPS RO is
currently the only satellite remote sensing technique that can profile the temperature
and water vapor with high vertical resolution inside the ABL. A typical well-
mixed boundary layer is capped by a temperature inversion and negative water
vapor gradient and therefore is characterized by a negative refractivity gradient. The
large refractivity gradient results in a sharp increase in bending angel observation,
produces large defocusing (e.g., reduction of mean signal amplitude) and multipath
propagation. The implementation of the open-loop receiver tracking along with the
application of radio-holographic retrieval method allow the routinely RO profiling
into the ABL with very high vertical resolution ( 100 m). Such well-mixed ABL
top height can be precisely determined from the RO bending angle or refractivity
profiles (Sokolovskiy et al. 2006b , 2007 ). The spatial and temporal variations
of ABL height climatology can also be derived based on the multiple-year RO
observations (von Engeln and Teixeira 2004 ; Sokolovskiy et al. 2007 ; Ratnam and
Basha 2010 ; Guo et al. 2011 ;Aoetal. 2012 ; Xie et al. 2012a ). Diurnal variation
of the ABL heights over subtropical eastern oceans (Sokolovskiy et al. 2010 ) and
Sahara desert (Ao et al. 2012 ) has also been derived from multiple year COSMIC
data. Xie et al. ( 2012a ) demonstrates the highly accurate ABL height detection from
COSMIC RO soundings over the subtropical eastern Pacific Oceans as comparing
with close-by high-resolution radiosondes, whereas the high-resolution ECMWF