QPE (Quantitative Precipitation Estimates)
Radar QPE is a realtime calibration approach for obtaining
rainfall rate through the ZR relation Z=aR^b.
The coefficients a and b can be either
static based on climatological values, or dynamically
calibrated.

Barnes analysis interpolates data at grids from discrete rain
gauge data with Gaussian weighting based on distance between
data and estimation point:

Cokriging analysis is a geostatistical technique to derive
an optimal estimate of rainfall using both rain gauge and
radar reflectivity data. While computationally intensive,
cokriging provides a sophisticated approach to estimate
rainfall amount taking into account the error characteristics
of rain gauge and radar measurements and their spatial
correlation.

Quality Control on Observations
QPF (Quantitative Precipitation Forecasts)
"Realtime Optical flow by Variational methods for Echoes of Radar
(ROVER)" is a realtime variational optical flow scheme, which
adopts 1) a preprocessing step to enhance radar reflectivity images;
and 2) a realtime variational optical flow technique.
1. Preprocessing step
In ROVER, the reflectivity fields are enhanced by transforming the
gridded value with the following function:
where Zc and ξ control the point of inflection and its sharpness.
Figure 1. Original (left) and Enhanced (right) reflectivity map
2. Realtime variational optical flow technique
To generate the motion field, SWIRLS adopts the open source codes titled
"VarFlow" (Harmat, 2014), developed based on the algorithm proposed in
Bruhn et al. (2003).
Figure 2. Motion field with arrows indicating motion vectors

After retrieval of the motion field, radar echoes are advected
with backward semiLagrangian advection scheme.
Figure 3. SemiLagrangian Advection scheme adopted in SWIRLS

Using ZR relationship with static or dynamically calibrated
parameters, radar reflectivity data are converted and aggregated
to rainfall intensity data, such as hourly rainfall, which in
turn are used by downstream applications to generate user
products.

