Climate-Scale Oceanic Rainfall Based on Passive Microwave Radiometry - Satellite-based Applications on Climate Change - page 224

An improved version that removes the wind trend (version 6, V6) was made
available by RSS in 2006. All data have been reprocessed using the V6 data.
A comparison of the version 4 and version 6 products showed a reduced linear
trend in the rain rate data (Chiu and Chokngamwong 2010 ). These products
are labeled V4 and V6, respectively, corresponding to the RSS versions. In the
following, we will restrict our discussion on the V6 2.5 product.

15.3.2 Sampling

The sampling errors associated with these products are examined using a simple
error model and different satellite combinations of the monthly products. In general
the error structure follows a power law of the form ERR ¼ aR b , where ERR is

the sampling error, R the rain rate, and a and b are empirical constants. The value of
b is between 0.25 and 0.33, while the constant a is determined by the particular
sensor (Chang and Chiu 1999 , 2001 ). Analyses also show that large errors are
observed for grid boxes with insufficient sample. This threshold is approximately
2,500, compared to typical averages of 4,000-4,500 for the 2.5 boxes. These grid
boxes are flagged in the output files.

15.3.3 Product Evaluation

Analyses of the early records have been presented elsewhere (Chang et al. 1993 ;
Chiu and Chang 1994 ). Figure 15.4 shows the time series of the domain
(65 N-65 S, ocean) average rain rates. Linear regression analyses show no signifi-
cant trend.

15.3.3.1 Rainfall Rate ( R ) (Unconditional)

Equation 15.4 shows that the total (unconditional) rainfall rate is the product of the
rain frequency ( p ) multiplied by the conditional rain rate (mean rain rate for the
lognormal distribution). Figure 15.5 shows the annual and monthly average rainfall
rates. Major features, such as that over the maritime continent, Intertropical Con-
vergence Zone (ITCZ) in the Pacific and the Atlantic, the South Pacific and South
Atlantic Convergence Zones (SPCZ, SACZ), and the storm tracks in the western
oceans, are quite distinct. Seasonally, the Pacific ITCZ is strongest in the boreal
summer to early fall (JJAS). During the JJA season, the SPCZ is extremely weak.
It intensifies and acquires its maximum strength in January. The intensification of
the Pacific ITCZ is accompanied by the decay of the SPCZ, and in March there is a
separation of the SPCZ and the ITCZ as the SPCZ is attached to the southern branch
of the double ITCZ. The double ITCZ is clearly present during the months of March