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SOLA, 2015, Vol. 11, 14−17, doi:10.2151/sola.2015-004

Early Evaluation of Ku- and Ka-Band Sensitivities
for the Global Precipitation Measurement (GPM)
Dual-Frequency Precipitation Radar (DPR)
Koichi Toyoshima1, Hirohiko Masunaga2 and Fumie A. Furuzawa2
1
Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan
2
Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japan

Abstract
The purpose of this paper is to quantify the sensitivity of the
Global Precipitation Measurement (GPM) mission core observatory Dual-frequency Precipitation Radar (DPR) with focus on the
Ka-band detectability of light rain and snow in comparison with
the Ku-band capability. In this work, storm top height (STH) isutilized exclusively as the metric of radar sensitivity. The GPM
DPR standard product level 2 version 3 is used in this analysis
for the period from April to August 2014. The Ka high sensitivity(HS) mode and Ku have little systematic difference in STH overa broad range of the histogram, implying that the advantage ofthe Ka HS mode may not be as distinct as expected. The nonRayleigh scattering effect may have partly offset the sensitivity
advantage of the Ka HS over the Ku.
(Citation: Toyoshima, K., H. Masunaga, and F. A. Furuzawa,
2015: Early evaluation of Ku- and Ka-band sensitivities for the
Global Precipitation Measurement (GPM) Dual-frequency Precipitation Radar (DPR). SOLA, 11, 14−17, doi:10.2151/sola.2015004.)

1. Introduction
The Global Precipitation Measurement (GPM) mission coreobservatory was launched in February 2014, carrying the Dualfrequency Precipitation Radar (DPR) developed by the Japan
Aerospace Exploration Agency (JAXA) with the National Institute of Information and Communication Technology (NICT). The
DPR consists of two radars with the microwave frequencies of Kaband (35.55 GHz) and Ku band (13.6 GHz). A Ku-band radar has
been demonstrated by the Tropical Rainfall Measuring Mission(TRMM) Precipitation Radar (PR) to be suitable for measuring
moderate to heavy rains typical of the tropical and mid-latitude
regions, while a Ka-band radar is expected to extend its ability tocapture weak rain and snow in higher latitudes as well (Hou et al.
2008, 2014). Several months of observations from the GPM coreobservatory are now available, allowing an early evaluation of
the instruments onboard. The purpose of this paper is to quantify
the sensitivity of DPR with focus on the Ka-band detectability oflight rain and snow in comparison with the Ku-band capability.
The techniques of rainfall retrieval with a dual-frequency radar
have been developed to reduce ambiguity in rain rate retrieval
in comparison with single wavelength radars (Chandrasekar
et al. 2003; Rose and Chandrasekar 2005; Liao et al. 2005; Liaoand Meneghini 2005; Seto et al. 2013; Kubota et al. 2014). In
this work, storm top height (STH) is utilized exclusively as themetric of radar sensitivity instead of more conventional measures
including surface rain rate. This is because STH is not affected byalgorithm assumptions such as drop size distribution and attenuation correction that are indispensable for rainfall retrieval andhence is deemed optimal for studying the instrument performance
separately from algorithm uncertainties.
Corresponding author: Koichi Toyoshima, Nagoya University, Furo-cho,
Chikusa-ku, Nagoya, Japan. E-mail: toyoshima@satellite.hyarc.nagoya-u.
ac.jp ©2015, the Meteorological Society of Japan.

In this paper, the analyzed data are first described in Section
2 and STH is defined in Section 3. The results are presented in
Section 4, where the individual Ku- and Ka-radar sensitivity is
examined in some detail. Finally, the findings are discussed andconcluded in light of potential physical explanations (Section 5).

2. Data
The GPM DPR standard product level 2 version 3 is used inthis analysis for the period from April to August 2014. The Kaband radar has two different scan modes of matched scan (Ka MS)
and high sensitivity scan (Ka HS). Ka MS is matched footprint
by footprint with the Ku-band radar and has 25 angle bins withthe range bin size of 125 m. Ka HS footprints are interlaced with
Ku footprints and has 24 angle bins with range bin size of 250 m.
The Japanese 55-year Reanalysis (JRA55) (Kobayashi et al. 2015)
surface air temperature at 2 m is used to isolate out the condition
that precipitation is considered to be entirely freezing.

3. Storm top height
STH in the DPR standard product is defined as the altitudeof the highest range bin that contains precipitating echoes, whereradar reflectivity is required to exceed a given threshold for 6
successive range bins to filter out noise. Noise analysis for spaceborne radars was given, for example, by Kumagai (1997) and
Takahashi and Iguchi (2008). The DPR minimum detectable echo
as measured in the pre-launch test is 14.5 dBZ, 16.47 dBZ, and10.2 dBZ for Ku, Ka MS, and Ka HS, respectively (Kubota, inprivate communication). These values are better than the nominal
sensitivities or 18 dBZ for Ku and 12 dBZ for Ka HS (Hou et al.
2014). These values, however, are not guaranteed to stay valid
after the instrument is launched into orbit. In the DPR product,
the radar sensitivity estimated with a signal-to-noise ratio
threshold is sought for each angle bin based on the noise level
determined from echo-free range bins (Kubota, in private communication). This definition of radar sensitivity is robust from anengineering perspective in that it is independent of any arbitrary
assumptions except for the additional requirement that 6 continuous bins should all contain echoes above the noise level todefine STH. In this paper, STH is recalculated in the same manneras done for the DPR product but by varying the threshold every
2 dBZ from 8 dBZ to 20 dBZ in order to ensure the robustness
of STH statistics. The analysis is limited to near-nadir (from 21st
to 29th) angle bins in order to minimize the influence of the siderobe clutters.

4. Results
The minimum detectable echoes of Ku and Ka radars generally do not precisely agree with the nominal values determined forthe success criterion of the GPM mission. The radar sensitivity
could be also slightly changed after launch from the measurements
at the pre-launch testing. We first evaluate the minimum detectableechoes in orbit with the actual observations and compare them

Toyoshima et al., Early Evaluation of Ku- and Ka-Band Sensitivities for the GPM DPR

Fig. 1. The normalized histogram of STH for global oceans within theobservational coverage of GPM DPR. The reference STH from the GPM
standard product is plotted in black, and STHs recalculated with different
reflectivity thresholds are delineated in colors as indicated for (a) Ku, (b)Ka MS and (c) Ka HS. (d) The same as (c) but with 3 instead of 6 successive range bins to define STH.

with the nominal and pre-launch values. Figure 1 shows the histogram of STH calculated with several thresholds with the standard
product as the reference for global oceans covered by the GPM
core satellite. Some noticeable features are evident in the STH
histogram. The Ka MS histogram has a sharp spike at 4−5 kmthat is absent for Ku, which is ascribed to the inability of Ka MS
to detect solid precipitation above the melting layer as describedlater. Note that a similar peak is observed also for Ka HS withartificially high threshold values but is only barely present fora realistic threshold of 12 dBZ. The Ka MS exhibits a striking
dependence on the threshold, showing a long tail toward high altitudes for lowest threshold values, which arises once the thresholddecreases to far below the noise level. A similar histogram tail isalso visible for Ka HS when the reflectivity threshold is decreased
to 8 dBZ. These results together suggest a guideline for estimating
the minimum detectable echo from the STH histogram. First, thethreshold would be too low compared to the actual noise level if
a histogram tail arises toward the high end. Second, the threshold
might be too high if a fictitious melting-layer spike appears,
except for Ka MS where the sensitivity is by design so poor thatfrozen precipitation is largely missed. A correct threshold should
lie between these two extreme cases. Bearing this in mind, thereflectivity threshold in the standard product (black dotted line in
Fig. 1) that best reproduces the reference STH roughly is hence
estimated to be between 12 and 14 dBZ for Ku (Fig. 1a), 18 dBZ
for Ka MS (Fig. 1b) and 12 dBZ for Ka HS (Fig. 1c). These results
suggest that estimated minimum detectable echoes in orbit are
roughly consistent with the pre-launch values noted above, although the Ku radar sensitivity is slightly better in orbit.
For Ka HS, the STH histogram appears to entirely lack STHs
lower than 1.5 km. This is unrealistic because observational evidence shown later (Fig. 2) implies a large population of precipitating clouds with STHs below 1.5 km. The failure to detect shallow
precipitation arises from the present constraint to define STH. For
Ka HS, a high sensitivity is achieved at the expense of a degradedvertical resolution from 125 m to 250 m, so that contiguous 6bins inevitably require that STH should be 1.5 km at minimum. In

15order to avoid this obvious deficiency, the Ka HS STH is recalculated with a series of thresholds defined under 3 successive bins
instead of 6 bins. As a result, shallower STHs less than 1.5 kmthat are undetected in the 6-bin thresholds can be detected for Ka
HS and its histogram seems to more naturally capture low-topped
precipitation (Fig. 1d). In the remainder of the paper, STH detection of Ka HS is defined as three successive bins with the 12 dBZ
threshold while the reference STH is adopted as it is for Ku and
Ka MS.
Figure 2 shows the height-latitude histogram of STH in orderto demonstrate the STH statistics in the context of global precipitation climatology. The overall distribution is similar across all
the radar products, although Ka MS somewhat underestimates theoccurrence frequency of high echo tops compared to Ku and Ka
HS as expected from Fig. 1. The higher frequencies around 5 km
in Figs. 2b, e is expected by Fig. 1b which has a spike aroundfreezing height. The STH frequency is concentrated below 2 km
for latitudes higher than 60° for the both hemispheres, where moderate precipitation from snow and light rain is typical (Behrangi
et al. 2014). Over tropical oceans (30°S−30°N, Figs. 2a, b, c), dual
peaks corresponding to shallow cumuli at 2 km and deeper clouds
at 5 km height are seen while the shallow peak is missing overland. This result is consistent with the TRMM PR study by Shortand Nakamura (2000). Subtropical oceans (15°−30°), more notably
in the southern hemisphere than in the northern hemisphere, only
exhibit a shallow peak as one might expect for regions cappedwith the trade inversion.
Figure 3 shows the STH histogram for three different meridional zones of the tropics (30°S−30°N), northern extratropics(30°N−65°N), and southern extratropics (65°S−30°S) with oceans
and lands separated for each zone. The Ka MS histogram exhibitsa notable difference from the other two, where the Ka MS modetends to miss high echo tops (4−7 km for Figs 3a, d, 7−12 km
for Figs. 3b, e, 4.5−7 km for Figs. 3c, f) and is as a result biased
toward low echo tops. The Ka MS fails to detect solid precipitation echoes above the freezing height more frequently than Ku and
Ka HS, while generally sensitive to the melting layer (Okamoto
et al. 2004). As a result, in the tropics (Figs. 3b, e) the Ka MS
histogram has a striking spike at 4−5 km, because STH settles
down on the melting layer whenever ice particles aloft remain
undetected. On the other hand, the Ka HS and Ku histograms stayclosely together, implying that the sensitivity advantage of the Ka
HS mode may not be as distinct as expected. A closer inspection,
however, reveals that the Ka HS better captures shallowest echo
tops below 1.5 km than the Ku as highlighted by green and red
shades in Fig. 3. Such precipitation with low STHs typically
produces light rain with small raindrops and drizzles or light snow
with fluffy snowflakes, which are generally difficult to capture at
low microwave frequencies. Moreover, one might argue that the
Ka HS is also superior in the ability to measure solid precipitation
at 11 km or higher in deep convection over tropical continents (Fig.
3e), whereas there is no such evidence outside the tropics. Overall,
the histograms suggest faint hints of the Ka superiority to the Ku
in terms of sensitivity, although the differences are minor. The Ka
HS sensitivity advantage was also pointed out by Kotsuki et al.
(2014).
Figure 3 contains both liquid and solid precipitation, but theradar detectability depends on the phase of hydrometeors. To
separate rain from snow, Fig. 4 shows the STH histogram withsnowfall isolated out, defined where surface air temperature at
2 m is below 0°C. The tropical latitudes never meet this criterion
and are not shown. The histograms become somewhat sharpened
around low STHs, i.e., snow tend to have lower STHs than rain,
but are otherwise similar to the previous figures. It follows that the
Ka sensitivity to snow may slightly outperform the Ku although
the advantage is small.

5. Discussion and conclusions
This paper is aimed at an early evaluation of the GPM DPR
performance with focus on exploring the expected difference in16

SOLA, 2015, Vol. 11, 14−17, doi:10.2151/sola.2015-004

Fig. 2. The height-latitude histogram of
STH. The frequency of occurrence is normalized at each latitude. (a) Ku Ocean, (b)Ka MS Ocean, (c) Ka HS Ocean, (d) Ku
Land, (e) Ka MS Land, and (f) Ka HS
Land. The contour lines are drawn ateach 0.03 step up to 0.15.

Fig. 3. The STH histogram for Ku, Ka
MS, and Ka HS for (a) 30°N−65°N
ocean, (b) 30°S−30°N ocean, (c) 30°S−
65°S ocean, (d) 30°N−65°N land, (e)30°S−30°N land, and (f) 30°S−65°S land.
For visual clarity, the difference between
Ku and Ka HS is shaded in red where Ku
exceeds Ka HS and in green otherwise.

precipitation detectability between the Ku- and Ka-band radars.
Our interest is to understand to what extent the Ka-band radar isable to detect weak rain and solid state precipitation compared to
Ku-band radar. The detectability of the Ku-band radar and twomodes of the Ka-band radar (Ka MS and Ka HS) is studied interms of STH. STH for the Ka HS is redefined with a modified
criterion so as to avoid the underrepresentation of shallow precipitation that was found in the original definition. While the Ka
MS is found to be least sensitive to high echo tops, the sensitivity
advantage of the Ka HS over the Ku is not very evident. A close
examination suggests that the Ka HS may be slightly better at
capturing weak rain and snow, although the differences are minor.

The difference in sensitivity among different radar frequencies
has been studied previously in various contexts. Berg et al. (2009)
investigated the precipitation detectability of the TRMM PR andCloudSat W-band radar in attempt to construct the histogram overa full range of precipitation rate beyond the capability of individual instruments. The possibility to separate the liquid and ice phasesof precipitation was explored by utilizing the sensitivity difference
between the Ku and Ka bands (Liao and Meneghini 2011).
The absence of apparent superiority of the Ka-band radar interms of precipitation detectability is in part owing to the nonRayleigh scattering effect. While the Rayleigh approximation
marginally holds at the Ku band with the exception of heaviest

Toyoshima et al., Early Evaluation of Ku- and Ka-Band Sensitivities for the GPM DPR

17

References

Fig. 4. The same as Figs. 3a, c, d, f, but only for the regions where surface
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ocean, (c) 30°N−65°N land, and (d) 30°S−65°S land.

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Acknowledgements
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Manuscript received 30 October 2014, accepted 27 December 2014
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