Regional Diurnal Variations of Precipitation Parameters Observedwith Tropical Rainfall Measuring Mission (TRMM) Microwave
Imager (TMI) and TRMM Precipitation Radar (PR)∗Fumie A. Furuzawa1 , Kenji Nakamura1(1: Hydrospheric Atmospheric Research Center, Nagoya University )* HyARC, Nagoya University, 464-8601, Furo-cho, Chikusa Nagoya, Japan. e-mail:akimoto@hyarc.nagoya-u.ac.jp

Abstract
We analyzed the TRMM PR and TMI data over the ocean, land and coastlines by using the global each-passage
data for June 1998, December 1998 - February 1999 (DJF) and obtained some results as follows. The PR reveals
that 1) rainfall with a low storm height over land is dominant in winter or from 6 LT to 14 LT. There exists
a convective rain more frequently from 12 LT to 20 LT and a stratiform rain exists more frequently in winterand from 6 LT to 8 LT. 2) precipitation has many larger drops from 18 LT to 24 LT for both convective andstratiform rains over the ocean and land on June and for both convective and stratiform rains over land on
DJF. The diurnal variation is small over the ocean on DJF. Moreover it has many larger drops at tropics andespecially lower in winter midlatitude, regardless of over the ocean and land on June and DJF. Especially,small size of precipitation is dominant over Tibetan plateau and the Andes. 3) monthly rainfall amount is
the maximum at 19 LT on June and 15 LT on DJF over land, though TMI shows that the time is delayeda few hours. Difference between PR and TMI diurnal variations of rainfall rate is appeared from 10 LT to
17 LT and TMI rainfall rate is smaller than PR at the period. This is probably due to frequently appeared
convective rain and/or frequently appeared shallow rain, as shallow rains have smaller TMI rainfall rate thanPR and convective rains have also smaller TMI rainfall rate. Moreover, this period is coincident with the period
without many large rain drops. This may imply that the TMI underestimates the surface rainfall rate at thecase of rainfall without many large drops. We will discuss about the relationship on characteristics of rainfall
and rainfall rates derived from PR and TMI.
Keywords: TRMM satellite, Precipitation, Regional variation, Diurnal variation

1. Introduction
Satellite observations of precipitation from space canget continuous, simultaneous and global informations.
These global observations of rainfall activities with satellites are important because measurements of rainfall allover the ocean or tropical forest can be hardly done byground-based observations. The TRMM satellite can detect directly obscured precipitation under the clouds byusing microwave more exactly than other infrared or optical satellites. TRMM satellite has two microwave sensors; a precipitation radar (PR) and a TRMM microwave
imager (TMI). The PR is an active radar which can determine 3 dimensional structure of precipitation systemand precipitation parameters such as rain type and storm
height. The rain rate is derived from TMI brightness temperatures and PR radar reflectivities, separately. The rainestimation algorithms for the PR and TMI are describedby Iguchi et al. (2000) and Kummerow et al. (2001),
respectively.
By the regional variations and diurnal variations of various precipitation parameters or regional dependencies ofthe diurnal variation observed with the PR, and then, we
tried to understand the difference between PR and TMI
rainfall rates, especially the difference of diurnal variation.

2. Data and Analysis
We analyzed the Level-2 data of the TRMM PR and
TMI data over the ocean, land and coastlines separately
by using the global each-passage data for June 1998, December 1998 − February 1999 (DJF). Level-2 data include meteorological parameters, such as rainfall rate,
storm height, freezing height, and rain type (convec-

tive/stratiform/others, warm-rain). These data were produced by the National Aeronautics and Space Administration (NASA) and the Japan Aerospace Exploration
Agency (JAXA) at the standard products version 5. The
global each-passage data of various parameters were converted into grid data with grid size of 0.2 degrees (longitude) × 0.2 degrees (latitude). We show the results of thestatistical analysis.

3. Results
3.1.
Storm Height andtive/Stratiform Fraction

Convec-At first, we focus on the PR storm height and rain type,because the difference between TMI and PR rainfall rateis large at the case of deep or shallow rain over land orconvective rain over all surface conditions (Furuzawa andNakamura 2004). Figure 1 shows the fraction of numberof rain events with each storm height over land and Fig. 2
shows the fraction of stratiform rain ratio over land. They
reveal that rainfall with a low storm height over land isdominant in winter or from 6 LT to 14 LT. There exists a
convective rain more frequently from 12 LT to 20 LT and astratiform rain exists more frequently in winter and from 6
LT to 8 LT. Instead of the number of rain events, we used
the rain amount for each storm height. The result has also
the same tendency as these figures (not shown). These
results are in June 1998. During DJF, the same tendency
was obtained except for the reverse of characteristics inthe north and south.

3.2. Drop Size Distribution
A Z-R relation depends on drop size distribution (DSD)
of precipitation. When the relation is written by the fol-Fig. 1: Fraction of number of rain events with storm
height, <3 km (between zero and the lowest red stepline), 3 – 4 km (between the lowest red step line and asecond lower green step line), 4 – 5 km, 5 – 6 km, 6 –7 km, 7 – 8 km, 8 – 9 km, 9 – 10 km, and >10 km (between the highest chartreuse step line and unity) overland in June 1998. The left panel shows dependence
on latitude, and the right one on local time every 3
hours. Cited from Furuzawa & Nakamura 2004.

the convective rain has no large rain drops. Moreover,for convective rain, the variations of both A-parameterand B-parameter over land are larger than those over theocean, and their averaged values are larger. For stratiform rain, the same tendency is obtained for B-parameter.
These results are for June 1998. During DJF, similar results are obtained except for over the ocean (not shown).

Fig. 2: Fraction of number of rainfall with eachstratiform rain ratio, < 10 % (between zero and thelowest red step line), 10 – 20 % (between the lowest
red step line and a second lower green step line), 20 –30 %, 30 – 40 %, 40 – 50 %, 50 – 60 %, 60 – 70 %, 70– 80 %, 80 – 90 %, and 90 – 100 % (between the highest chartreuse step line and unity) over land during
June 1998. The left panel shows dependence on thelatitude, and the right one shows on local time withevery 3 hours. Cited from Furuzawa & Nakamura
2004.
lowing equation;
R = AZ B ,(1)

two parameters, A and B, depend on DSD. It is known
that the DSDs of each precipitation differ depending onrain type (Tokay and Short 1996, Iguchi et al. 2000,
etc). We investigated the variation of Z-R relation foreach region or latitude and for each local time on convective rain and stratiform rain, respectively. As cloud
microphysical processes such as evaporation and collisionrate, are functions of the DSD, DSD statistical studies canplay an important role in cloud modeling studies. Those
two parameters, A and B, were obtained by fitting alldata of a PR reflectivity Z near the surface and PR rainfall rate R near the surface, within each grid with thesize of 0.2◦ × 0.2◦ during one satellite passage, using theZ-R relation. These parameter values can be classified
based on each hourly local time or each grid position.
Figure 3 shows the diurnal variations of these parameters, for two rain types and for over the ocean and land.
For all 8 cases, we can see the clear diurnal variations.
These variations are similar with each other. During 10–
17 LT, A-parameters are smaller than the mean values
and B-parameters are larger. On the other hand, during 20–24 LT, A-parameters are larger and B-parameters
are smaller. For convective rain, however, A-parameters
are much larger than those for stratiform rain, and Bparameters are slightly smaller, as is well known (Tokayand Short 1996, Iguchi et al. 2000, etc). This means that

Fig. 3: Diurnal variations of A-parameter (left panels) and B-parameter (right panels) fitting Z-R relation in June 1998. Top 2 panels are for convective
rain over the ocean, the next for convective rain overthe land, the next for stratiform rain over the ocean,and bottom 2 panels are for stratiform rain over theland. Averaged values are shown by lines.
Relations between A-parameter and B-parameter for
convective rain and stratiform rain are shown in Fig. 4.
The larger A-parameter is, the smaller B-parameter is. It
is consistent with the results of diurnal variations shownin Fig. 3. This tendency means that when A-parameteris large and B-parameter is small, that is, 20 – 24 LT, forstrong rain, large drops are contained and Z is large, on
the contrary, for weak rain, large drops are not contained
and Z is small. On the other hand, when A-parameter issmall and B-parameter is large, that is, 10 – 17 LT, forstrong rain, large drops are not contained and Z is small,and for weak rain, large drops are contained and Z is large.
Although the relation for stratiform rain is simple, thatfor convective rain has two streams. The inclination of therelation for stratiform rain is between two inclinations of

Fig. 4: Correlation between A-parameter and Bparameter fitting Z-R relation in June 1998. Leftpanel is for convective rain and right one is for stratiform rain. These are only over the ocean. The sametendency was obtained for over land. The best fitted
model, B ∼ −0.4log10 A is shown in right panel. In
left one, two streams are shown by dotted lines, which
are B ∼ −0.32log10 A + 0.19, −0.62log10 A − 0.22.

The results are shown in bottom two panels of Fig. 6.
At the latitude of over 30◦ N, A-parameter increases andparameter-B also drastically decreases, for both convective and stratiform rain. These maps of Fig. 7 arefor June 1998. In Fig. 8, we show the same maps for
DJF. Parameter-A is also low at winter midlatitude andA-parameter is very low over Tibetan plateau and overaround the Andes. Eventually, it is likely that large drops
do not exist over Tibetan plateau, the Andes and at winter midlatitude.

Fig. 5: Model Z-R relations, Z = 316R1/B , obtained
from the best fitting equation for stratiform rain over
the ocean in June 1998, shown in right panel of Fig.
4. 7 cases for B-parameters of 0.2 – 1.4 are shown.
All lines are through the point with 25 dBZ and 1
mm hr−1 . Orange dotted line is referred from Marshall & Palmer (1948), Z = 200R1.60 , representing
for midlatitude stratiform condition. The NEXRAD
relation; Z = 300R1.40 is nearly equall with the resultof B-parameter of 0.7 (not shown).
these streams for convective rain.
At the case of stratiform rain over the ocean, the relation between A-parameter and B-parameter are describedby B ∼ −0.4log10 A with deviation of B < 0.1. Thismeans that the Z-R relation depends on only one parameter. We show the relation for various B-parameters offrom 0.2 to 1.4 in Fig. 5. This figure indicates that critical rain rate, which is stated in the previous paragraph,is 1 mm hr−1 . Therefore, for almost all cases (R > 1
mm hr−1 ), the previous statement for strong rain is valid.
That is, at the case of small B-parameter (20 – 24 LT),large drops exist, on the contrary, and at the case of large
B-parameter (10 – 17 LT), large drops do not exist.
In Fig. 6, we show the dependences of A-parameter andB-parameter on latitude for convective rain and stratiform rain over the ocean and land, respectively. Over theocean, A-parameter is smaller and B-parameter is larger
at midlatitude for both convective and stratiform rain.
Especially, the tendency is remarkable at winter midlatitude.
To understand the global DSD map, we show the mapsof A-parameter for convective rain and stratiform rain,
respectively in Fig. 7. At first glance, A-parameter is low
at winter midlatitude and A-parameter is very low overTibetan plateau. These tendencies are seen for both convective rain and stratiform rain and are consistent with
Fig. 6. We excluded the data over Tibetan plateau (70◦<longitude < 110◦ ) and checked the dependencies of Aparameter and B-parameter on latitude over land again.

Fig. 6: Dependence of A-parameter (left panels)
and B-parameter (right panels) fitting Z-R relationon latitude in June 1998. Top 2 panels are over theocean, the other panels are over the land. Bottom 2
panels are for selected-longitude over the land. Red
step lines and open circles are for convective rain, andgreen step lines and crosses are for stratiform rain,
respectively.

3.3. Diurnal Variation of Rainfall Rate
TMI and PR observe a rain system nearly simultaneously, that is, within less than one minute due to the difference in the scan geometries. The quasi-simultaneous measurement gave us a significant opportunity to improve therain intensity estimates by radiometers and rain radars.
In Fig. 9, we show the difference of diurnal variation between TMI and PR over land. TMI rainfall rate is smaller
than PR from 10 LT to 17 LT and larger than from 20
LT to 24 LT. Therefore, TMI diurnal variation looks delay a few hours. At least, TMI rainfall-maximum-time isobserved to be delayed a few hours.

4. Discussion
The cause of differences between surface rainfall rates
estimated by TMI and PR is thought to be TMI underestimate of rainfall rate for shallow rain over land due tolack of ice scattering or convective rain and TMI overestimation of deep rain due to evaporation or tilting system.

is dominant over land and larger drops do not exist or ata period that a convective rain is dominant, difference between PR and TMI diurnal variations of monthly rainfall
rate happens due to TMI underestimation of rainfall rate.
At a period that a deep rain is dominant over land andlarger drops exist, the difference happens due to TMI overestimation. That is, uncertainty of TMI rainfall rate estimation probably depends on the drip size distribution,
storm height and rain type.

Fig. 7: Maps of A-parameter fitting Z-R relation
for stratiform rain (top panel) and convective rain(bottom panel) in June 1998. Unit is 10−6 .

Fig. 9: Dependence of rain amount RT on local
time over land in June 1998. Open circles show the
TMI-RT, and crosses show the PR-RT. The TMI-RT
is smaller than PR-RT in the day time and larger
than PR-RT in the late evening. This means that thediurnal variation is different between PR and TMI.
Cited from Furuzawa & Nakamura, 2004.

Fig. 8: Same as Fig. 7 but for DJF.
Moreover, over land, the period that TMI rainfall rate issmaller than PR (10 – 17 LT) is consistent with that large
drops do not exist, and the period that TMI rainfall rateis larger (20 – 24 LT) is consistent with that large drops
exist. This means TMI weakness for rain estimation ofshallow rain or deep rain over land may be due to DSD.
At winter midlatitude, DSD variation is remarkable
over the ocean, in addition to over-land case. On the other
hand, the relation between TMI and PR rainfall rate over
winter midlatitude is different from that over tropical region for both over-the-ocean case and over-land case. In
Fig. 10, we show the scatter plots between TMI and PR
rainfall rate for stratiform rain over the ocean. This figure
shows that TMI rainfall rate is smaller than PR at strong
rain, and TMI is larger than PR at weak rain at wintermidlatitude. This tendency may be due to different DSD,
that is, a Z-R relation with small A-parameter and large
B-parameter, when there is no large drops for strong rain
but weak rain has large drops. If so, in TMI estimation
algorithm, the effect of DSD variation must be considered.
For convective rain, the relation between A-parameterand B-parameter has two streams. This may result in the
TMI underestimation of rainfall rate.

5. Conclusions
We investigated the relationship between characteristics of rain system and surface rainfall rates estimated by
TMI and PR by using the global each-passage instantaneous data.
The PR reveals that, at a period that a low storm heightFig. 10: Relation between TMI and PR rainfall rates
for stratiform rain over the ocean in June 1998 at thelatitude of 0 – 10◦ N (left) and 30◦ S – 37◦ S (right).

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