Intraseasonal and diurnal variations in the Indian sector*Kiyotoshi Takahashi(Meteorological Research Institute/Japan Meteorological Agency)
Climate Research Department,MRI,Nagamine 1-1,Tsukuba,Ibaraki,305-0052,Japan
E-mail: ktakahas@mri-jma.go.jp
Abstract
Relationship between intraseasonal and diurnal variabilities in the Indian sector is investigated using daily rainfall dataand cloudiness observed by geosynchronous satellites. Relative diurnal activity defined as the ratio of variance of
diurnal activity to the total one tends to be enhanced just before the onset of monsoon and in inactive phases of
intraseasonal variation.
Keywords: Intraseasonal, diurnal, cloudiness1.Introduction
Intraseasonal variation (ISV) and diurnal one ofconvective activity are known as most dominantvariabilities in the tropical monsoon regions. Many
studies have been done about both variabilities.
However, their relationship and a role of diurnal
variation in the monsoon processes are notyetunderstood fully. It isn’t still clear what a role diurnal
activity plays in the monsoon processes. On that point,
revealing the relationship of these both modes is
considered to be important in understanding thevariability in the tropical monsoon.
The Indian sector is the region where the ISV isobserved most prominently. Taking this into
consideration, we selected this region for investigating
the relationship between ISV and diurnal variation.
The purpose of this research is to depict
observational aspects of their relationship and examine arole of diurnal variations in monsoon processes.

Analyses were done based on pentad data. Phase of
ISV is monitored by pentad-accumulated precipitation
observed at surface stations. And relative activity ofdiurnal variation is evaluated as the ratio of variance ofdiurnal activity to the total variance of 3-hourly
cloudiness derived from the IR observation by satellites
in each pentad.

3. Result
Figure 1 shows a time-latitude section for 71-75E
for 30-60day band-passed anomaly of C255.

2. Data and method
In this study, to investigate the relationship between
ISV and diurnal activities of convection, we usedfollowing data.
To examine convective activity, we used cloudinessdata obtained from geostationary satellite IR observation
by GMS and Meteosat-5.
Especially, availability of Meteosat-5 data is one ofmajor reasons to conduct this study. Metosat-5 wasrelocated on 63E to support the INDOEX project in June
1998. Since then, it has been providing useful data which
can cover the whole Indian sector.
Cloudiness data are compiled 3 hourly into one
degree gridded data with several thresholds, 270K, 255K,240K, 225K, 210K, together with mean TBB (equivalentblack body temperature) for both GMS and Meteosat-5.
Here each cloudiness data with each threshold value as
mentioned above is denoted as C270, C255, C240, C225and C210 respectively. An advantage of these data areproduced based on original pixcel data to reduce
affection by the ground surface information.
Rainfall data observed at surface stations are used
to identify active phases of ISV, and some sorts ofreanalysis data (ERA40,GAME-reanalysis, JRA-25, etc)
are used for the large scale features of related
atmospheric conditions and circulation field.

Fig. 1: Time-latitude section of 30-60 day band-passed
anomaly of C255 for 71-75E. Blue (Red) indicates more(less) cloudiness.

In this Fig.1, you can see that ISV activity isenhanced in every summer season. Particularly, ISV isstronger in summer of 2000 than in other years.
Next we examined the sequence of relative diurnal
activity in association with ISV indicated in pentad
accumulated rainfall.
Fig.2 shows the time-series of these quantities at
Poona (located at 18.5N,73.8E).
There are four major active phases of ISV in 2000
summer season. The first one corresponds to monsoon

onset around June of the year. As can be seen clearly inthis figure, relative activity of diurnal variability tends tobe enhanced just before the onset and in each inactive
phases of ISV.

Fig. 4: Horizontal distribution of 30-60 day band-passed
anomaly of C255 at the pentad (26May-30May,2000).

4. SummaryFig. 2: Time-series of pentad accumulated rainfall(yellow bar), total variance of C255 (TVC255: orange),
diurnal variance (DVC255: green) and relative diurnal
activity defined as a ratio (=DVC255/TVC255: black)within each pentad period at Poona. This is a case for2000.

The phase of ISV identified with pentad accumulated
rainfall can be confirmed with cloudiness data, too. Fig.3
shows the situation of C255 anomalies corresponding to
the peak phase of relative diurnal activity before themonsoon onset of monsoon in 2000. On the other hand,
fig.4 corresponds to the peak phase of ISV in the end ofMay 2000. Anomaly is defined as deviation from a
simple average of 5 years (1998-2002).
These figures confirm validness of identification of
ISV phase by rainfall from the view point of cloudinesschange.

Fig. 3: Horizontal distribution of 30-60 day band-passed
anomaly of C255 at the pentad (11May-15May,2000).
Positive anomalous region of C255 is showed withbluish coloring and negative is with reddish coloring.

We investigated the relationship of ISV and diurnal
variation using satellite data, station data and so on.
Results of this research are summarized as follows.
1) Diurnal convective activity tends to berelatively enhanced in monsoon season.
2) Diurnal activity tends to be enhanced ininactive phases or just before and after rainy
pentads.
3) At one month before the onset of monsoon,enhancement of diurnal activity was alreadystarted.