Saturday, July 21, 2012

Position of BB2 as on 5.30pm Saturday, (IST).
IMD satellite image superimposed with vagaries Graphics.Shown: Movement till now, Expected Track, Monsoon Axis, UAC  and BB2.


10 comments:

Abhijit Modak said...

@ Rajesh Sir.Means there are two Low system near by near area ! One located over Chattisgarh/ Orissa border which will travel WNW direction. And second one in form of UAC over Vidarbha region ! I want know what is pressure difference between LOW & UAC ? At what mb pressure we confirm it is developed into low ! And UAC pressure slightly higher over Low ! Or what makes UAC & LOW difference from each other ! And also what is the pressure of off shore trough of west coast !

GSB said...

The MJO seems to be turning negative from 22nd July till about 2nd Aug. After that too some models are negative thru august. Am I right ? Not an expert in this type of analysis....

Rajan Alexander said...

Yeah. From MJO point of view, August looks a very bleak month for rainfall. But I suppose it depends on which model you look at for the "worst" case scenario.

I too is not very well verse with MJO.

But can MJO alone over-ride other favourables?

Can the MJO be negative and we still get rains? If so, what other conditions?

Rajan Alexander said...

Did some quick research on MJO and the IMD has a good research report on this subject. http://www.imdpune.gov.in/ncc_rept/RESEARCH%20REPORT%2010.pdf

It seems rather the amplitude (intensity) of MJO it is its phase that is more important.

What I culled out from the report hoping it could be useful for those trying to forecast the monsoon are given below. If this throws additional light on August monsoon behaviour, pls share:

1. During MJO Phases of 1 & 2, break monsoon type rainfall distribution was observed over India. Subsequently, as the MJO propagated eastwards, a gradual northward shift of the above normal rainfall band from south Peninsula to north India was observed.

2. During Phases 5 & 6, the above normal rainfall band was observed along monsoon trough region and active monsoon type rainfall distribution was observed.

3. During the subsequent Phases (7 & 8) a general decrease in the rainfall was observed over most parts of the country.

4. The observed intraseasonal variation in the rainfall can be explained by the changes in the convective and circulation anomalies observed in association with the formation of MJO induced positive convective anomalies over equatorial Indian Ocean during Phases 1 & 2 and its northward propagation. Earlier studies have also reported northward propagation of cloud bands over Indian monsoon region with MJO periodicity (Yasunari 1979, Sikka and Gadgil 1980, Gadgil et al. 2003).

5. The MJO induced above normal convection over the equatorial Indian Ocean during Phases 1 & 2 strengthened the OTCZ over the region. The associated anomalous rising motion over the region caused large scale anomalous subsidence
over the monsoon trough region as manifested by the low level anomalous anticyclone over the region. These factors weakened the monsoon circulation and resulted in break monsoon type situation. Associated negative convective anomalies
over monsoon trough region extended eastwards to date line indicating weaker than normal northern hemisphere ITCZ and formed dipole like pattern with convective anomalies of opposite sign along active OTCZ.

6. Around Phase 4 MJO induced OTCZ was transformed as monsoon trough over Indian monsoon region and activated western part of the ITCZ.

7. Meanwhile, the eastward propagating MJO associated convective anomaly activated eastern part of the ICTZ. During Phase 5 & 6, the activated monsoon trough moved northward to its normal position and active monsoon type convective and circulation anomaly patterns were set in.

8. During Phases 5 & 6, the dipole like pattern in the convective anomalies was reversed with negative anomalies over equatorial Indian Ocean and positive anomalies along ITCZ. Associated anomalous subsidence motion over equatorial Indian Ocean and anomalous rising motion over monsoon trough region caused stronger than normal monsoon circulation.

9. observed strong impact of MJO on the onset of break and active monsoon events over India and their duration. Though
the amplitude of the MJO plays a significant role in this association, the Phase of the MJO seems to be more deciding factor.

rajesh said...

abhijit: The UAC is associated with the low, meaning the low is tilting upwards with height towards the SW direction. It sometimes occirs with a low. Low is at Mean Sea level (pressure) while a UAC will be in the upper atmosphere, 850 mb, 700 mb, 500 mb, or higher...
Rajan: the IMD note is generalised..dont you think so..i mean it is a general note, a sort of an education on MJO vis-a- vis Indian Monsoon..again like i mentione on phone, it may vary from situation to situation. But a overall understanding of MJO is relevent also..Thanks.

Rajan Alexander said...

That's right Rajesh. Its general but for beginners this could be a Guide for Dummies.

Correct me if I am wrong. The MJO is now between Phase 4-5. Even if its amplitude is weak, what I understand from these general principles is that the Phase could over-ride its amplitude viz we can expect good rains despite poor amplitude. Is this behind the IMD's rather bullish forecast for August? Or is this still a very simplistic or even a wrong understanding of the basic principles?

I have taken till end of the month for granted - the best rains for the season so far. Trying to get understand the possible monsoon behaviour in August.

Rajan Alexander said...

That's right Rajesh. Its general but for beginners this could be a Guide for Dummies.

Correct me if I am wrong. The MJO is now between Phase 4-5. Even if its amplitude is weak, what I understand from these general principles is that the Phase could over-ride its amplitude viz we can expect good rains despite poor amplitude. Is this behind the IMD's rather bullish forecast for August? Or is this still a very simplistic or even a wrong understanding of the basic principles?

I have taken till end of the month for granted - the best rains for the season so far. Trying to get understand the possible monsoon behaviour in August.

sset said...

Rajesh/Rajan,

Are there any good websites to understand weather terminologies? For instance difference between low,vortex,uac,mojo???

Thanks

Rajan Alexander said...

Hi SSET: Try this link. http://theweatherprediction.com/basic/guide/

It looks very useful for me too. Must read

ganapathy said...

Thank you very much Sset / Rajan / Rajesh for the nice link and info about weather.- regards, rajugana