Application of RASS data for understanding the dynamics of THORPEX events

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Application of RASS data for understanding the
dynamics of THORPEX events

• J. R. Kulkarni
• Indian Institute of Tropical Meteorology

2nd WP/RASS Workshop November 28, 2005
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• The skilful prediction of high-impact weather is
  one of the greatest scientific and societal
  challenges of 21 st century.
• THORPEX is a response to this challenge

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THORPEX

• For skillful prediction, the understanding of the
  dynamics of the events is essential

Events studied
Heat wave conditions during March 2004 and 2005
Exceptional Heavy precipitation over Santacruz on
26/27 July 2005
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404 MHz UHF Wind profiler system at IMD Pune
Antenna Array Aperture 13 m X 13 m
Radar Control Room houses RF transmitter,
duplexer and receiver
Schematic of the operation of a wind profiler
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• Merits of Wind profiler system
• The principal merit of the profiler is the high
  temporal height resolution which can be
  achieved.
• Wind profiles can be obtained as often as
  necessary.
• Real time computations and display is available.
• Frequent observations with a single profiler
  enables a time section to be prepared for study
  of movement of weather systems.
• Unattended operation is possible.
• Besides wind, vertical motion, divergence and
  turbulence can be monitored.
• The data represents a profile over a limited area
  of a few km across, over which the wind can be
  assumed to be constant in many situations.
• The height resolution is better than the
  radiosonde and a number of samples can be
  averaged to obtain greater accuracy.
• Because of the time integration the profiler
  effectively samples a much larger volume than the
  radiosonde.

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Atmospheric subsidence and the surface
temperature variability in the pre-monsoon month
over a semi arid north peninsular Indian station
A case study ContributorsSachin Deshpande, R.
R. Joshi, S. S. Damle and Narendra Singh

• Objectives
• The heat wave is one of the high-impact weather
  phenomenon's over India.
• Climatologically, duration of the heat wave over
  the country is generally 5 to 6 days and there
  are two or three such episodes.
• March 2004 showed the similar behavior, however
  there was a long spell nearly of month duration
  of above normal temperatures over Pune.
• To understand the maintenance of the long spell
  of above normal temperatures over Pune, using the
  vertical velocities measured by wind profiler.

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Maximum temperature distribution over Pune during
March 2004
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• The favorable factors for heat wave conditions to
  occur over a particular region are
• (1) large region of warm dry air prevailing in
  the surrounding of that region and appropriate
  flow pattern for transporting hot air into the
  region of the study.
• (2) absence of moisture over a depth of
  atmospheric column.
• (3) large amplitude anticyclonic flow in the
  vertical levels above a place (Chaudhury et al.,
  2000)
• The anticyclonic flow pattern in the vertical
  produces a chain of processes for generating and
  maintenance of the heat wave conditions.
• First it produces a large-scale subsidence which
  makes the atmospheric columns stable. The
  stability inhibits the formation of clouds and
  ventilation of the heat. The clear sky conditions
  help surface to get more irradiance, which
  increase the surface temperatures.
• Thus the key factor in the process is the
  subsidence or in more general terms 'vertical
  velocity.
• Direct measurements of the vertical velocities
  became available at Pune after installation of
  UHF wind profiler radar.

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Data

• In the present study, the vertical velocity data
  measured by the wind profiler in the month of
  March 2004 is utilized.
• Hourly averaged profiler velocity profiles were
  obtained four times a day viz. at 0800, 1100,
  1400 and 1700 IST.
• The data of daily maximum temperature of Pune in
  the month of March 2004 has been collected from
  the bulletin of Daily Weather Report published by
  India Meteorological Department.

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Role of advection in the surface temperature
variability

• Generally heat waves develop in the
  northwestern parts of India, or northern parts of
  Pakistan. From these areas, waves expand to the
  neighboring subdivisions of country.

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• Two approaches of understanding the surface
  temperature variability viz. (1) physical and (2)
  dynamical. In the physical approach, the surface
  temperature at any place is determined by surface
  energy balance.
• In the dynamical approach, the temperature
  variability is determined by the processes of
  advection adiabatic and diabatic heatings.
• The climatological latitudinal distribution of
  horizontal daily isolation in the month of March
  is plotted from the data given by Iqbal (1983).
• The higher temperatures are observed over the
  northern parts of the country. The reason for
  this type of temperature distribution lies in the
  land-sea distribution.

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• The latitude-time cross section of the maximum
  temperature distribution
• The tilting of temperature isolines indicates the
  high temperatures are developed first in the
  northern latitudes and gradually moved towards
  the southern latitudes.
• Three episodes are clearly seen. In the first one
  i.e. on 4th March a region of high temperatures
  is developed at latitude 28.31 N and after 5 days
  the high temperatures are observed at 18.53 N on
  9th March. The second episode is from 16 to 20
  March and the third episode is from 23 to 27
  March.
• There was an advection of warm air from northern
  to southern latitudes which makes the temperature
  distribution uniformly high.

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• Role of subsidence in the surface temperature
  variability
• In order to quantitatively understand the role of
  subsidence, in the surface temperature
  variability, the variability in the vertical
  motion has been studied.
• The height-time cross section of profiler
  vertical velocity in March 2004
• The shallow convection topped on by subsidence is
  clearly seen.

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• Monthly mean distribution of vertical
  velocity- March 2004
• Existence of two cell structure in the vertical.
• The lower cell consists of upward motion
  extending up to 2 - 3 km and the upper cell
  consists of the subsidence motions confined
  between 3 to 6 km.
• This mean structure of the vertical circulation
  of the atmosphere is similar to that found over
  heat low observed by Blake et al (1983) during
  summer MONEX period.
• However, large variations (s.d. 20 cm/sec) are
  observed in the individual profiler velocity
  profiles.

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• The upward and downward motions can be considered
  to cause the cooling and warming effects on the
  surface heat budget and hence on the temperature.
  
• The upward motion mixes the air in the lowest
  levels and thus tries to prevent the surface
  temperature becoming high.
• The subsidence acts towards the increasing
  temperatures in two ways
• (i) it prevents the upward motion in the lower
  levels from reaching higher heights, and thereby
  reducing the ventilation of heat.
• (ii) it warms and stabilizes the atmospheric
  column. The atmosphere becomes cloud free, which
  helps more solar radiation to reach the surface
  levels and increase the temperature.

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• Variation of Wind profiler (triangles) and NCEP
  (circles) vertical velocity at 11.00 a. m. IST.

• Wind profiler velocity is one order higher than
  the reanalysis velocity on most of the times.
• The profiler velocity have larger variability in
  time and height than the reanalysis velocity.
• The reason
• Reanalysis velocity is representative of
  synoptic scale motion whereas the profiler
  velocity is representative of mesoscale motions.
• This is in agreement with the findings of Nastrom
  et al. (1985)

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• The subsidence motion is not occurring
  continuously, but there are layers of upward and
  downward motions laid on each other.
• This is in agreement with the dynamics of the
  atmosphere i.e. Dynes compensation principle
  which states that the boxes of convergence and
  divergence are laid on each other.
• This feature is not observed in reanalysis wind
  structure, since the computations of vertical
  velocities are carried out for discrete layers of
  the atmosphere of relatively large thicknesses,
  in which the fine structures are averaged out.

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• The factors governing the variability of surface
  temperature are
• (1) solar radiation, (2) advection and (3)
  subsidence.
• The advection dominates in the initial period.
  When the horizontal temperature gradient
  vanishes, the effect of advection becomes small.
• 
  The
  incoming solar radiation at Pune
  is computed using the formulation
  given in Racz and Smith (1999)
• The effect of the solar radiation on the
  variability of the temperature is removed by
  fitting a straight line (shown in figure by
  dotted line) to the daily anomalies in the
  maximum temperature and subtracting the
  contributions of the solar radiation heating
  obtained through the equation of the fitted
  straight line.
• The straight line fit captures most of the
  original variance ( 95) and represents a
  reasonable approximation to the original curve.
  The residual temperature anomalies can be thought
  to be arising from subsidence and noise.

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• Association between subsidence depth and
  maximum surface temperature anomaly
• The subsidence occurs in the form of
  alternating boxes overlaid on each other. The
  total depth of the column, even if it is not
  continuous, adds to the warming and stability of
  the atmosphere.

• The correlation coefficients are 0.40, 0.39, 0.53
  and 0.46 at 8, 11 a. m., 2 and 5 p.m. IST
  respectively.
• The highest relationship is observed with the
  subsidence at 2 p.m. IST which is quite obvious.

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• Conclusions
• First time instrument-measured-vertical
  velocities has been used to bring out the role of
  vertical velocities in the maximum temperature
  variability.
• Existence of two cell structure in the vertical
  in the pre-monsoon season over India.
• Two cell structure is similar to that found over
  heat low observed by Blake et al (1983) during
  summer MONEX period. However, large variations
  (s.d. 20 cm/sec) are observed in the individual
  profiler velocity profiles.
• The positive anomalies in the maximum temperature
  over Pune, are related to the heating processes
  by advection, radiation and subsidence.
• Three distinct episodes of advective warmings
  viz. on 4 to 9, 16 to 20 and 23 to 27 March 2004.
  
• After removing the effects of advection and
  incoming solar radiation, the variability in the
  anomalies in the maximum temperature is found to
  be related with the total depth of the
  atmospheric columns in which subsidence occurs.
• Application of study
• A simple regression model may be developed to
  forecast the maximum temperature anomaly at local
  scale a few hours ahead.
• The two cell structure and the order of the
  vertical velocity brought out in this study will
  be found useful in the validation of the meso
  scale models over the Indian region and in turn
  will be useful in improving the short range
  temperature forecasts over the region

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A comparative study of structure of atmospheric
subsidence over Pune in March 2004
and 2005 using Wind Profiler? Hourly averaged
vertical wind velocity profiles were obtained
four times a day from 0800 to 1700 hrs
IST in March 2004 and 2005. ? Mesoscale
vertical velocities have been used to understand
the maintenance of long spell of above (in March
2004) and below (in March 2005) normal surface
temperatures.? The structure of atmospheric
subsidence in Mar 04 and Mar 05 is compared
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Distribution of the anomalies of the surface
temperatures (0C) in March 2004 and 2005 over
India
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Maximum temperature distribution over Pune during
March 2004 and 2005
On every day of March 2004 the daily maximum
temperature was above normal while it was below
normal nearly for half of the month during March
2005.
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Latitude-time cross section of the maximum
temperature distribution
Mar 04 ?Tilting of temperature isolines ?
Advection makes the temperature distribution
uniformly high. Mar 05 ? Temperature isolines
nearly vertical weak meridional temperature
gradient. ? Weak or absence of temperature
advection from north in March 2005.
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Comparison of mean vertical velocity (March
2004 and March 2005)
? In Mar 04, two cell structures upward motion
up to 2-3 km and downward motion in the 3-6 km
? In Mar 05, upward motion extending all the
way up to 6 km ? In Mar 04, with the
progress of the day, the subsidence penetrates to
the lower levels reaching around 1 km in the
afternoon hours. ? In Mar 05, upward motion
existed in the layer 2.0 to 3.5 km and subsidence
penetration was confined to levels above 1 km
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• References
• Blake DW, Krishnamurti TN, Low-Nam SV and Fein JS
  (1983) Heat low over Saudi Arabia desert during
  May 1979 (summer MONEX). Mon Wea Rev 111
  1759-1775
• Chaudhury SK, Gore JM, Sinha Ray KC (2000) Impact
  of heat waves over India. Current Science 79 No.
  2 153-155
• Iqbal M (1983) An Introduction to Solar
  Radiation. Canada Academic Press, 67 pp
• Nastrom GD and Gage KS (1983) A brief climatology
  of vertical air motions from MST radar data at
  Poker Flat, Alaska. 21st Conf. Radar Meteor. Amer
  Meteor Soc, 135-140
• Racz Z and Smith RK (1999) The dynamics of heat
  lows. Q J R Meteorol Soc 122 1-22

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Exceptional heavy rainfall over Santacruz on
26/27 July 2005

• Contributors
• R. Vijayakumar, S. B. Morwal and R. S. Maheshkumar

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Map of Mumbai
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Ridge
Mumbai
Easterly Jet
Cyclonic circulations
Low level Jet
Monsoon trough
Offshore trough
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3 hourly accumulated rainfall over Santacruz
from TRMM satellite imageries
UTC
26Jul
27 Jul
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Table 1.6 Santacruz heavy rainfall and its
association with the Colaba heavy rainfall
Category of heavy rainfall Common occurrence at Santacruz and Colaba both places Occurrence only at Santacruz and absent at Colaba Total of common occurrence
1 6.5 12.9 cm 125 174 299 44
2 13.0 19.9 cm 17 60 77 22
3 20.0 cm and above 10 22 32 31
Total 152 256 408 37
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• These results can be interpreted in terms of the
  horizontal scale of the convection. As the
  rainfall amounts are increased, the percentages
  of common occurrences decrease indicating
  localized growth of the cloud to super cell
  structures and corresponding reduction in the
  horizontal scale of the cloud system.
• On 26/27 July there was formation of
  mesocyclone over Santacruz which had subsidence
  motion in the surrounding regions. This inhibited
  the formation of the tall clouds in the
  surrounding region which explains the 7.3 cm
  rainfall over Colaba compared to 94.4 cm over
  Santacruz.

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• Simulation of heavy rainfall
• using 2-D cloud model.

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• Model requirements
• Vertical Mesoscale convergence / divergence
  structure
• Temperature bubble

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The vertical distribution of convergence and
vertical velocities at different hours on 26 July
2005
Height (Km) 1500 IST Divergence (s-1) Vertical velocity (m/s) 1700 IST Divergence (s-1) Vertical Velocity (m/s) 2000 IST Divergence (s-1) Vertical Velocity (m/s)
4.65 1.2 x 10-3 2.59 -1.8 x 10-3 2.27
4.95 -4.7 x 10-3 1.18 3.0 x 10-4 2.37
5.25 -6.3 x 10-3 1.11 -1.6 x 10-3 0.69 -3.7 x 10-3 1.27
5.55 -7.5 x 10-3 -1.13 -5.9 x 10-3 -1.08 -4.2 x 10-3 0.02
5.85 1.0 x 10-3 -0.85 9.0 x 10-4 -0.83 -1.0 x 10-3 -0.3
6.15 1.1 x 10-3 -0.52 1.3 x 10-3 -0.43 -1.3 x 10-3 0.09
6.45 8.0 x 10-4 -0.27 5.0 x 10-4 -0.28 -1.0 x 10-3 -0.21
6.75 2.0 x 10-4 -0.20 -3.0 x 10-4 -0.36 -1.3 x 10-3 -0.61
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The composite of the vertical distribution of
vertical velocities
Height Km.
Santacruz
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Schematic of mesocyclone (from Stull 2000)
7 Structure of mesocyclone
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• Thank you !