Nearcasting Convection using GOES Sounder Data

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Nearcasting Convection using GOES Sounder Data

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ROBERT M. AUNE AND RALPH PETERSENNOAA/ASPB/STAR

JORDAN GERTH AND SCOTT LINDSTROMSSEC / CIMSS

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Requirement, Science, and Benefit

Requirement/Objective• Mission Goal: Weather and water

– Increase lead time and accuracy for weather and water warnings and forecasts– Improve predictability of the onset, duration, and impact of hazardous and severe

weather and water events– Increase development, application, and transition of advanced science and

technology to operations and services

Science• Can observations from a geostationary IR sounder be used

to predict severe weather outbreaks 1 to 6 hours in advance, filling the gap between radar nowcasts and NWP models?

Benefits• Reduce loss of life, injury and damage to the economy• Better, quicker, and more valuable weather and water information to support

improved decisions• Increased customer satisfaction with weather and water information and services


Nearcasting uses GOES Sounder Data

• The GOES Sounder includes three separate water vapor channels

• The water vapor channels have weighting functions that peak in different parts of the troposphere (longer wavelengths see farther down into the atmosphere)

• Therefore have a three-dimensional look at atmospheric moisture

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Note how sounder yieldsdata at three levels!

http://cimss.ssec.wisc.edu/goes/wf/faq.html


Note that the peak in the weightingfunction descends as the sounding dries out – you are looking at the radiation emitted by water vapor.As the sounding dries, less watervapor aloft to emit, so the sensor‘sees’ farther down into the atmosphere

(compare this page withthe previous page)


Imager Water Vapor for weighting function slides


Imager Water Vapor for weighting function slidesNote Brightness Temperature values at CHS and LBF


Imager Water Vapor for weighting function slidesImager Water Vapor for weighting function slidesNote Brightness Temperature values at CHS and LBF



• Retrievals transform observed Sounder radiances to more common meteorological variables (e.g. temperature, dewpoint) that can then be used to compute other variables (e.g. Lifted Index, CAPE)

• Retrievals require clear skies

• Is there a way to ‘move’ the clear pixels now to future positions that may be cloudy?

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Nearcasting Severe Convection Using the GOES Sounder

• GOES sounder provides hourly snapshots of layer-averaged stability parameters (for example, e). These observations can be assimilated at multiple levels into a Lagrangian model to provide fast, short-term projections of atmospheric stability.

• Lagrangian model uses model winds (u,v) and geopotential heights to guide motion of observations.

• Model output and sounder retrievals are blended together to yield t = 0 observations – thus, there is more horizontal coverage at t = 0 than just from sounder retrievals alone (cloudy regions and eclipse regions can be included)


Premise: Sounder gives information on distinct layers in atmosphere at observation timeWinds from a numerical model can move those slabs of moisture around

Question: Where does Convective Instability develop because of the moving slabs?

Very Moist Layer

Somewhat Moist Layer

Very Dry Layer


Observations at this time are limited over the East Coast by plenty of cloudiness


00-h fields include informationfrom previous runs; arealextent of information on EastCoast is greater.


Cloud-free observationsinside black curve – otherobs are from earlier runs


Observations shown again


How is nearcasting done?

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1 2 3 4 5Data 6

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5 6

4 5 6

3 4 5 6

2 3 4 5 6

Data

Data

Data

Data

Data

etc.

Data include winds and sounder observationsof e and e that has moved to a point at time=0and geopotential heights att=0, 3 and 6h

ob

s time in

creasing

fcst time increasing

1 2 3 4 5

1 3 4

3

2

1

1

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2

Start at an initial time. Use a Lagrangian model.Step forward 6 hours.Output hourly forecastsUse hourly output as input

into later forecasts

etc.

etc.


Benefit

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As clouds develop in the daytime heated boundary layer, you still can track information from earlier observations.

Retrievals aren’t made when clouds appear, but earlier information is still present in the advected fields

There will be more coverage in the 00-h image than a sounder dataset for that same time because the 00-h fields include output from (up to) the previous 6 runs.


Example:

• Yazoo City, MS tornado from 24 April 2010

• Supercell developed in a region of extensive cloudiness, making Sounder data sparse

• However, available data and nearcast model output did suggest a region of strong convective instability in the region of tornadogenesis

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Tornado on ground in northeast Louisiana


Tornados on ground in northeast Mississippi


Sounder data ignored in the presence ofclouds, but information still there in the holes in the cloud deck and in regions where data has moved from earlier times


Minimum in stability indicated


Convective Instability indicated(Tornado location and eventual track shown)


Forecasts for 1800 UTC showexcellent run-to-run continuity(See next six slides!)


Next Case:

• 2009 Case over NW Iowa

• Convection aligned in an arc similar in structure to that suggested by region of instability in nearcast output

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How does this area of instability evolve?



• Lagrangian moisture transport controlled by RUC winds/height fields and GOES-East Sounder moisture.

• Moisture information is from three different levels in the atmosphere for Sounder, and for ABI (vs. 1 for Imager)

• Information can be moved into regions not covered by sounder because of clouds, eclipse, KOZ, etc.– Thus, you have information where you need it!

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Conclusions


Hail case from Wisconsin, 2006

• Presented as a difference in Precipitable water between the two layers

• 6-h forecast shows strong increase in drying aloft in region where severe weather was reported

• Very large hail in Madison -- >2” in diameter

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How do individual points move in the Lagrangian model?

• Note regions of convergence – points are coming closer together – and regions of divergence – points are moving farther apart with time. Lagrangian model is interpolated to a grid for hourly output

• Only one level is shown. Differing rates of convergence and divergence at different levels.

• For product to be computed, must have data at both levels

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How do individual points move in the Lagrangian model?

• In regions of divergence, number points in Lagrangian model output should decrease. Subsequent interpolation from Lagrangian Model to Grid used for output: Requires at least 2 Lagrangian points near the Grid to be considered ‘valid’

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What are error sources?

• Winds in the Lagrangian model are from the RUC. The level chosen to move moisture around must be correct

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In this 6-h forecast,note that the axis ofinstability is near TUL


However, the axis of observed convection at that time is still back in central Oklahoma


Forecasts for 2200 UTCdo converge to a solutionshowing max instability where observed.

Nearcasting Convection using GOES Sounder Data

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