Inferring Mesoscale Information From Synoptic Scale NWP Guidance Interactions of Synoptic Scale...

Inferring Mesoscale Information

From

Synoptic Scale NWP Guidance

Interactions of Synoptic Scale features

can createResponses on the

Mesoscale

0 1 5 6 hours

What are we looking for?

1 – An environment that is already conducive to (will support) thunderstorm growth

Remember, Convective Parameterizations do NOT predict realistic convection patterns


1 – An environment that is already conducive to (will support) thunderstorm growth

Classic Severe Thunderstorm Sounding


1 – Environments that are already conducive to (will support) thunderstorm growth

Or

2 - Synoptic flow patterns that will produce this kind of environment in the near future

Need to develop (for thunderstorms):a) Abundant low level moisture

b) A capping inversion (to prevent convection from forming too early)c) Dry air at middle levels

d) Diurnal heatinge) Development of Low-level lift

f) Development of Upper-level divergence

g) For heavy precipitation - add a continuous supply of additional low-level moisture

h) For strong winds – a continuous supply of mid-level dryness

The US NWS has developed ‘check lists’ for use in various parts of the country

How do we find What are we looking for?


Does the NWP guidance already have the signatures that we need?

Look at model soundings and their evolution, but . . .Remember that if the Convective Parameterization in the model has been triggered

the atmospheric stability was becoming too weak, the model will 1) stabilize and 2) moisten the sounding.

Check to see if the model is producing convective precipitation

Model Soundings show important information in addition to Temperature, Moisture and Wind Profiles, including

surface and multi-level stability parameters(In severe weather situations, Model Soundings will typically underestimate strengths

of inversions (temp and moisture), wind shears (vertical and horizontal), . . .



Or

2 - Synoptic flow patterns that will produce this kind of environment in the near future(even if the models don’t depict correct it at small enough scales)


(Model may make area of moisture too wide and moist high enough)

Display of low-level moisture (mixing ratio, g/kg), winds and surface topography,with high amounts of moisture highlighted.

Plot done for 850 hPa layer of model – good for mean moisture in low levels

Display of low-level moisture (mixing ratio, g/kg), winds and surface topography,with high amounts of moisture highlighted.

Plot done for bottom-most 30-hPa deep layer of model to capture near-surface moisture sources - good in areas affected by sloping terrain



Or



Another measure of the total thermal energy in Equivalent Potential Temperature

E

which combines: the temperature of a parcel of air and the total latent heat present in the parcel

Display of low-level Equivalent Potential Temperature (C), winds and surface topography, with high E highlighted.

Plot done for bottom-most 30-hPa deep layer of model to capture near-surface moisture sources - good in areas affected by sloping terrain



Or



b) A capping inversion (to prevent convection from forming too early)(Model will probably underestimate strength of both temperature and moisture inversions)

Diagnosed either from model sounding at a point

Or

By looking at the difference in Temperature from 850 to 700 (or 600) hPa

Or

By looking at the difference in Potential Temperature from 850 to 700 (or 600) hPa

Display of Temperature difference between 850 and 700 hPaLower values indicate more stable Lapse Rates, since temperature normally decreases with height

Display of Potential Temperature difference between 850 and 700 hPaLower values indicate more stable Lapse Rates,

since potential temperature always increases with height



Or



b) A capping inversion (to prevent convection from forming too early)c) Dry/cold air at middle levels + High tropopause

(Model may underestimate extreme dry values and smooth out horizontal gradients)

Display of Tropopause Pressure (hPa)Lower values indicate potential for deeper storm growth

Display of 600 hPa mixing ratio (green, g/kg), Winds (blue) andMixing Ratio difference between 600 and 850 hPa (<-10 g/kg – dashed white)

Lower values indicate greater drying between lower to upper levels

Display of 600 hPa Equivalent Potential Temperature difference between 600 and the layer 30 hPa above the ground (C)

Larger values (highlighted) indicate greater drying from lower to upper levels

Vertical circulations ahead and behind a propagating Jet Streakcan produce localized upper-level convergence and produce drying by subsidence below the jet level.

Contributes both to mid-level drying and warming needed to create ‘capping inversion’

Southern Hemisphere Ideal Jet

Parcels enteringJet Streak accelerateand move toward lower heights

Parcels leavingJet Streak decelerate

and move toward higher heights

Entrance RegionThermally Direct Circulation

Warm air rises Cold air sinks

Exit RegionThermally Indirect Circulation

Warm air sinksCold air rises

Vertical circulations ahead and behind a propagating Jet Streakcan produce localized upper-level convergence and

Subsidence of dry air below the jet level. Contributes both to mid-level drying and ‘capping inversion’

EntranceRegion

ExitRegion

J ConvergenceConvergence

Divergence

Divergence

As a Jet Streak moves past a point (X) that was originally below the left exit region,

It transitions from a thermally direct regime of upper-level convergence and subsidence of dry air which supports the “capping inversion”or “Lid”to

A dynamically forced, thermally indirect regime with upper-level divergence, which forces low-level convergence and lifting of the column of air

X

X

Convective Available Potential Energy (CAPE) is a measure of parcel buoyancy, while Convective INhabition (CIN) is a measure of “Lid” strength

CAPE

CIN



Or




d) Diurnal heating(Model may under- or overestimate daily temperature range do to

incorrect clouds, soil moisture, vegetation, . . . )

Display of near surface Temperature difference between 00 and 12 UTC Larger values highlighted

Beware – incorrect clouds or precipitation in model can cause surface temperature to be too cool!!!



Or





Development of Low-Level Lifting

Here’s where things get interesting

Remember, we are NOT ONLY looking for

areas where the model has forecast rain,

but also

Areas where convection is likely to occur



Options: Model low-level Vertical Motion

Can be affected by model precipitation fields – are they correct?



Options: Model low-level Convergence

Can be affected by model precipitation fields – are they correct?



Options: Model low-level cyclonic circulations/vorticity

Not affected as much by model precipitation fields



Options: Orographic lifting diagnosed from model (upslope)

Calculate uplift using low level wind (above friction layer) and surface topography



Options: Model low-level Vertical Motion

Small-scale cyclonic circulations and short waves

Orographic lifting diagnosed from model (upslope)

In bands head of and parallel to cold fronts

Right-exit or left-entrance region of jet streak

Along outflow boundaries of previous convection

Along “sea breeze” fronts (likely to extend MUCH too far inland)

Water / LandWet Soil / Dry SoilIn afternoon between areas that had beencloudy (foggy) and clear in morning

Gravity Waves, . . .

Jet Streak Entrance Region

Jet Streak Exit Region



Or





f) Development of Upper-level divergence(Model may underestimate wind maxima,

wind shears (especially on equator-ward (left) side of jet), ageostropic circulations, . . .)

Vertical circulations ahead and behind a propagating Jet Streakcan produce localized upper-level convergence and

Subsidence of dry air below the jet level. Contributes both to mid-level drying and ‘capping inversion’

EntranceRegion

ExitRegion

J ConvergenceConvergence

Divergence

Divergence



Or







What are we looking for? For heavy precipitation - add a continuous supply of additional low-level moisture

Combine low-level mixing ratio and wind field


Combine low-level mixing ratio and wind field to determine Moisture Flux


Combine low-level mixing ratio and wind field to determine Moisture Flux and Moisture Flux Convergence (at small scale)



Or







h) For strong winds – add a continuous supply of mid-level dryness

What are we looking for? For strong winds – add a continuous supply of mid-level dryness

Combine mid-level mixing ratio and wind field to determine moisture (dryness) advection

Summary of factors for diagnosing Severe Storms

SurfaceHourly surface mesoscale analysis is critical in severe forecasting and necessary to monitor model accuracy. Detailed analysis can uncover features such as boundaries, mesolows, bubble highs, strong pressure falls, and moisture pooling.

Optimal surface features for severe weather include:

· Dew Points 18°C· Theta E ridge and positive Theta-E advection· Low-level moisture flux convergence· Thermal ridge over or west of the moisture axis· Areas experiencing strong temperature and dew point rises· Rapidly developing cumulus congestus within areas· Areas reaching convective temperature· Focusing mechanisms (fronts, troughs, gust fronts, dry lines, outflow boundaries, etc.)· Surface pressure 1005 mb· Areas with concentrated pressure falls of 5 mb over 12 hours.

AloftForecasters should start with a 4-dimensional mental picture of the atmosphere (Doswell, 1982). Upper air and surface maps can and should be enhanced to emphasize features of importance to convective storm forecasting (Maddox, 1979b).

Optimum upper air features for severe weather include:

1. 925 mb (near surface)· Areas under and just west of the low level jet (winds 25 kts)· Thermal ridge west of moisture axis· Significant warm air advection· Strong moisture flux convergence· Focusing mechanisms (fronts, troughs, and dry lines)

2. 850 mb (above surface)· Areas under and just west of the low level jet (winds 35 kts) or on the nose of the jet· Thermal ridge west of moisture axis· Dew Point 8°C· Significant warm air advection· The greater the angle of the winds from dry to moist air, the greater the instability.· Strong moisture flux convergence· Focusing mechanisms (fronts and troughs)· Moisture transport axis



3. 700 mb· Wind veering 30 degrees between surface and 700 mb· Dry air intrudes at a 40 degree angle and speeds of at least 25 knots. Look at Skew- T’s, model soundings, and gridded data for significant entrainment.· Dew point depression > 6°C. Significant dry air in mid levels may signal possibility of strong downdrafts.· Winds cross 12 hr temperature no change line at > 40 degrees· Focusing mechanisms (fronts and troughs)· Significant upward vertical velocity (UVV)

4. 500 mb· Wind speeds of 50 knots· Short Waves, especially negatively tilted rapidly moving short waves· Cyclonic Vorticity Advection with contours crossing vorticity pattern > 30 degrees· Significant cold pool aloft (-16oC June-Aug; -14°C Sept-Oct and Apr-May; -12 °C Nov- Dec; -10°C Jan-Mar) – (changed to be applicable to southern hemisphere)· Horizontal shear over 90 miles is 30 kts



5. 300 and 200 mb (corrected for sougthern hemisphere)· Wind speeds 40 ms-1 (Probably less in sub-tropics)· Diffluent areas· Right exit region and left entrance region of straight jet streaks; right exit region of cyclonically curved jet streak; left entrance region in anticyclonic jet streak· Most severe weather occurs north of the polar jet, and south of the subtropical jet (coupled jet) or in the right exit region of a jet· Severe weather outbreaks often occur in the diffluent zone between the polar and subtropical jet streams· Long wave troughs and strong synoptic scale lift· Significant height falls and or a deepening of an upper level low

FORECASTING LARGE HAIL

The four primary forecasting keys for hail are:

· Strength of Updraft (CAPE)· Height of freezing level (WBZ)· Environmental wind structure· Remembering that supercells (especially HP supercells) can produce prodigious hail

Some operational considerations include the size and distribution of CAPE, using a reasonable lifted parcel, and the environmental lapse rates.

Another important factor is the melting of the hailstone as it falls to the surface from the freezing level.

Forecasters should look for a cold pool at 500 mb (usually associated with a closed low) as it moves into an area of moderate low level moisture

FORECASTING DAMAGING WINDS

The four primary forecasting keys for damaging winds with environments with weak shear are:

· Amount of dry mid-level air· Strength of the updraft· Amount of low-level moisture· Downdraft instability.

When the environmental shear is weak, the thermodynamic profile is a primary signal for identifying when strong convectively induced winds are likely to occur.

Operational forecasters should look for Inverted-V soundings, steep low level Theta-E lapse rates, and the strength of the CAPE.

Sometimes high CAPE and weak shear environments can lead to derecho development.

FORECASTING DAMAGING WINDS

The four primary forecasting keys for damaging winds of air-masses with moderate to strong shear are:

· Amount of low and mid level helicity· Degree of instability· Amount of dry mid level air· Rapid storm motion (> 40 knots)

Precipitation loading and negative buoyancy due to evaporative cooling are recognized factors in initiating and sustaining downdrafts.

Once a downdraft is established, continued entrainment of unsaturated air in the mid levels aids in evaporation and consequently stronger downdrafts.

Storms in moderate to strong shear can turn into supercells, bow echos, and derecho.

Summary of what are we looking for to

Inferring Mesoscale Information

From

Synoptic Scale NWP Guidance

1 – Environments that are already already conducive to (will support) thunderstorm growth

Or







h) For strong winds – a continuous supply of mid-level dryness

Must use real time data to monitor model performance and to detect a variety of trigger (low level lifting) mechanisms

Inferring Mesoscale Information From Synoptic Scale NWP Guidance Interactions of Synoptic Scale...

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