401 Flashcards
1
Q
Isallobar
A
A line connecting points that have equal changes in pressure over a given period of time.
2
Q
Pressure tendency can provide a good estimation of what?
A
How the pressure centre will evolve with time
3
Q
3 factor that affect the central value of a low pressure system:
A
- Topography
- Diurnal pressure changes
- diabatic influences
4
Q
Topography
A
-Higher elevations reduced to MSL can give inaccurate central pressure values
5
Q
Diurnal pressure changes
A
-a 1-2 hPa is calculated for for central values for the 1600 lcl low pressure cycle
6
Q
Diabatic influences
A
-Difficult to assess quantitatively.
7
Q
6 factors which contribute to divergence and convergence in a developing low pressure system
A
- horizontal thermal advection
- heat transfer by vertical motion
- sensible heat transfer
- latent heat transfer
- horizontal vorticity advection
- surface vorticity
8
Q
Horizontal thermal advection:
Maximum WAA or Minimum CAA
Maximum CAA or Minimum WAA
A
- Divergence aloft
- Convergence aloft
9
Q
Heat transfer by vertical motion
A
- Sinking air (compression) will contribute to -atmospheric divergence
- Rising air (expansion), local cooling will produce convergence. difficult to evaluate
10
Q
Sensible heat transfer
A
-Areas of maximum heating will produce divergence -Regions of maximum cooling causes convergence.
11
Q
Latent heat transfer
A
- Local heating due to heat release will contribute to atmospheric divergence.
- Local cooling due to latent heat storage will generate convergence
12
Q
Horizontal vorticity advection
A
- NVA is associated with divergence
- PVA is associated with convergence
13
Q
Surface vorticity
A
- air flows towards a low center (convergence)
- In free atmosphere upper level divergence and lower level convergence tends to lower surface pressure
14
Q
The atmosphere is said to be baroclinic when there is?
A
-a horizontal temperature gradient on an isobaric surface.
15
Q
What type of baroclinic region exclusively favours large changes in surface pressure?
A
-those which favour thermal development
16
Q
The atmosphere is said to be barotropic when?
A
-there is no variation in it’s temperature along an isobaric surface. (at a given pressure)
17
Q
The development of a low depends on what?
The strength of it depends mainly on what?
A
- the associated baroclinic zone
- the thermal contrast or temperature gradient
18
Q
The _______thermal contrast, the ________ the baroclinic zone.
A
-Larger, stronger
19
Q
5 stages of a life cycle of a baroclinic depression
A
- formation (baroclinic leaf)
- development
- maturity
- occlusion
- dissipation
20
Q
Cyclogenisis
A
-any development or strengthening of cyclonic circulation around a surface pressure system.
21
Q
Indicators of cyclogenesis pt.1
A
- New vorticity centre developing at 500hPa
- Intensification of vorticity and thickness advection over an area where no centre was previously analyzed
- a kink developing on a baroclinic zone or indication of a new cyclonic flow by the surface winds
- awareness of favourable areas for cyclogenesis
eg. warm water in winter - New shortwave trough and positive vorticity area moving across a baroclinic zone.
22
Q
Indicators of cyclogenesis pt.2
A
- New pressure fall or pressure rise centres developing away from a low centre.
- new height rise or falls developing at 500 hPa.
- Topographical influences. eg.new lows can form in the lee of mountains
- to the east of long wave troughs, especially when when a short wave trough connects to it.
- Changing edges of mid level cloud (new s shape curvature
23
Q
Baroclinic or frontal depressions originate on a stationary portion of a baroclininc zone, other points that should be noted:
A
- Some areas favour formation more others
- –along arctic coast lines
- –cA over warm water
- -areas of subsidence -lee side of mountains
- -areas of decreased friction from land to water-hudson bay, along east and west coast
24
Q
Most baroclinic lows are associated with what?
A
- A 500 hPa short wave trough located 3 to 5 degrees of longitude behind the atmospheric low
- Surface lows may or may not be affected by the shortwave trough
- Surface lows are generally guided by the 500hPa flow.and move through long wave troughs
25
Baroclinic instability occurs when:
there is a lag between thickness and contour patterns of about 1/4 wavelength
26
Features associated with a mature baroclinic depression:
-the low has instensified considerably
-wave amplitude has grown on the baroclinic zone and is more pronounced
-Isotherms are curved in the direction toward which the air is moving, like sails on a ship
-the amplitude of the upper trough and ridge is increasing and will reach it's peak at this stage.
east of the low the WAA has increased considerably
-Comma shape
27
Dynamic features of a mature baroclinic depression
- the direction of the motion is parallel to the isobars in the warm sector
- the thermal wave is rotating counter-clockwise about the centre
28
Indicators that an occlusion is forming: (pt1)
- Deep UPR TROF due to CAA
- a few closed contours form in the UPR TROF
- Shear and curvature increase UPR vorticity centre
- UPR centre departs from the baroclinic zone
- SFC low loses thermal supprt
- SFC low keeps contact with PVA support
29
Indicators that an occlusion is forming: (pt2)
-Cold front appears to undercut warm front (TROWAL)
-apparent piching of FNTL crest
-CLD stretches along TROWAL/SFC TROF
CLD further wraps around these intense UPR vorticity centre
30
Indicators of Baroclinic dissipation:
- Surface low moves toward lower 500 hPa heights
- the air column cools down producing a cold low
- Convergence occurs in the entire air column
- Central pressure is usually rising (filling low)
- spiral cloud band s become more diffuse around cold low.
- cirrus leaf moves away with baroclinic zone
31
5 methods that can be used to indicate the motion of a low pressure system:
- History
- isallobaric pattern,
- 500 hPa flow
- isobaric pattern
- baroclinic zone
32
History of low pressure system
- Accurate historical positions can help to position the center more accurately
- The more sites, the better the accuracy
- can calculate average speed for small differences of low center
- 500 hPa contours are very good indicators of direction of motion of the surface
33
Isallobaric pattern
- Surface low usually moves from an area of maximum rises to an areas of maximum falls. best for circular isobars
- With an asymmetrical low (elongated trough, the centre will move along the longest axis with very little adjustment for the isallobaric field if it conflicts with the axis direction
34
500 hPa
-In some cases short wave troughs will move a ittle faster than a surface low
Surface wind will be about 50% of 500 hPa winds
-a 6 decameter change should equal a 8hPa on the surface
35
Isobaric pattern
-Surface low will ussually move ina direction which is parallel to the isobars in warm sector.
36
Baroclinic zone
-If baroclinic zone is falt, the motion of the surface low will be parallel to the orientation of the isotherms or thickness lines
500 hPa,700 hPa or 850 hPa can be used to analyse isotherm pattern
37
Winter Severe weather
-Weather occurring in the winter that can have significant effect on humans and/or the environment
38
4 thermodynamic factors favourable for the development of summer severe weather
- Warming in the upper levels
- Cooling at the upper levels
- Adding moisture to the lower levels
- Drying at the upper levels
39
The difference between destabilization and a trigger
-A trigger contributes to the release of energy that was previously stored through destabilization
40
WAA at low levels due to a low level jet and CAA aloft can cause:
- Instability
| - low level convergence of moisture due to a LLJ can also destabilize
41
instability can be analyzed by looking for:
- Pools or troughs of cold air aloft (700 hPa chart)
- Pools or ridges of warm air at low levels
- indications of lapse rates higher than wet adiabatic lapse rate
42
3 dynamic factors favourable for the development of severe summer weather
- Upper level divergence
- Vertical windshear
- low level convergence
43
Upper level divergence
- creates updraft in a region below
- surface pressures fall
- enhanced by low level convergence
44
Vertical windshear
- enhances strength of updrafts
- organizes storm to maintain itself for a longer time
- nocturnal LLJ increases vertical wind shear
- Strong vertical windshear correlates to to severe summer wx
45
Low level convergence
-air moves upwards, collecting moisture and bringing upwards, enhancing the bad weather
46
Topography and summer severe wx
- Vegetation and water provide moisture
- Hilly terrain causes differential heating
- Channelling causes low level convergence
- Sloping terrain adds lift
- roughness discontinuity can cause convergence
47
Tornado facts:
- travel at about 70 km/h
- rotating winds can reach 480 km/h
- most travel 15-60 km before they subside, can reach 450 km
- trajectory of 300m wide but widths of 10m to 2 km are possible
- most frequent in canada between may and june, between 1500 and 1800 lcl
- from frequently with supercells associated with cold fronts and squall lines
- Mammatus is a good indicator
48
Associated hazards of a tornado:
- Damage to buildings, vehicles, and power lines
- impassable areas for aircraft
- leaves debris on runway
- death and injury
49
Characteristics of a tornado:
- Mesocyclone (microcyclonic, extends from mesocyclonic activity)
- very intense wind vortex (64 km/h to 179 km/h)
- small diameter
- extends from a cumulonimbus
- irregular track
- accompanied by other thunderstorm phenomena
50
Stability index
- an objective method for diagnosing the occurence of and severity of summer convective activity
- a measure of atmospheric stability
51
Uses of Stability indices
- Indices are regional due to different meteorological influences, each weather centre uses it's own
- used for quick assessment of severe weather potential
52
limitations of stability indices
- not always portable
- often do not take all factors into account
- based on forecast parameters, only effective as the forecast they are based on.
53
5 stability indices used in canada
- Lift Index (LI)
- Showalter Index (SI)
- total-totals (TT)
- george index (K)
- Sly index
54
Lift Index
- Equal or less than -6 means severe thunderstorms
- -9 reflects extreme instability
- Compares 500 hPa temp with the temp of a parcel of air lifted adiabatically from surface to 500 hPA
- (LTsfc - 500)
55
Showalter index
- CB will fail if index is greater than 4
- ompares 500 hPa temp with the temp of a parcel of air lifted adiabatically from 850 hPa to 500 hPA
- negates influences of very high moisture that LI includes
56
Total-Totals
- calculated directly from plotted values and dew points
| - greater than 48 indicates favourable conditions for severe thunderstorms, 55 for tornadoes
57
George index
- useful for predicting air mass thunderstorms
| - calculated from plotted values
58
Sly index
- convective indicator of CB development
| - used in alberta, where it was developed
59
Winter severe weather
- Blizzards
- Cold wave
- Heavy snowfall
- freezing rain
- strong wind
- heavy blowing snow
- freezing spray
- wind chill
- heavy rain
- snow squalls
60
Blizzard
- VIS less than 1 km (5/8 sm) in SN and or BLSN
- wind greater than 40 km/h
- temp less than 10c
- All factors must occur for 6 hours or more
- criteria may differ slightly
61
Cold Wave
- rapid fall in temperature in a 24 hour period
| - determined by the rate of temperature drop and the minimum to which it falls
62
Heavy snow fall
- important due to restrictions to visibility, runway closure and airframe icing
- 15 cm or more in 24 hour period
- 3/8 sm
- may differ i different regions
63
Freezing rain
- Freezing surface temperature with warm air aloft
- larger accumulations arise near and parallel to a stationary or slow moving frontal zone
- considerable width
64
Strong winds
-directly relates to reduced wind chill values, snow squalls, freezing spray and BLSN
65
+BLSN
-visibility is reduced to 1/4 sm or less
66
Snow squall
- Sudden moderately heavy snow fall with blowing snow, strong gusty surface winds
- associated with lake effect snow or frontal snow.
67
3 factors necessary for the formation of winter convective clouds
- Unstable temperature profile
- Long fetch
- Little windshear
68
Unstable temperature profile
- difference of 13c or greater between lake temp andd 850 hPa level provides absolute instability (streamers)
- no inversion below 1 km
69
Long fetch
- Distance an air mass travels over water is called fetch-
- 100 km or more required for lake effect precipitation
- 150 km or more for streamers
70
Little wind shear
- shear is 30 degrees or less below 3km need for streamers
| - speed shear should be relatively uniform
71
5 characteristics associated with a snow squall
- shallow systems (< 10000 ft)
- narrow band of precipitation
- snowfall can be heavy (white out)
- duration of several hours to days
- island penetration up to 200 km
72
Pacific Lows
- Broad surface trough often associated with the Aleutian low.
- baroclinic zone (warm water, cold land) associated with frontal jet streams (A/M)
- Occluding lows associated with Polar jet stream tracking toward the gulf of alaskaor filling along the coastline
73
Alberta Lows
- Reformation of Pacific lows, occasionally in a broad trough extending from deep surface lows over the ocean
- Arctic baroclinic zone
- snowfalls rarely exceed 15 cm
- arctic frontal jetstream associated
74
Nebraska Lows
- Maritime or polar baroclinic zone
- M or P jetstream
- weak friction over a relatively flat surface
75
Colorado Lows (lows over rockies)
- Maritme or polar baroclinic lows
- M or P jetstream occasionally associated
- moderate to heavy snowfalls
76
Great lakes low
- Lows in the vicinity of the great lakes
- are crossroads for migrating lows
- a frontal jetstream is often associated
77
Atlantic lows
- Baroclinic zones (warm water/cold land) associated with frontal jet
- divergence corridor between the continental long wave
- increased jet streams with the enforcement of baroclinic zones over the gulf stream
- Abundance of all types type of precipitation
- heavy snowfalls in Labrador and N. Quebec
78
Newfoundland lows
- Broad surface trough often associated with icelandic low
- crossroads for lows originating in great lakes
- Baroclinic zones at times reinforced by labrador current
79
Davis straight low
Dead end for the tracks of migrating lows
- broad surface trough often associated with icelandic low
- Arctic baroclinic zones at times intensified at the ice/open water boundary
