ATOC 184

THE SCIENCE OF STORMS

WINTER 2022

TR 10:05-11:25
Department of Atmospheric and Oceanic Sciences (AOS)
Faculty of Science

Course Instructor : Lily Ioannidou

Evangelia.Ioannidou@mcgill.ca

Office hours : 10:00 -11:00 am SATURDAYS on zoom

 Evangelia Ioannidou 2022 © Unpublished work

The slides that follow are instructional material for the Winter 2022
ATOC184 course and may not be reproduced, distributed,
transmitted, displayed, published or broadcast outside the class
without the prior written permission of the author.
 RECAP
Falling raindrop breaking
A RAINDROP is not TEAR-SHAPED !! apart as it grows large

•it is Spherical if diameter less than < 2mm

• it is slightly elongated, mushroom shaped, when diameter > 2mm
Because air pressure as droplet falls is greater on the bottom and
least on the sides.
RECAP
REAL PHOTO
RECAP
SLEET

Consider temperature as in the diagram.

A hydrometeor begins its life as a snowflake.

In the layer where temperature is above 0𝑜 𝐶 it

begins to melt.

Then, the partially melted snowflake enters the

deep layer near the surface of subfreezing
temperatures (less than 0 degrees) and

it becomes a tiny ice pellet called sleet.

Sleet bounces when it strikes the ground.

Typically it has diameter equal of less to

5mm
 RECAP
MEASURING PRECIPITATION
RAIN GAUGES
Consists of:

Funnel shaped collector

Measuring tube with

scale: the wet portion of
the scale indicates the
depth of the collected
water.

An outer cylinder
 PRINCIPLES of OPERATION

Targets are hydrometeors

i.e raindrops, snowflakes,
hailstones, cloud drops
within a cloud, as well as
other things ..

An electromagnetic signal is generated by the weather radar and emitted towards a target. The
wave is reflected and its reflection is detected by the radar receiver system.
More precisely: radar signal reflected by a hydrometeor in the cloud
 Radar screen

Radar beam revolves by 360 degrees around the

vertical axis to make measurements of all cloud
formations in the vicinity of the radar.
 The intensity of the reflected signal is called ‘radar reflectivity’ 𝑍.
It is measured and displayed on the radar screen.

IN GENERAL a high value of reflectivity indicates large size and high concentration of hydrometeors
within the volume of the cloud and a low value of reflectivity indicates small size and low
concentration of hydrometeors within the cloud volume.

What would generate

large hydrometeors ??
 Hailstones grow
A cloud droplet rising then falling through a warm cumulus for several
cloud can grow by collision and coalescence, and emerge from minutes as they
the cloud as a large raindrop. are moved
around in the
convective cell by
vigorous updrafts
and downdrafts
 IN GENERAL a high value of reflectivity indicates large size and high concentration of hydrometeors
and a low value of reflectivity indicates small size and low concentration of hydrometeors

And, large size and high concentration of hydrometeors is the result of intense, long-lasting updrafts in a cloud
RELATION BETWEEN intensity of PRECIPITATION and REFLECTIVITY value
𝜃 𝜃
 RANGE HEIGHT INDICATOR or RHI

𝜃2
𝜃1
RECAP

OVERCAST- CLOUD ABOVE RADAR SITE – RHI IMAGES – RADAR REFLECTIVITY

APPROACHING STORM – RHI IMAGE – RADAR REFLECTIVITY
APPROACHING HURRICANE – PPI IMAGE – RADAR REFLECTIVITY
MOVING STORM – PPI IMAGE – RADAR REFLECTIVITY
SNOW BANDS – PPI IMAGE – RADAR REFLECTIVITY
SMALL SCALE WINDS
GEOSTROPHIC WIND
 KATABATIC WINDS

Very strong downslope winds that rush down elevated slopes at hurricane wind speeds.

Typical setting: elevated plateau surrounded by mountains, with an opening that

slopes rapidly downhill.

Snow accumulates on the plateau in winter

cold

creates a shallow high pressure centre

 VIEW FROM ABOVE
Snow accumulates on the plateau in winter
996hPa

cold

1004hPa PGF
creates a shallow high pressure cell
H

PGF force causes cold wind to flow

across the isobars and downhill from
any openings into the valleys. Gravity
strengthens the downslope flow.

If, in addition a strong Low pressure system, with intense horizontal pressure gradients, passes
over the location, the katabatic winds increase in strength and become destructive.
 REGIONS KNOWN for COLD KATABATIC FLOWS

Former Yugoslavia The Rhône valley, France

BORA wind MISTRAL wind

Invasions of cold air masses from Russia that descend the Descends the mountains, blows over
slopes and reach the lowlands. Known as the BORA winds. the Rhône valley and then into the
Wind speeds > 100𝑘𝑡𝑠 Mediterranean.
 REGIONS KNOWN for COLD KATABATIC FLOWS

Cold air masses flowing down the Cold air masses flowing down the
Greenland ice caps. Antarctica plateau
SEA BREEZES and LAND BREEZES

Thermal circulations
Sea breeze: wind that blows from the sea
towards the coast in daytime.
 COL_1 COL_2

Same mass of air → same pressure at the surface

𝑚1 = 𝑚2
𝑝𝑆𝐹𝐶_1 = 𝑝𝑆𝐹𝐶_2
𝑝𝐴_2 𝑝𝐴_1

COL_2 COL_1
Same mass of air → same pressure at the surface

𝑚1 = 𝑚2
𝑝𝑆𝐹𝐶_1 = 𝑝𝑆𝐹𝐶_2
Land has small heat capacity therefore air mass over
land will warm more rapidly as sun rises.

PGF

𝑝𝐴_2 𝑝𝐴_1
𝑝𝐴_2 𝑝𝐴_1

WATER LAND
WATER LAND
 When the PGF first develops

PGF

𝑝𝐴_2 𝑝𝐴_1

WATER LAND
 As PGF keeps acting

Progressively, deficit of air mass

at level A over land.

This will reduce the weight of

the warm column.
PGF

𝑝𝐴_2 𝑝𝐴_1

Progressively, accumulation of air

mass at level A over water.
WATER LAND
This will increase the weight of
the cool column
 Some time later

The weight of the warm column

has been reduced and

A low pressure L at the surface

over the land has been created.
PGF
The weight of the cool
column has increased and 𝑝𝐴_2 𝑝𝐴_1

A high pressure H at the sea

surface over water was H L
created.
WATER LAND
 PGF

𝑝𝐴_2 𝑝𝐴_1

H L
WATER LAND

This will in turn creates a PGF at low levels in the reverse direction to the PGF at level A.

The sea-breeze is the wind that blows at surface levels from sea to land as a result of the PGF.

Sea-breeze is strongest in the afternoon.

Why?
 PGF

𝑝𝐴_2 𝑝𝐴_1

H L
WATER LAND

The afternoon is the time of the strongest temperature

contrast between water and land.
 SECONDARY EFFECT No 1: SEA BREEZE FRONT

The leading edge of the cool ocean air advancing

inland is called the sea-breeze front.

As the front moves inland, the humidity of the

air over the land rises.

Since there are a lot of sea salt particles in this

humid air that can serve as condensation nuclei
the water vapour carried in the air mass
condenses at 𝑅𝐻 < 100% . This leads to

production of haze or fog at the

leading edge of the sea-breeze front
the cool air rises over coast.
SECONDARY EFFECT No 1 of THE SEA BREEZE

WATER LAND

Humid air is warmed over

land, so becomes buoyant
and rises → clouds tend to
form over land in daytime.
CONDENSATIONAL HEATING

Release of large amounts of

heat

Additional heating of the

warm column over land !!!
 SEA BREEZE CONVERGENCE

Over the Florida peninsula helps produce its abundant rainfall

From the Gulf shore it From the Atlantic side the sea
moves in from the West breeze blows from the East

Their convergence inland: air masses rise, giving cloudy,

showery weather over land. While over nearby water skies
remain cloud-free.
 SURFACE WEATHER MAP.

Showing a Low pressure system with its warm and cold front
We learned that the way the wind blows is determined by the geostrophic
balance that is a balance of 2 forces the PGF and the CF.

Because of the geostrophic balance the flow around a Low pressure centre
and a High pressure centre in the Northern hemisphere is as shown.

1004hPa 1008 hPa

996hPa 1020 hPa

L H
IS GEOSTROPHIC BALANCE ALWAYS VALID ?
 GEOSTROPHIC BALANCE

LOW pressure

PGF
CF
HIGH pressure

We saw that at each point downstream CF acts at right angles to the curved
path and deflects the particle to the right of its original path.
So, the path becomes progressively more bent.
And eventually this gives a flow that is parallel to the isobars, with the high pressure on the right in
the Northern hemisphere and proportional in strength to the pressure gradient.

LOW pressure

𝑉𝑔

HIGH pressure

Are the PGF and the CF the only forces acting on the air mass ?
When an air is moving near the surface of the Earth it is also subject to the
force of friction.

Friction is a result of exchange of momentum between molecules.

When the speed of two layers of air is different, there is a strong exchange
of momentum at the boundary between the molecules of the slow moving
and the fast moving air mass.

layer_2

layer_1
 So,

1. Exchange of momentum happens when there is a marked

change of the wind speed or the wind direction in the vertical

Wind 𝑉
𝑧

2. Exchange of momentum also happens in the lowest 1km

of the atmosphere, called the Planetary Boundary Layer, as the air
flows over terrain or water.

This exchange is particularly strong for flow over rough terrain.

The friction or frictional drag FR acts in the direction opposite to the wind speed
and is strong for strong wind speed.

FR Wind 𝑉
So, we consider that since FR is so strong near the surface of the Earth the
moving air-mass in the lowest 1km of the troposphere is under the effect of
the FR as well as the PGF and the CF forces.

and, for the flow to be steady, we need that these three forces balance out.

PGF + CF + FR = 0

LOW
PGF
𝑉

FR CF
HIGH
THREE-FORCE BALANCE GEOSTROPHIC BALANCE

LOW 𝑝 LOW 𝑝

PGF PGF
𝑉 𝑉

FR CF CF

HIGH 𝑝 HIGH 𝑝

When the balance of the 3 forces happens the wind

a) is no longer parallel to the isobars but is directed at an angle to the isobars

towards the Low pressure

b) is weaker than the geostrophic wind would have been, for the same PGF.
So, this wind is called subgeostrophic.
to be continued …..