Explain the typical weather conditions in New Zealand when affected by cold and warm advection. 

Cold advection:
– usually means sub-polar air moving North or North East which results in a South or South West wind
– when these winds encounter the mountain ranges of the North and South Islands; orographic lifting enhances the instability factor so major cumulus and cumulonimbus clouds develop

Warm advection:
– tropical air moves South or Southeast; causing a Northerly or North-Westerly wind
– cooling of the lower layers increases the relative humidity and stability of the air leading to stratiform cloud with low bases and rain or drizzle and poor visibility

Describe what is meant by:
a)  cold advection; 
b)  warm advection. 

a) Cold Advection
This is cold air travelling towards warmer latitudes

b) Warm advection
This involves warm air travelling to colder latitudes and

8.36.16  Describe the typical associated factors for a Northerly flow onto NZ

(a) Stability; 

A northerly flow from the tropics or sub-tropics will be warm and very moist. As it moves south over a progressively
colder sea surface temperature, the airmass is cooled from below. This causes a low-level inversion to develop which
is a major indicator of stable atmospheric conditions. 

(b) Cloud types;
Even though the environment is stable, cloud can still form if there is a trigger to cause forced lifting. In a flow
originating in the sub-tropics, the lifting mechanism will be either frontal lifting caused by a warm front, or the
converging circulation surrounding a low of sub-tropical origin (both collectively fall under the heading of
‘widespread ascent’). In addition, further lifting may be caused over the New Zealand landmass by orographic lifting.
The clouds generated in this stable flow will be stratiform in nature i.e. clouds that form in layers: Stratus (St),
Stratocumulus (Sc), Altostratus (As), Nimbostratus (Ns), and Cirrostratus (Cs). 

(c) Likely precipitation; Any precipitation that does form within a stable flow must fall from stratiform cloud. Thus, the precipitation will be in
the form of continuous or intermittent rain or drizzle. 

(d) Visibility reductions; With an inversion close to the surface, the visibility will slowly decline prior to the arrival of any precipitation due to
pollutants getting trapped at low-levels. Prior to the rain, the visibility may drop over time to between 15 and 20 km.
Once the rain sets in, the visibility will drop to between 2000 m and 5000 m, and this condition may last for many
hours. 

(e) Turbulence; The severity of turbulence generated depends on the isobar spacing delivering this flow onto New Zealand. Often
the isobars are reasonably widely spaced, and so the turbulence generated is nothing more than light to moderate in
intensity. However, every now and then, a low of tropical or sub-tropical origin reinvigorates as it moves into New
Zealand latitudes, and the isobars can become very close. This of course, will generate severe turbulence when
orographically lifted.
In addition, there is likely to be a narrow zone of wind shear at the inversion, which will also generate light to
moderate turbulence.

So, to summarise:

In a northerly flow onto New Zealand… 

➡ The warm air is moving over a progressively colder sea surface…

     ➡ Therefore, it cools down near the surface… 

        ➡ This develops an inversion and increased stability…

            ➡ Any lifting results in stratiform clouds forming… 

                ➡ St, Sc, Ns, As and Cs, with very low bases possible… 

                    ➡ This results in continuous RA or DZ and … 

                        ➡ Poor visibility… 

                            ➡ With mostly light to moderate turbulence,
but severe if the isobars are close together.

8.36.14 Describe the typical associated factors for a Southerly flow onto NZ

(a) Stability; 

A southerly flow onto New Zealand will deliver cold, moist maritime polar (mP) air onto the country. As this cold air
flows northward, it moves over a progressively warmer sea or land surface. Thus, the air at low-levels is warmed from
below. This destabilises the airmass. 

(b) Cloud types; 

Convection begins to occur within this unstable airmass, and Cumulus (Cu), Towering Cumulus (TCu), and
Cumulonimbus (Cb) clouds form in the flow. These clouds, by their very nature, form in individual, separated, but
closely spaced convection currents. 

(c) Likely Precipitation; 

Cumuliform clouds like this produce ‘showers’ of rain, hail, snow or sleet. These showers may be very heavy and
frequent, but they fall in short bursts with short gaps between them when the weather is often quite good. 

(d) Visibility reductions; 

In showers, particularly heavy showers associated with Cb clouds, the visibility may be very poor – perhaps as low as
1000 m at times. In between showers however, the visibility will generally be very good. 

(e) Turbulence. 

Southerly flows are often quite strong, and on occasion, they reach storm force. Over land, moderate to severe
mechanical turbulence may exist, particularly in the lee of any ranges. Within the flow itself, turbulence within
cumulonimbus cells may be severe at times as is always the case with any Cb cloud.
The modification of a Southerly flow onto New Zealand is best summarised as follows:

A southerly flow onto New Zealand… 

➡ The cold air is moving over a progressively warmer sea surface… 

    ➡ Therefore, it warms up near the surface… 

        ➡ This results in increased instability… 

            ➡ Pockets of air break away from the surface (convection)… 

                ➡ Cu, TCu and Cb cloud form… 

                    ➡ Resulting in showers of rain, hail, snow & sleet 

                        ➡ Poor visibility at times…and 

                            ➡ It is often turbulent.

8.36.12 State the similarities between cold and warm fronts with reference to changes in the following;

Temperature      Cold Front                                                           Warm Front

                           Steady – then falling post front                              Rising then steady post front

Pressure           Falling then rising                                                  Falling then steady

Wind                 NW backing to W or SW                                        E or NE backing to NW

• Cloud            St and Cb in a narrow band                                  Inc layer cloud / lowering base followed by clearing skies                                    followed by Cu / Tcu

Precipitation      TSRA and poss hail followed by showers           Virga then light rain followed by clearing skies

8.36.10 Describe the range of weather conditions associated with fronts in the NZ region

Cold; 

Cold fronts are most likely to produce thunderstorms and/or heavy showers because of their unstable nature. The
poor weather associated with cold fronts generally lasts no more than 2 – 3 hours (although there are exceptions).
Cold fronts are most active, and therefore most dangerous, when they are fast moving; 25 to 40 knots, or when the
temperature difference across the front is greater than 5 degC. 

Warm;
Because warm sectors are stable in nature, the cloud types produced are layer-type clouds (St Sc, As, Ns and Cs).
These clouds produce extended periods of rain or drizzle, frequently accompanied by very poor visibility and
extensive very low cloud.

 Occlusions;
Occlusions will tend to have only a weak temperature gradient across the surface front. However, there is likely to be
extensive middle and upper cloud associated with the elevated frontal boundary. Precipitation is usual, but mostly
only light or moderate in intensity. 

 Stationary;
Stationary fronts are often weakening fronts, however on occasion; they can result in an extended (2-3 day) period of
extremely poor weather if they stall overhead.

8.36.8 Define the formation processes of the following frontal types

(a) Cold; 

Cold fronts are the most common of the frontal types. Cold fronts form when a cold mP (polar maritime) airmass moves more quickly
than and displaces a mT (tropical maritime) airmass. The colder airmass will force the warmer air to rise quickly at the frontal surface.
Cold fronts are unstable features. 

(b) Warm (or warm sectors); 

Classical warm fronts as found and defined in the Northern Hemisphere are rare if not non-existent in the Southern
Hemisphere. Instead, we experience what forecasters have termed ‘warm sectors’. Southern Hemisphere warm
sectors are drawn with the same symbols as the classical warm fronts, however the length of the front tends to be
much shorter than Northern Hemisphere examples.
Warm sectors occur when moving warm mT tropical maritime) air comes up against the much colder mP (polar maritime air) airmass. Rather than rise
abruptly, the warm air slides very slowly and very gently in a shallow wedge over the top of the cold air. Warm sectors
are stable frontal features. 

(c) Occluded;

 Occluded fronts are drawn on weather maps when there is a band of precipitation, but it lacks the characteristics of a
cold or a warm front. The classical occlusion forms when a cold front catches up with a preceding warm front, lifting
the frontal boundary completely of the surface. They therefore tend to have a weak temperature gradient at low levels. However, precipitation continues from the elevated frontal boundary, and the precipitation still falls to the
surface. 

(d) Stationary;

Officially, any front that slows down to less than 5 knots ground speed is redrawn as a stationary front. This explains
why stationary fronts drawn on a series of weather maps often appear to move substantially over a 24-hour period.

Define source region and state the typical areas where New Zealand's source regions are found. 

Source regions are polar and subtropical regions where there is anticyclonic subsidence belts that contain areas of air stagnation defined from global air movement studies

8.36.6   Define a “front”

Fronts mark the boundaries between airmasses with different temperature and moisture characteristics.

In New
Zealand latitudes, when mT and mP airmasses come together, a front is formed. The type of front depends on which
airmass is moving the quickest