Weather Radars

684 Unique Browsers visited my first blog and you spent an average of 2m.18s reading the blog, which is not bad for a first effort. I find the statistics about ones site incredible and while this blog is not about attracting advertising revenue, it is important to know the stats and demographics of your blog if you want it to be successful. Apparently if you want lots of people to read your blog you need to write some crazy stuff …….. but I also might lose my job :)

Thank you for all the emails suggesting different topics, I will try and get to each one.

Weather radars was a topic that came up from several people. In 2007 our weather radar network looked like the picture below. The dark part of the circle is the area / range in which the radar is very accurate and the lighter range is useful as a heads-up. New modern radars send out both vertical and horizontal scans allowing us to not only determine the presence of rain but the volume as well. This is really handy for forecasting and tracking storms and severe weather. Severe weather forecasts are free to everyone and we provide this information to all media BUT they don’t have to use it. If you have an email address, one assumes if you are reading this blog you do, then you can register to receive warnings and watches direct to your email for the areas you wish to receive. Weather Watches are a “heads-up” ,we are not completely 100% sure something is coming, it’s your choice to be safe or sorry. They are usually published a couple of days before the main event.

2007 Radar Network

By the end of 2012 the network will increase to cover most of NZ with some patches in Nelson, Otago, several islands and upper Manawatu / Rangitikei / Wairarapa region. We are still working on funding for these areas.

 

2012 Radar Network

One of the key questions asked was, who pays for these and why aren’t they on metservice.com in real-time?

Each radar costs around NZ$3m to install and then we have a number of operating costs e.g. data communications and maintenance. The payback to NZ is obvious, just ask the insurance council the cost of one storm, or a farmer the cost of losing his best ewes, the cost of a life…the list goes on. As the ferryman says “someone has to pay” but in this case it is a bit more than two gold coins.

The radars are partly funded by the NZ government and partly by our customers e.g television, large construction firms, industry groups, local government etc etc. If the radars were fully funded by the government we would give everything away for free, but we can’t do this as we would lose all external revenue and this would end up costing the tax-payer more. If you did the financial analysis the best decision would be to fully government fund the radars and provide all data for free, assuming NZ would benefit, BUT this also assumes our government has endless amounts of money. Now I am not a politician but I know they don’t have endless amounts of money so if we receive more, someone else has to miss out. I do believe that when we are in better times that we will get funding to complete the Weather Radar Network.

In my next blog I want to talk about recreational marine forecasts and why we want to forecast more areas and the approximate cost. As a country that is surrounded by water we want to make it a safer place to enjoy. Also I might be able to talk about a new JV that will see better beach / swell forecasts for recreational purposes to be launched on metservice.com in time for summer.

The Foehn Wind

Last Saturday many parts of New Zealand, especially South Island, experienced Foehn winds. As summer approaches eastern parts of both islands will get more warm Foehn winds under suitable conditions. In this blog post I’ll describe what the Foehn wind is and explain how it comes about.  

We’ll start by looking at temperature reports from weather stations on Saturday 16 Oct 2010.  

     western stations    Time &
Temperature  
                   eastern stations   Time  &
Temperature  

   Westport:  

  3pm: 15 °C
  4pm: 15 °C
  5pm: 15 °C
  6pm: 14 °C 
  Darfield:     3pm: 20 °C
  4pm: 25 °C
  5pm: 24 °C
  6pm: 22 °C  

Hokitika:  

  3pm: 15 °C
  4pm: 14 °C
  5pm: 14 °C
  6pm: 14 °C
Ashburton:     3pm: 20 °C
  4pm: 24 °C
  5pm: 25 °C
  6pm: 23 °C

Haast:  

  3pm: 14 °C
  4pm: 14 °C
  5pm: 14 °C
  6pm: 14 °C
Dunedin Airport:     3pm: 26 °C
  4pm: 27 °C
  5pm: 24 °C
  6pm: 22 °C

 

Note the big temperature difference from west to east. 

As was explained in the post on Saturation water exists in three phases: liquid, ice and vapour. When a change between any of the phases occurs (for example, when ice melts), energy in the form of latent heat is either released into the surrounding environment or taken from the surrounding environment. The key to the Foehn wind process is this release of latent heat.  

Now, when a northwesterly airstream flows from the Tasman Sea onto the western sides of hills and mountains of New Zealand, it is forced to rise.   

  

As the air ascends it cools (as mentioned in the post on Ridge-Top winds). If there is enough moisture in the rising air, cloud will form in it. As the water vapour in the air condenses into water droplets (cloud), it releases latent heat. This extra heat then reduces the rate of cooling of the air.

Northwesterlies arriving on NZ’s western coasts are often moist enough to produce not just cloud, but rain. The rain falls where the air has risen and cooled the most; usually, this is west of the main mountain ranges. Because the winds over NZ are most often from the west or thereabouts, western districts are generally wetter than places east of the main divide.

So, when the air advances to the eastern side of the hills or mountains, it is drier because much of the water has been lost from it as rain. As the air sinks now, it warms (the same effect described in the context of  anticyclones) at a faster rate than the earlier cooling – because there’s less water to heat. In other words, the latent heat released in the moist rising air upstream of hills and mountains has a big effect on the dry descending air on the downstream side.

Have a look at the animated diagram below, where an air parcel starts on the west coast at 15 ºC, ascends and cools to 3 ºC at the top of the mountain range, then subsides and warms (faster than it cooled) to 27 ºC on the east coast.  

Schematic animation of how air temperature changes in a northwest flow across a mountain range. Note the "lenticular" cloud on the eastern side remains anchored to the mountains.

Once the air reaches the eastern side of the range it has dried out, and the relative humidity can be very low. Your skin might feel dry and irritated as a result, but it’s great for drying your washing on the line!  

All this is very well, but I have noticed a few situations where there is a Foehn-warming effect without any rainfall on the west coast! How can this be?  

The answer lies in the 3-D nature of wind-flow. Sometimes the effect of a mountain barrier is to draw an air-stream aloft down towards the ground on the downwind side, as shown in the diagram below.  

How warmer air can occur at low levels on the eastern side of a mountain range without any rainfall on the western side.

In this case, warming still occurs by the same subsidence process and, provided the temperature is high enough aloft (e.g. 3 ºC as in the previous example) it will be warm on the eastern side (27 ºC in the previous example). No rainfall is needed in this process, but rather a mechanism to draw the air downwards – such as an unstable airmass.  

Everything I’ve discussed in this blog post applies quite generally around the world – whenever moist air flows up and over a hill or mountain, the temperature will be higher on the downwind side.  

In NZ, the principle also works when the flow is reversed, such as a southeasterly airstream flowing onto Canterbury that crosses the Southern Alps and flows over Westland. But bear in mind that the originating airmass has come from the south so it will be colder everywhere. I’ve experienced the southeasterly after it’s crossed the mountains, and its dryness and gustiness can make it feel uncomfortably cold.