If you've ever looked at the weather forecast,
you might be familiar with a percentage.
It's the probability of precipitation.
For example, sixty percent rain means that when similar
forecasts have occurred in the past, only sixty percent
actually rained.
Uncertainties can sometimes leave us unprepared for storms
and floods.
But predictions improve as we monitor storms in greater detail
and now NASA's newest precipitation satellite
GPM is measuring the individual water drops that make up a storm.
GPM's the first time we've been able to get this type
of information on a global scale.
To understand why this matters, it helps to know how weather predictions are made.
First, computer programs known as weather models turn the
processes in the atmosphere into math equations,
governed by the laws of physics.
These equations receive measurements of the land, sea, and air
to get a view of the atmosphere's current state
then predicts how it will change over time.
To ensure predictions are correct, equations need
to represent the atmosphere accurately.
But one thing that's uncertain in the equations is how
precipitation is structured within storms.
A storm is made up of water drops of different sizes but without
knowing these measurements many weather models operate under an
assumption. What they do instead is have some assumption
about how that's distributed. So as an example, for every one
hundred small drops, there would be ten medium sized drops or
just one large drop. Assumptions are made because
researchers have only studied drops in isolated areas.
Now for the first time, GPM is measuring the size and
distribution of drops around the world.
If we go into a storm we'll see varying drops sizes labeled
different colors.
Near the top, there are many small drops around .5,1 and
2 millimeters. As they fall to the middle of the storm, some drops
bump into others causing them to grow in size.
Right at the bottom, drops can grow around four, five and six millimeters.
This 3-D mosaic of water drops is called drop size distribution.
It shows a high concentration of small drops colored in blues and
greens near the top and lower concentrations of big
drops colored in reds and yellows near the bottom.
A storm with a higher ratio of red and yellow will contain more
water than a higher ratio of blue and green. Without knowing
the relationship or ratio of those large drops to the smaller or medium-sized drops.
We can have a big error in how much
rain we know fell and that can have some big implications for
knowing long term accumulations which can help for flash flood prediction.
Not only does it give a more accurate measurement of rainfall
but drop sizes also give insight into the winds within a storm.
Thunderstorms have a lot of wind associated with them and we all
know this.
But the strength of that wind actually depends on the size of
the drops that are falling from that storm in some ways.
Because a storm with small drops will have more evaporation,
which cools the air more, which creates stronger winds.
We've never been able to see how water droplet sizes vary
globally until now.
So what causes them to vary in different places?
One factor is the temperature of the environment drops grow in.
In the mid-latitudes a lot of those raindrops actually
originated as snowflakes or even hailstones and snowflakes can
grow a lot larger than cloud droplets can.
So you have these big snowflakes that then melt into big
raindrops.
In contrast, over the oceans and in the tropics, they tend to be
smaller and the reason is because smaller raindrops tend
to originate from clouds that don't have any ice in them.
It's worth noting, however, these measurements are only a
small part of the equation. The drop size distribution is one of
many factors that determines how big a storm will grow, how long
it will last and how much rain it will ultimately produce.
As GPM improves our understanding of precipitation
from space, that information will be vital in improving
weather models and thus helping us better predict and prepare
for our weather.