Friday, November 21, 2014

Rain Droplet Growth: Collision-Coalescence Efficiency

When rain drops grow by collision and coalescence not every drop of water it hits gets absorbed by the bigger droplet. We therefore need to include an efficiency term in our equation of droplet growth.

Equation for the change in droplet radius over time. E represents the collection/coalescence efficiency. v_T is the terminal velocity of the drop, LWC is the liquid water content of the cloud, and ro (the funny looking p) is the density of water. We can integrate and solve this equation to determine the length of time required for a droplet to grow to some determined size.

Below is a video by Reid Wiseman, an astronaut at the International Space Station, demonstrating the collection efficiency of a raindrop.

Thursday, November 20, 2014

Welcome back to the prolonged inversions and poor air quality

Air quality along the Wasatch Front has been getting worse the last few days. Salt Lake City is the winter home of temperature inversions and bad air. And this is just be beginning of the season. Below are shown measurements of PM 2.5 (Particulate Matter smaller 2.5 micrometers in size).
The air is not super unhealthy yet, but the pollution concentration is increasing. You may have noticed the decreased visibility last time you tried looking at the mountains across the valley. This weekend a storm should blow all this gunk away before it gets much worse, but be prepared for more days like this and try to drive less during these inversion periods if you can.
November 19, 2014 @ 2:50 PM
Photo: kbkb
November 20, 2014 @3:30 PM
Source: WBB webcam
Above is another look at the smog today. The sun is beginning to set. The concentration of the smog actually looks worse due to the fact that we are looking in the direction of the sun. When particles scatter light, they always scatter with constructive interference in the forward direction, toward your eyes if you are looking toward the sun. This optical phenomenon makes the haze look brighter, making it seem there is more stuff in the air. But still, the air is dirty.
source: atmospheric radiation course notes

More Snow for Buffalo

Another round of lake effect snow is pounding the area just south of Buffalo, New York. Some places report over six feet of snow in the last three days. 

And now a bunch of pictures from Twitter...

Wednesday, November 19, 2014

HRRR Forecast Skill

Curtis Alexander from NOAA/ESRL gave a webinar talk titled "Progress and plans for the operational HRRR model." The talk can be found here.

Below is just one slide from his presentation. It shows forecast skill of different methods of forecasting the weather. A persistence forecast, saying the weather will be the same later as it is now, has no skill beyond an hour. Extrapolation, or saying the weather trend will continue, does pretty good for an hour, but skill quickly decays. The HRRR model (dark green line) produce a better forecast than extrapolation after the first hour. After the first forecast hour the HRRR model is more accurate than persistence and extrapolation. As with all weather models, the forecast skill of the HRRR degrades over time, but still there is more skill in a HRRR forecast at six hours than a simple persistence or extrapolated forecast.
HRRR Forum

Growth of Snow Crystal

Time lapse of snow crystals grown in a laboratory