Past IEM Features tagged: radiation
The featured chart displays model estimates of daily solar radiation for Ames. The brown area represents the maximum potential based on long term estimates from the NARR model. The green area represents what was estimated for this year. Since there is an availability lag of the NARR data, a short term weather model (HRRR) is used to supplement this gap allowing for estimates up until yesterday. While the daylight time and solar declination changes are easily noticeable for this time of year, the actual difference in solar energy received is some 3-5 times less than what we get during the summer time. This energy difference makes it much more difficult to warm the ground during the winter time and thus warm the air. Recently, we have also had plenty of clouds, which has further limited the amount of radiation received. For warming to occur this time of year, we need help from the transfer of warmer air masses to our south into Iowa.
Much of Iowa has been struggling to see the sun these past few days and so temperatures have been struggling as well. When the sun is seen, the ground is typically experiencing the potential maximum amount of energy received for that day and time of the year. This produces heating that drives our near surface air temperatures warmer during the daylight hours. The featured chart presents one model's estimate of daily maximum shortwave solar radiation received at the surface and what actually happened this year. You can consider the tan area showing up until today as the effect of clouds. The last two bars in the chart show the minimal amount of solar radiation experienced by Ames for the past two days. An increase in sunshine is expected today, which will help warm temperatures out of the 40s and 50s we've been stuck in.
The featured graph and images are from the Ames SchoolNet Site and Webcamera. The time series is of instantaneous solar radiation sampled every minute. Notice how the curve transitions from smooth to choppy in the afternoon and then smooth again just before sunset. The shape of this curve tells the story shown in the featured 3 webcam images, a transition from clear skies to party clooudy to mostly clear at sunset. The solar radiation curve becomes choppy as the sun plays peekaboo with the clouds as opposed to being a pure function of location in the sky during optically clear situations.
Tags: sun radiation
The featured graph has a lot going on. The three data lines are from the NSTL Flux site near Ames for Tuesday morning. The green line is long wave radiation and simply shows the period up until 5 AM which was cloudy (clear sky values are around -100 W m**-2). The orange line is short wave (solar) radiation and its obviously near 0 until sunrise around 8 AM increasing to around 400 W m**-2 at midday. The air temperature trace in red shows three distinct regimes during this period.
- Between midnight-5AM: Cloudy skies trap heat near the surface (decreased long wave) and temperatures slowly cool.
- Between 5AM-8AM: Skies clear, long wave radiation increases, and air temperatures cool rapidly at 4 times the rate with clouds.
- 8AM - after: The sun rises and short wave radiation exceeds long wave radiation (net radiation positive) and the result is heating.