The acronym “OLR” stands for Outgoing Longwave Radiation.

This parameter is a direct output of WRF system and it is used to simulate an infrared satellite image.

What is Outgoing Longwave Radiation?

Visible light, radio waves, and X-rays are all forms of electromagnetic radiation. Electromagnetic waves are self-propagating oscillations of electric and magnetic fields that travel through space at the speed of light (c ~ 300,000km/s). Unlike mechanical waves, they do not require a physical medium to propagate. Electromagnetic radiation carries energy both as waves and as discrete packets called photons. It is produced by accelerating electrically charged particles, either naturally (e.g., by the Sun and other celestial bodies) or artificially for a wide range of scientific, industrial, and medical applications [1].

Figure 2 illustrates the electromagnetic spectra of both solar radiation and the Earth’s thermal emission. The vertical axis has been scaled so that the two spectra can be compared more easily. It can be seen that the Earth’s thermal emission is concentrated at wavelengths longer than approximately 4 μm (horizontal axis), a region of the electromagnetic spectrum generally classified as longwave infrared (LWIR). In contrast, the Sun emits most of its radiation at much shorter wavelengths, primarily in the visible and near-infrared regions of the spectrum.

Radiation intensity (both solar incoming and earth’s outgoing) is measured by Watts per square meter (W/m²). Using the Stefan-Boltzmann law [3] for black bodies (F = σT⁴) and assuming that Earth’s average temperature is 15ºC (288ºK), we can calculate the average Outgoing Longwave Radiation of Earth, which is roughly 240W/m² .

Below, two examples of U-Model’s Outgoing Longwave Radiation forecasts are presented. 1st image shows the forecast for 18th of July 2026, 17:00 local time and 2nd shows the forecast for 19th of July, 05:00 local time. Both forecasts use initial and boundary conditions from NCEP/GFS model’s run of 17/7/26 – 12Z.

1.

2.

By comparing both charts, we can obtain useful information. For example land colors appear to be lighter in the morning (2nd image) than in early afternoon (1st image). This is due to the fact that, the land is cooler during this time, so radiation flux is also lower. The most interesting observation though, is that using OLR parameter, we can easily observe various cloud types. The higher the cloud tops, the colder they are, and therefore the less outgoing longwave radiation they emit. So for example in image 1, we can clearly observe the characteristic rounded tops of thunderstorms, whose temperatures often reach as low as −60°C (or even lower). This is confirmed from the image’s legend, as we can observe that the color of those areas corresponds to 100W/m² radiation (or even lower). If we apply Stefan-Boltzmann’s low (for a black body material), we can calculate that cloud top temperatures are indeed around -65ºC.

As there is a direct relationship between cloud tops temperatures and outgoing longwave radiation, those charts actually represent a “forecast” of an infrared satellite image, which is widely used in meteorology to determine cloud-top temperatures, estimate cloud height, and monitor the development and movement of weather systems during both day and night. Therefore OLR forecasts are directly comparable to those images and can be a useful analysis tool.

An important limitation is that a standard infrared image mainly shows the temperature of the uppermost opaque surface. It cannot easily reveal lower cloud layers hidden beneath higher clouds. Besides this, cloud-top temperature alone does not prove that precipitation or severe weather is occurring (cirrus clouds which do not produce precipitation are also at the same level).

For this reason, another forecasting product has been developed, named “clouds”, easily accessible through umeteo’s tool (https://umeteo.com/maps) aiming to distinguish between high, middle and low clouds, using a clear color pallete.

OLR maps, are provided for both the Mediterranean (25km horizontal resolution) and Greece (5km horizontal resolution) domain of U-Model .

In the link provided (https://umeteo.com/maps), one can explore forecasts for the next 72 hours, update twice per day.

All ºU-Model charts are created using NCAR Command Language (NCL), V6.3.0 [4]

Sources

[1] – https://en.wikipedia.org/wiki/Electromagnetic_radiation

[2] – https://en.wikipedia.org/wiki/Outgoing_longwave_radiation

[3] – https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law

[4] – https://www.ncl.ucar.edu/