Scatter plots of Long wave radiation budget ΔLW=LWdown-LWup at the three master stations (where long wave radiation is availlable).
Every point is a 10 minute average. Colors indicate hour of day. Larger symbols mark values during fog events.
Fog events are identified via a wet leaf wetness sensor.
Single fog events are joined if seprated by less than 30minutes and fog events must be longer than 30minutes.
If you see fog symbols with RH or leaf_wet below their thresholds these are due to short gaps between fog events.
Click on images to enlarge.
See also ΔLW wet and RH
| Station | N3 | C3 | S3 |
|---|---|---|---|
| ΔLW vs RH | |||
| ΔLW vs leaf wetness |
A very negative ΔLW indicates clear sky emitting at temperature well below zero degC and a warm surface, which is the typical situation during daytime.
A value of ΔLW close to zero indicates fog (down welling LW radiation is emitted at a similar temperature as the surface).
Intermediate values above ΔLW 100Wm-2 indicate either a cool surface or an elevated cloud emitting long wave radiation at temperatures only somewhat lower than the surface.
ΔLW has under clear sky conditions a typical diurnal course: it starts a 00UTC (1-2hours after sunset arount 23UTC) with values around -100wm-2 (blueish dots).
During the course of the night the surface cools and ΔLW accordingly increases slightliy to values around -60W/m-2 (greenish dots). With sunrise (11-12UTC) the surface warms up and ΔLW to values below -150Wm-2 (yellow to red dots). Towards evening the surface cools and ΔLW tends again towards -100Wm-2.
During fog ΔLW is above -50W/m-2 except for the end of fog events when the sensor is still wet and air is still humid but the fog layer becomes either shallower, or contains less water droplets, or has disolved.
One can estimate the temperature difference related to a certain ΔLW by linearizing Stefan-Boltzmans law
If we assume an emissivity equal to one we have:
LW = σT4 = σT04 ×( 1 + 4×(T-T0)/T0 ) + …
The difference in radiation emitted at two different temperatures T0 and T is accordingly
ΔLW / LW0 ≅ 4 ×(T-T0)/T0
with LW0 = σT04
For a temperature T0 of 15°C we get at T=5°C, T=-4°C and T=-35°C values of ΔLW=-54Wm-2, ΔLW=-103Wm-2, and ΔLW=-271Wm-2, respectively.
If we assume an adiabatic temperature profile with 1k/100m decrease, this is equivalent to an effective height of emission in 1km, 1.9km and 5km, respectively.