我读过关于固定砧温(FAT)假设,想知道为什么辐射冷却随着水蒸气混合比例的降低而降低。这难道不是与事实相反吗?水蒸汽是一种温室气体,因为它吸收LW辐射。那么它如何让大气降温呢?
最诚挚的问候Jan
https://climate-protecion-hardware.webnode。page/english/
@David "这有助于冷却对流层,因为凝结是一个放热过程。“;凝结作为放热过程使对流层变暖,而不是像你说的那样使它变冷。大气通过TOA的向外热辐射和降水冷却
这解释了大气层如何有助于冷却,但没有解释为什么冷却随着水蒸气混合比的降低而降低。答案可以用大气的递减率来解释。大气干绝热递减率约为$\Gamma_d = 10$ K/km。湿绝热递减率约为$\Gamma_w = 6$ K/km。水蒸气混合比越低,递减率越接近$\Gamma_d$。 And thus the upper most layer of the troposphere will be colder compared to a situation where the water vapor mixing ratio is high. A colder top layer will radiate less and therefore cooling decreases, since there is less outgoing long wave radiation.
The FAT assumes that the temperature of this upper atmospheric layer is independent of surface temperature and thus, would amplify warming. What changes is the height of the upper layer - the troposphere has a larger vertical extent in a warming climate.
Hartmann, Larson - An important constraint on tropical cloud - climate feedback