 |
| Titel |
The negative shortwave cloud feedback at high latitudes: mechanisms and observational constraints |
| VerfasserIn |
Paulo Ceppi, Daniel McCoy, Dennis Hartmann, Mark Webb |
| Konferenz |
EGU General Assembly 2016
|
| Medientyp |
Artikel
|
| Sprache |
en
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250128021
|
| Publikation (Nr.) |
 EGU/EGU2016-7961.pdf |
|
|
|
|
|
| Zusammenfassung |
| Climate models agree on a negative shortwave cloud feedback at high latitudes, driven by
increases in cloud optical depth and liquid water path (LWP), but the mechanisms remain
uncertain. We assess the importance of microphysical processes for the negative optical
depth feedback by perturbing temperature in the microphysics schemes of two
aquaplanet models, both of which have separate prognostic equations for liquid
water and ice. We find that most of the LWP increase with warming is caused by a
suppression of ice microphysical processes in mixed-phase clouds, resulting in reduced
conversion efficiencies of liquid water to ice and precipitation, and yielding an enhanced
reservoir of cloud liquid water. Hence, in climate models, the suppression of ice-phase
microphysics that deplete cloud liquid water is a key mechanism of the LWP increase
with warming and of the associated negative shortwave cloud feedback in cold
clouds.
In support of these findings, we show the existence of a very robust positive relationship
between monthly-mean LWP and temperature in CMIP5 models and observations in
mixed-phase cloud regions only. In models, the historical LWP sensitivity to temperature is a
good predictor of the forced global warming response poleward of about 45∘, although
models appear to overestimate the LWP response to warming compared to observations.
Historical cloud optical depth–temperature relationships are shown to provide an
observational constraint on the modeled cloud feedback, and support the prediction of a
negative cloud feedback at high latitudes. Because optical thickening with warming is
supported by simple temperature-dependent mechanisms and dominates over cloud amount
changes, we conclude that the shortwave cloud feedback is very likely negative in mid to high
latitudes. |
|
|
|
|
|
|