Dinh, Tra, and Stephan Fueglistaler, November 2019: On the Causal Relationship between the Moist Diabatic Circulation and Cloud Rapid Adjustment to Increasing CO2. Journal of Advances in Modeling Earth Systems, 11(11), DOI:10.1029/2019MS001853. Abstract
General Circulation Models (GCMs) predict that clouds in the atmosphere rapidly adjust to the radiative perturbation of an abrupt increase in atmospheric CO2 concentration on a short time scale of about 10 days. This rapid adjustment consists of an increase of clouds in the boundary layer and a decrease of clouds in the free troposphere. Our focus is the mechanism for the decrease of clouds in the free troposphere, which is the dominating component of cloud rapid adjustment in most GCMs. We propose that the decrease in clouds in the free troposphere arises from the causal relationship between the moist diabatic circulation and the production of condensates that forms clouds in moist processes. As CO2 concentration increases, tropospheric radiative cooling is reduced, resulting in weakening of the moist diabatic circulation and a decrease in precipitation. As the hydrologic cycle weakens and the moist processes involving phase change of water vapour to form the condensates in the atmosphere lessen, the mass of cloud condensates decreases. This decrease in cloud condensates can be predicted from the decrease in the radiative subsidence mass flux, which is a metric for the strength of the moist diabatic circulation in the free troposphere.
Dinh, Tra, and Stephan Fueglistaler, November 2017: Mechanism of Fast Atmospheric Energetic Equilibration Following Radiative Forcing by CO2. Journal of Advances in Modeling Earth Systems, 9(7), DOI:10.1002/2017MS001116. Abstract
In energetic equilibrium, the atmosphere's net radiative divergence (math formula) is balanced by sensible (math formula) and latent (math formula) heat fluxes, i.e. math formula. Radiative forcing from increasing CO2 reduces math formula, and the surface warming following an increase in CO2 is largely due to the reduction in atmospheric energy demand in math formula and math formula, with only a smaller surface radiative budget perturbation. With an idealized General Circulation Model, we show that the fast atmospheric adjustment at fixed surface temperature produces the required decrease in the sum of math formula and math formula through changes in the near-surface temperature and specific humidity. In layers near the surface, the reduced radiative cooling forces a temperature increase that leads to a negative Planck radiative feedback and, because of the reduced surface-atmosphere temperature difference, also to a reduction in sensible heat flux. In the free troposphere, the reduced radiative cooling leads to a weakening of the tropospheric circulation. Consequently, there is a decrease in the water flux exported from the layers near the surface, and as such in precipitation. By mass conservation, the near-surface specific humidity increases and surface evaporation decreases until it balances the reduced export flux. Other processes can amplify or dampen the responses in math formula and math formula and change the partitioning between these two fluxes, but by themselves do not ensure math formula.
Dinh, Tra, et al., January 2016: Effect of gravity wave temperature fluctuations on homogeneous ice nucleation in the tropical tropopause layer. Atmospheric Chemistry and Physics, 16(1), DOI:10.5194/acp-16-35-2016. Abstract
The impact of high-frequency fluctuations of temperature on homogeneous nucleation of ice crystals in the vicinity of the tropical tropopause is investigated using a bin microphysics scheme for air parcels. The imposed temperature fluctuations come from measurements during isopycnic balloon flights near the tropical tropopause. The balloons collected data at high frequency, guaranteeing that gravity wave signals are well resolved.
With the observed temperature time series, the numerical simulations with homogeneous freezing show a full range of ice number concentration (INC) as previously observed in the tropical upper troposphere. In particular, low INC may be obtained if the gravity wave perturbations produce a non-persistent cooling rate (even with large magnitude) such that the absolute change in temperature remains small during nucleation. This result is explained analytically by a dependence of the INC on the absolute drop in temperature (and not on the cooling rate). This work suggests that homogeneous ice nucleation is not necessarily inconsistent with observations of low INC.
We use two-dimensional numerical simulations to study the impact of cloud radiative heating on transport timescales from the tropical upper troposphere to the stratosphere. Clouds are idealized as sources of radiative heating, and are stochastically distributed in space and time. A spatial probability function constrains clouds to occur in only part of the domain to depict heterogeneously distributed clouds in the atmosphere.
The transport time from the lower to upper boundaries (age of air) is evaluated with trajectories. We obtain bi-modal spectra of age of air, with the first mode composed of trajectories that remain in the cloudy part of the domain during their passages from the lower to upper boundaries. For these trajectories only, the mean age scales inversely the time-mean radiative heating in cloudy air, and the one-dimensional advection-diffusion equation provides an adequate model for transport. However, the exchange between the cloudy and cloud-free regions renders the mean age over all trajectories (including those that visit the cloud-free region) much longer. In addition, the overall mean age is not inversely proportional to the time-mean heating rate in cloudy air. Sensitivity calculations further show that the sizes, durations, and amplitudes of the individual clouds are also important to the transport time.
Our results show that the frequently used decomposition of radiative heating into clear-sky and cloud radiative heating may lead to misleading interpretations regarding the timescale of transport into the stratosphere.
Dinh, Tra, and Stephan Fueglistaler, October 2014: Microphysical, radiative and dynamical impacts of thin cirrus clouds on humidity in the tropical tropopause layer and lower stratosphere. Geophysical Research Letters, 41(19), DOI:10.1002/2014GL061289. Abstract
Cloud-resolving numerical simulations are carried out to study how in situ formed cirrus affect the humidity in the tropical tropopause layer and lower stratosphere. Cloud-induced impacts on the specific humidity are evaluated separately in terms of (i) the dehydration efficiency and (ii) the increase in the saturation mixing ratio associated with cloud radiatively induced temperature adjustment. The numerical results show that the dehydration efficiency of cirrus clouds, which is measured by the domain average relative humidity, varies within 100 ± 15% in all model configurations (with/ without heterogeneous ice nucleation, and with / without cloud radiative heating and cloud dynamics). A larger impact on the specific humidity comes from temperature increase (of a few Kelvins) induced by cloud heating. The latter is found to scale approximately linearly with the domain average ice mass. Resolving the cloud radiatively induced circulations approximately doubles the domain average ice mass and associated cloud-induced temperature change.
Dinh, Tra, Stephan Fueglistaler, D R Durran, and T P Ackerman, November 2014: Cirrus and water vapour transport in the tropical tropopause layer – Part 2: Roles of ice nucleation and sedimentation, cloud dynamics, and moisture conditions. Atmospheric Chemistry and Physics, 14(22), DOI:10.5194/acp-14-12225-2014. Abstract
A high-resolution, two-dimensional numerical model is used to study the moisture redistribution following homogeneous ice nucleation induced by Kelvin waves in the tropical tropopause layer (TTL). We compare results for dry/moist initial conditions and three levels of complexity for the representation of cloud processes: complete microphysics and cloud radiative effects, likewise but without radiative effects, and instantaneous removal of moisture in excess of saturation upon nucleation.
Cloud evolution and moisture redistribution are found to be sensitive to initial conditions and cloud processes. Ice sedimentation leads to a downward flux of water, whereas the cloud radiative heating induces upward advection of the cloudy air. The latter results in an upward (downward) flux of water vapour if the cloudy air is moister (drier) than the environment, which is typically when the environment is subsaturated (supersaturated).
Only a fraction (~25% or less) of the cloud experiences nucleation. Post-nucleation processes (ice depositional growth, sedimentation, and sublimation) are important to cloud morphology, and both dehydrated and hydrated layers may be indicators of TTL cirrus occurrence. The calculation with instantaneous removal of moisture not only misses the hydration but also underestimates dehydration due to (i) nucleation before reaching the minimum saturation mixing ratio, and (ii) lack of moisture removal from sedimenting ice particles below the nucleation level.
The sensitivity to initial conditions and cloud processes suggests that it is difficult to reach generic, quantitative estimates of cloud-induced moisture redistribution on the basis of case-by-case calculations.
