S. R. de Roode

Understanding Convective Extreme Precipitation Scaling Using Observations and an Entraining Plume Model

Previously observed twice-Clausius–Clapeyron (2CC) scaling for extreme precipitation at hourly time scales has led to discussions about its origin. The robustness of this scaling is assessed by analyzing a subhourly dataset of 10-min resolution over the Netherlands. The results confirm the validity of the previously found 2CC scaling for extreme convective precipitation. Using a simple entraining plume model, an idealized deep convective environmental temperature profile is perturbed to analyze extreme precipitation scaling from a frequently used relation based on the column condensation rate. The plume model simulates a steady precipitation increase that is greater than Clausius–Clapeyron scaling (super-CC scaling). Precipitation intensity increase is shown to be controlled by a flux of moisture through the cloud base and in-cloud lateral moisture convergence. Decomposition of this scaling relation into a dominant thermodynamic and additional dynamic component allows for better understanding of the scaling and demonstrates the importance of vertical velocity in both dynamic and thermodynamic scaling. Furthermore, systematically increasing the environmental stability by adjusting the temperature perturbations from constant to moist adiabatic increase reveals a dependence of the scaling on the change in environmental stability. As the perturbations become increasingly close to moist adiabatic, the scaling found by the entraining plume model decreases to CC scaling. Thus, atmospheric stability changes, which are expected to be dependent on the latitude, may well play a key role in the behavior of precipitation extremes in the future climate.

Factors Controlling Rapid Stratocumulus Cloud Thinning

The relationship between the inversion stability and the liquid water path (LWP) tendency of a vertically well-mixed, adiabatic stratocumulus cloud layer is investigated in this study through the analysis of the budget equation for the LWP. The LWP budget is mainly determined by the turbulent fluxes of heat and moisture at the top and the base of the cloud layer, as well as by the source terms due to radiation and precipitation. Through substitution of the inversion stability parameter k into the budget equation, it immediately follows that the LWP tendency will become negative for increasing values of k due to the entrainment of increasingly dry air. Large k values are therefore associated with strong cloud thinning. Using the steady-state solution for the LWP, an equilibriumvalue keq is formulated, beyond which the stratocumulus cloud will thin. The Second Dynamics and Chemistry of Marine Stratocumulus field study (DYCOMS-II) is used to illustrate that, depending mainly on the magnitude of the moisture flux at cloud base, stratocumulus clouds can persist well within the buoyancy reversal regime.

An LES model study of the influence of the free tropospheric thermodynamic conditions on the stratocumulus response to a climate perturbation

Twenty-five large-eddy simulations are performed to study how free tropospheric thermodynamic conditions control equilibrium state solutions of stratocumulus-topped marine boundary layers. In particular, we systematically vary the lower tropospheric stability (LTS) and a similar measure for the bulk humidity difference between the 700 hPa level and the surface, inline image. For all simulations, a completely overcast boundary layer is obtained in which the turbulence is mainly driven by cloud top radiative cooling. The steady state liquid water path (LWP) is rather insensitive to the LTS, but increases significantly and almost linearly with the free tropospheric humidity. In a second suite of runs, the response of the stratocumulus layer to an idealized global warming scenario is assessed by applying a uniform warming of 2 K to the initial temperature profile including the sea surface while the initial relative humidity profile is kept identical to the control case. The warming of the sea surface acts to increase the latent heat flux, which invigorates turbulence in the boundary layer. The steady state inversion height therefore increases, despite the competing effect of a more humid free troposphere that increases the downwelling radiative flux and hence tends to decrease the entrainment rate. The stratocumulus layer nevertheless thins for all free tropospheric conditions as cloud base rises more than cloud top. This implies a positive stratocumulus cloud-climate feedback for this scenario as thinner clouds reflect less shortwave radiation back to space. The cloud thinning response to the climate perturbation is found to be mostly controlled by the change of inline image.

A single‐column model intercomparison on the stratocumulus representation in present‐day and future climate

Six Single-Column Model (SCM) versions of climate models are evaluated on the basis of their representation of the dependence of the stratocumulus-topped boundary layer regime on the free tropospheric thermodynamic conditions. The study includes two idealized experiments corresponding to the present-day and future climate conditions in order to estimate the low-cloud feedback. Large-Eddy Simulation (LES) results are used as a benchmark and GCM outputs are included to assess whether the SCM results are representative of their 3-D counterparts. The SCMs present a variety of dependencies of the cloud regime on the free tropospheric conditions but, at the same time, several common biases. For all the SCMs the stratocumulus-topped boundary layer is too shallow, too cool, and too moist as compared to the LES results. Moreover, they present a lack of clouds and liquid water and an excess of precipitation. The disagreement among SCMs is even more distinct for the response to a climate perturbation. Even though the overall feedback is positive for all the models, in line with the LES results, the SCMs show a rather noisy behavior, which depends irregularly on the free tropospheric conditions. Finally, the comparison with the host GCM outputs demonstrates that the considered approach is promising but needs to be further generalized for the SCMs to fully capture the behavior of their 3-D counterparts.

Single‐Column Model Simulations of Subtropical Marine Boundary‐Layer Cloud Transitions Under Weakening Inversions

Results are presented of the GASS/EUCLIPSE single-column model inter-comparison study on the subtropical marine low-level cloud transition. A central goal is to establish the performance of state-of-the-art boundary-layer schemes for weather and climate models for this cloud regime, using large-eddy simulations of the same scenes as a reference. A novelty is that the comparison covers four different cases instead of one, in order to broaden the covered parameter space. Three cases are situated in the North-Eastern Pacific, while one reflects conditions in the North-Eastern Atlantic. A set of variables is considered that reflects key aspects of the transition process, making use of simple metrics to establish the model performance. Using this method some longstanding problems in low level cloud representation are identified. Considerable spread exists among models concerning the cloud amount, its vertical structure and the associated impact on radiative transfer. The sign and amplitude of these biases differ somewhat per case, depending on how far the transition has progressed. After cloud breakup the ensemble median exhibits the well-known “too few too bright” problem. The boundary layer deepening rate and its state of decoupling are both underestimated, while the representation of the thin capping cloud layer appears complicated by a lack of vertical resolution. Encouragingly, some models are successful in representing the full set of variables, in particular the vertical structure and diurnal cycle of the cloud layer in transition. An intriguing result is that the median of the model ensemble performs best, inspiring a new approach in subgrid parameterization.