SENSITIVITY OF PERTURBATION GROWTH TO FLOW CHARACTERISTICS AND SAMPLING STRATEGY

The chaotic nature of the weather/climate attractor intrinsically limits the deterministic skill of weather forecasts by promoting rapid growth of errors. In this study, such error growth is simulated by artificially perturbing the atmosphere at initial time, and its sensitivity to the chosen perturbation methodology and to the flow characteristics is investigated. The different simulations are integrated with the limited-area model LM run on a convection-resolving grid. Results demonstrate that the locations of growing disturbances are insensitive to the definition of the initial temperature perturbation. This can be explained through an analysis of the perturbation growth and propagation mechanisms. In particular, rapid radiation of the imposed initial disturbance through a sound wave and presence of specific flow characteristics (e.g. convective instability) appear to force localized error growth far remote from the initial perturbation

Entrainment and its dependency on environmental conditions and convective organization in convection-permitting simulations

In this study, we estimate bulk entrainment rates for deep convection in convection-permitting simulations, conducted over the tropical Atlantic Ocean, encompassing parts of Africa and South America. We find that, even though entrainment rates decrease with height in all regions, they are, when averaging between 600 and 800 hPa, generally higher over land than over ocean. This is so because, over Amazonia, shallow convection causes an increase of bulk entrainment rates at lower levels and because, over West Africa, where entrainment rates are highest, convection is organized in squall lines. These squall lines are associated with strong mesoscale convergence, causing more intense updrafts and stronger turbulence generation in the vicinity of updrafts, increasing the entrainment rates. With the exception of West Africa, entrainment rates differ less across regions than across different environments within the regions. In contrast to what is usually assumed in convective parameterizations, entrainment rates increase with environmental humidity. Moreover, over ocean, they increase with static stability, while over land, they decrease. In addition, confirming the results of a recent idealized study, entrainment rates increase with convective aggregation, except in regions dominated by squall lines, like over West Africa

Effect of soil moisture on diurnal convection and precipitation in large-eddy simulations

A determination of the sign and magnitude of the soil moisture-precipitation feedback relies either on observations, where synoptic variability is difficult to isolate, or on model simulations, which suffer from biases mainly related to poorly resolved convection. In this study, a large-eddy simulation model with a resolution of 250m is coupled to a land surface model and several idealized experiments mimicking the full diurnal cycle of convection are performed, starting from different spatially homogeneous soil moisture conditions. The goal is to determine under which conditions drier soils may produce more precipitation than wetter ones. The methodology of previous conceptual studies that have quantified the likelihood of convection to be triggered over wet or dry soils is followed but includes the production of precipitation. Although convection can be triggered earlier over dry soils than over wet soils under certain atmospheric conditions, total precipitation is found to always decrease over dry soils. By splitting the total precipitation into its magnitude and duration component, it is found that the magnitude strongly correlates with surface latent heat flux, hence implying a wet soil advantage. Because of this strong scaling, changes in precipitation duration caused by differences in convection triggering are not able to overcompensate for the lack of evaporation over dry soils. These results are further validated using two additional atmospheric soundings and a series of perturbed experiments that consider cloud radiative effects, as well as the effect of large-scale forcing, winds, and plants on the soil moisture-precipitation coupling

Weak cooling of the troposphere by tropical islands in simulations of the radiative-convective equilibrium

We assess whether tropical islands tend to warm or cool the troposphere. To this end, we use idealized simulations of the Radiative‐Convective Equilibrium employing a simulation domain that contains flat tropical islands represented by a land surface scheme. Results show more frequent precipitation over land as coastal breezes establish, and gravity waves triggered by afternoon convection propagating away from the islands. These waves horizontally homogenize density and in doing so communicate convectively‐induced temperature anomalies from the islands onto the ocean. What is the influence of the islands on tropospheric temperature? The diurnal surface warming of the islands tends to push the afternoon convection over land towards a warmer moist adiabat, and along with it, the temperature profile of the troposphere. However, at the same time, drying of the land surface pulls it towards a colder moist adiabat. All in all, we find that islands rather cool than warm the troposphere. More specifically, we obtain a weakly colder domain‐mean troposphere during episodes with a larger share of precipitation over land, or when the prescribed land fraction is increased. In particular, we find that the cooling becomes more pronounced over large islands. Overall, the results indicate that the inability of evaporation to keep up with the daytime surface warming over land, in contrast to the ocean, is of key relevance for understanding land effects on the mean climat

The formation of wider and deeper clouds as a result of cold-pool dynamics

This study investigates how precipitation-driven cold pools aid the formation of wider clouds that are essential for a transition from shallow to deep convection. In connection with a temperature depression and a depletion of moisture inside developing cold pools, an accumulation of moisture in moist patches around the cold pools is observed. Convective clouds are formed on top of these moist patches. Larger moist patches form with time supporting more and larger clouds. Moreover, enhanced vertical lifting along the leading edges of the gravity current triggered by the cold pool is found. The interplay of moisture aggregation and lifting eventually promotes the formation of wider clouds that are less affected by entrainment and become deeper. These mechanisms are corroborated in a series of cloud-resolving model simulations representing different atmospheric environments. A positive feedback is observed in that in an atmosphere where cloud and rain formation is facilitated, stronger downdrafts will form. These stronger downdrafts lead to a stronger modification of the moisture field which in turn favour further cloud development. This effect is not only observed in the transition phase but is also active in prolonging the peak-time of precipitation in the later stages of the diurnal cycle. These findings are used to propose a simple way for incorporating the effect of cold pools on cloud sizes and thereby entrainment rate into parametrization schemes for convection. Comparison of this parameterization to the cloud-resolving modeling output gives promising results

A simplified model of precipitation enhancement over a heterogeneous surface

Soil moisture heterogeneities through the triggering of mesoscale circulations influence the onset of convection and subsequent evolution of thunderstorms producing heavy precipitation. However local evaporation also plays a role in determining precipitation amounts. Here we aim at disentangling the effect of advection and evaporation on precipitation over the course of a diurnal cycle by formulating a simple conceptual model. The derivation of the model is inspired from the results of simulations performed with a high-resolution (250 m) Large-Eddy Simulation model over a surface with varying degrees of heterogeneity. Key element of the model is the representation of precipitation as weighted sum of advection and evaporation, each weighted by its own efficiency. The model is then used to isolate the main parameters that control the variations of precipitation over spatially drier patches. It is found that these changes surprisingly do not depend on soil moisture itself but instead purely on parameters that describe the atmospheric initial state. The likelihood for enhanced precipitation over drier soils is discussed based on these parameters. Additional experiments are used to test the validity of the model

Transition in the fractal geometry of Arctic melt ponds

pre-printDuring the Arctic melt season, the sea ice surface undergoes a remarkable transformation from vast expanses of snow covered ice to complex mosaics of ice and melt ponds. Sea ice albedo, a key parameter in climate modeling, is determined by the complex evolution of melt pond configurations. In fact, ice-albedo feedback has played a major role in the recent declines of the summer Arctic sea ice pack. However, understanding melt pond evolution remains a significant challenge to improving climate projections. By analyzing area-perimeter data from hundreds of thousands of melt ponds, we find here an unexpected separation of scales, where pond fractal dimension D transitions from 1 to 2 around a critical length scale of 100m2 in area. Pond complexity increases rapidly through the transition as smaller ponds coalesce to form large connected regions, and reaches a maximum for ponds larger than 1000m2, whose boundaries resemble space-filling curves, with D 2. These universal features of Arctic melt pond evolution are similar to phase transitions in statistical physics. The results impact sea ice albedo, the transmitted radiation fields under melting sea ice, the heat balance of sea ice and the upper ocean, and biological productivity such as under ice phytoplankton blooms