Abstract

The paper investigates the enhancement of surface fluxes by atmospheric mesoscale motions. The authors show that horizontal wind variabilities induced by these motions (i.e., gustiness) need to be considered in the parameterization of surface fluxes used in general circulation models (GCMs), as they always occur at subgrid scale.

It is argued that there are two different sources of gustiness: deep convection and boundary layer free convection. The respective scales (time and length) and the convective patterns are very different for each of these sources. A general parameterization of the gustiness distinguishing these two effects is proposed.

For boundary layer free convection, the gustiness is related to the free convection velocity. To establish this relationship, both observations and numerical simulations are used. Revisiting the Coupled Ocean–Atmosphere Response Experiment data, the authors propose a new value of the proportionality coefficient that links the free convection velocity and the gustiness.

For deep convection, the dominant source of gustiness is the occurence of downdrafts and updrafts generated by convective cells. It is shown that these motions produce large enhancement of surface fluxes and should be parameterized in GCMs. Results indicate that the gustiness can be related either to the precipitation or to the updraft and downdraft mass fluxes.

Abstract

The 3-hourly brightness temperatures (BTs) at 10.8 μm from the Meteosat Second Generation (MSG) satellite were used to document the cloud system variability over West Africa in the summer of 2006 and to evaluate the quality of the Méso-NH model forecasts of cloud cover in the African Monsoon Multidisciplinary Analysis (AMMA) framework. Cloud systems were observed over the Guinean and Sahelian bands with more frequent occurrence and patchier structures in the afternoon. Some intraseasonal variations of the number of cloud systems were found, partly related to the intermittency of the African easterly wave (AEW) activity. Compared to the MSG observations, the Méso-NH model reproduces the overall variation of the BT at 10.8 μm well at D + 1 forecast. The model captures the BT diurnal cycle under conditions of clear-sky and high-cloud cover, but misses the lowest BT values associated with deep convection. Forecasted cloud systems are more numerous and smaller, hence patchier, than those observed. These results suggest some deficiencies in the model’s convection and cloud parameterization schemes. The use of meteorological scores further documents the skill of the model to predict cloud systems. Beyond some systematic differences between simulations and observations, analysis also suggests that the model high-cloud forecast is improved under specific synoptic forcing conditions related to AEW activity. This indicates that room exists for improving the skills of weather forecasting over West Africa.

Abstract

This study evaluates the predictions of radiative and cloud-related processes of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5). It is based on extensive comparison of three-dimensional forecast runs with local data from the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site collected at the Central Facility in Lamont, Oklahoma, over a seasonal timescale. Time series are built from simulations performed every day from 15 April to 23 June 1998 with a 10-km horizontal resolution. For the one single column centered on this site, a reasonable agreement is found between observed and simulated precipitation and surface fields time series. Indeed, the model is able to reproduce the timing and vertical extent of most major cloudy events, as revealed by radiative flux measurements, radar, and lidar data. The model encounters more difficulty with the prediction of cirrus and shallow clouds whereas deeper and long-lasting systems are much better captured. Day-to-day fluctuations of surface radiative fluxes, mostly explained by cloud cover changes, are similar in simulations and observations. Nevertheless, systematic differences have been identified. The downward longwave flux is overestimated under moist clear sky conditions. It is shown that the bias disappears with more sophisticated parameterizations such as Rapid Radiative Transfer Model (RRTM) and Community Climate Model, version 2 (CCM2) radiation schemes. The radiative impact of aerosols, not taken into account by the model, explains some of the discrepancies found under clear sky conditions. The differences, small compared to the short timescale variability, can reach up to 30 W m⁻² on a 24-h timescale.

Overall, these results contribute to strengthen confidence in the realism of mesoscale forecast simulations. They also point out model weaknesses that may affect regional climate simulations: representation of low clouds, cirrus, and aerosols. Yet, the results suggest that these finescale simulations are appropriate for investigating parameterizations of cloud microphysics and radiative properties, as cloud timing and vertical extension are both reasonably captured.

Abstract

The capacity of wavelets to effectively represent atmospheric processes under compression is tested by a dataset generated by a cloud-resolving model simulation of deep convective events observed during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE).

Generally, more than 90% of the total variance is reproduced by retaining only the top 10% of the modes. The compressed data does not drastically deteriorate for graphic purposes, even when only the top 1% of modes are retained. The performance of compression is overall comparable for all the wavelets considered and also does not strongly depend on the type of physical variables.

Conventional quantitative measures do not distinguish the compression errors arising from different characters of individual wavelets well, although different wavelet modes filter out different structures as “noises” depending on their characteristics. The importance of choosing wavelets that represent the shape of signals physically expected is emphasized. Analytical discontinuities of wavelets are not necessarily undesirable for all the purposes, but must be consistent with our physical picture for the system. For this reason, the Haar wavelet may be acceptable because of its piecewise-constant structure, whereas the Daubechies wavelets of lower degrees are less appropriate because of their highly irregular structures.

Some preliminary analyses are performed for assessing the capacity of wavelets to represent the full physics of meteorological systems. It is suggested that the loss of magnitudes in vertical fluxes under high compression can be recovered by a kind of “renormalization” to a good extent. The mass continuity is found to be reasonably satisfied under the proposed compression method, although the latter is not explicitly constrained by the former.

Abstract

Over the recent decades, extreme precipitation events (EPEs) have become more frequent over the Sahel. Their properties, however, have so far received little attention. In this study the spatial distribution, intensity, seasonality, and interannual variability of EPEs are examined, using both a reference dataset based on a high-density rain gauge network over Burkina Faso and 24 precipitation gridded datasets. The gridded datasets are evaluated in depth over Burkina Faso while their commonalities are used to document the EPE properties over the Sahel. EPEs are defined as the occurrence of daily accumulated precipitation exceeding the all-day 99th percentile over a 1° × 1° pixel. Over Burkina Faso, this percentile ranges between 21 and 33 mm day⁻¹. The reference dataset show that EPEs occur in phase with the West African monsoon annual cycle, more frequently during the monsoon core season and during wet years. These results are consistent among the gridded datasets over Burkina Faso but also over the wider Sahel. The gridded datasets exhibit a wide diversity of skills when compared to the Burkinabe reference. The Global Precipitation Climatology Centre Full Data Daily version 1 (GPCC-FDDv1) and the Global Satellite Mapping of Precipitation Gauge Reanalysis version 6.0 (GSMaP-gauge-RNL v6.0) are the only products that properly reproduce all of the EPE features examined in this work. The datasets using a combination of microwave and infrared measurements are prone to overestimate the EPE intensity, while infrared-only products generally underestimate it. Their calibrated versions perform better than their uncalibrated (near-real-time) versions. This study finally emphasizes that the lack of rain gauge data availability over the whole Sahel strongly impedes our ability to gain insights in EPE properties.

Abstract

An approach for convective parameterization is presented here, in which grid-scale budget equations of parameterization use separate microphysics and transport terms. This separation is used both as a way to introduce into the parameterization a more explicit causal link between all involved processes and as a vehicle for an easier representation of the memory of convective cells. The equations of parameterization become closer to those of convection-resolving models [cloud-system-resolving models (CSRMs) and large-eddy simulations (LESs)], facilitating parameterization development and validation processes versus a detailed budget of these high-resolution models.

The new Microphysics and Transport Convective Scheme (MTCS) equations are presented and discussed. A first version of a convective scheme based on these equations is tested within a single-column framework. The results obtained with the new scheme are close to those of traditional ones in very moist convective cases [like the Global Atmospheric Research Programme (GARP) Atlantic Tropical Experiment (GATE) Phase III, 1974]. The simulation of more difficult drier situations [European Cloud Systems Study/Global Energy and Water Cycle Experiment (GEWEX) Cloud System Studies (EUROCS/GCSS)] is improved through more memory due to higher sensitivity of simulated convection to dry midtropospheric layers; a prognostic relation between cloudy entrainment and precipitation evaporation dramatically improves the prediction of the phase lag of the convective diurnal cycle over land with respect to surface heat forcing.

The present proposal contains both a relatively general equation set, which can deal continuously with dry, moist, and deep precipitating convection, and separate—and still crude—explicit moist microphysics. In the future, when increasing the complexity of microphysical computations, such an approach may help to unify dry, moist, and deep precipitating convection inside a single parameterization, as well as facilitate global climate model (GCM) and limited-area model (LAM) parameterizations in sharing the same formulation of explicit microphysics with CSRMs.

Abstract

The present assessment of the West African monsoon in the models of the Coupled Model Intercomparison Project (CMIP) phase 5 (CMIP5) indicates little evolution since the third phase of CMIP (CMIP3) in terms of both biases in present-day climate and climate projections.

The outlook for precipitation in twenty-first-century coupled simulations exhibits opposite responses between the westernmost and eastern Sahel. The spread in the trend amplitude, however, remains large in both regions. Besides, although all models predict a spring and summer warming of the Sahel that is 10%–50% larger than the global warming, their temperature response ranges from 0 to 7 K.

CMIP5 coupled models underestimate the monsoon decadal variability, but SST-imposed simulations succeed in capturing the recent partial recovery of monsoon rainfall. Coupled models still display major SST biases in the equatorial Atlantic, inducing a systematic southward shift of the monsoon. Because of these strong biases, the monsoon is further evaluated in SST-imposed simulations along the 10°W–10°E African Monsoon Multidisciplinary Analysis (AMMA) transect, across a range of time scales ranging from seasonal to intraseasonal and diurnal fluctuations.

The comprehensive set of observational data now available allows an in-depth evaluation of the monsoon across those scales, especially through the use of high-frequency outputs provided by some CMIP5 models at selected sites along the AMMA transect. Most models capture many features of the African monsoon with varying degrees of accuracy. In particular, the simulation of the top-of-atmosphere and surface energy balances, in relation with the cloud cover, and the intermittence and diurnal cycle of precipitation demand further work to achieve a reasonable realism.

Abstract

A set of nighttime tethered balloon and kite measurements from the central Sahel (15.2°N, 1.3°W) in August 2005 were acquired and analyzed. A composite of all the nights’ data was produced using boundary layer height to normalize measured altitudes. The observations showed some typical characteristics of nocturnal boundary layer development, notably a strong inversion after sunset and the formation of a low-level nocturnal jet later in the night. On most nights, the sampled jet did not change direction significantly during the night.

The boundary layer thermodynamic structure displayed some variations from one night to the next. This was investigated using two contrasting case studies from the period. In one of these case studies (18 August 2005), the low-level wind direction changed significantly during the night. This change was captured well by two large-scale models, suggesting that the large-scale dynamics had a significant impact on boundary layer winds on this night. For both case studies, the models tended to underestimate near-surface wind speeds during the night, which is a feature that may lead to an underestimation of moisture flux northward by models.

Abstract

The core of the mass flux formulation, on which the majority of the current cumulus parameterizations are based, is to transport physical variables by the so-called mass flux for individual physical components, such as convective updrafts, downdrafts, and environment. These parameterizations use horizontal means over the subdomains occupied by these physical components to define the mass fluxes and transported variables. However, evaluations of the mass flux formulation against high-resolution spatial data obtained from explicit numerical models reveal that it substantially underestimates vertical transport of heat, moisture, and momentum by deep convection.

The present paper proposes an alternative approach, in which the effective values weighted toward extreme values are used both for the mass flux and the transported variable to obtain an accurate estimate of vertical transport. Statistically, the distribution of convective variables is so widely distributed within individual subdomains that the vertical transports are controlled by extreme values, rather than by simple means. Evaluation for these effective values are facilitated by considering four categories depending on the sign of both the vertical velocity and the transported variable, instead of the conventional convective-type classifications. A best estimate of the effective value is obtained empirically by weighting the variable by a power of one-quarter during the averaging.

A major consequence of this alternative approach is that the mass fluxes must be defined differently for the individual variables. Thus, chemical species would not be transported by the same mass flux as that for temperature or moisture. With this extra elaboration, the proposed formulation provides more robust estimation of the subgrid- scale convective transports.

Abstract

A method to analyze the daily cycle of evapotranspiration over land is presented. It quantifies the influence of external forcings, such as radiation and advection, and of internal feedbacks induced by boundary layer, surface layer, and land surface processes on evapotranspiration. It consists of a budget equation for evapotranspiration that is derived by combining a time derivative of the Penman–Monteith equation with a mixed-layer model for the convective boundary layer. Measurements and model results for days at two contrasting locations are analyzed using the method: midlatitudes (Cabauw, Netherlands) and semiarid (Niamey, Niger). The analysis shows that the time evolution of evapotranspiration is a complex interplay of forcings and feedbacks. Although evapotranspiration is initiated by radiation, it is significantly regulated by the atmospheric boundary layer and the land surface throughout the day. In both cases boundary layer feedbacks enhance the evapotranspiration up to 20 W m⁻² h⁻¹. However, in the case of Niamey this is offset by the land surface feedbacks since the soil drying reaches −30 W m⁻² h⁻¹. Remarkably, surface layer feedbacks are of negligible importance in a fully coupled system. Analysis of the boundary layer feedbacks hints at the existence of two regimes in this feedback depending on atmospheric temperature, with a gradual transition region in between the two. In the low-temperature regime specific humidity variations induced by evapotranspiration and dry-air entrainment have a strong impact on the evapotranspiration. In the high-temperature regime the impact of humidity variations is less pronounced and the effects of boundary layer feedbacks are mostly determined by temperature variations.