Cloud horizontal size distributions of cloud clusters were analyzed for global cloud resolving simulations with the global nonhydrostatic model NICAM whose mesh interval is about 3.5 km and 7 km. The 3.5 km-mesh simulation was performed for 7 days starting at 00 UTC 25 Dec 2006 by giving an initial condition of reanalysis data, while the 7 km-mesh simulation was run for 32 days from 00 UTC 15 Dec 2006. We used outgoing long-wave radiation (OLR) simulated by NICAM to calculate size distributions of deep convection, and compared them with those analyzed using equivalent blackbody temperature (TBB) of the infrared channel of 11 µm of the Japanese geostationary meteorological satellite (MTSAT-1R). We selected two threshold temperatures, 208 K and 253 K, to identify deep convective areas including anvil cloud. Specifically, we call clouds defined by the 208 K-threshold “deeper” convective clouds. Over the tropical region covering the maritime continent and the western tropical Pacific (10S-10N, 90E-160W), we examined the size of cloud areas defined by the two BB threshold values and corresponding threshold values of OLR of 90 W m-2 and 210 W m-2, which were chosen by comparing cumulative histograms of BB and OLR in this region. Resolution dependency by NICAM shows that the overall cloud size distribution of the 3.5 km-mesh simulation is much closer to that of the MTSAT-1R observation than that of the 7 km-mesh simulation. Size distributions of deep convection in both simulations indicate nearly lognormal as is seen in the MTSAT-1R observations. The 3.5 km-mesh simulation shows slightly less frequency than the MTSATR observation for smaller size of deeper convection, and it does not reproduce very large clouds. When comparing cloud characteristics over land and ocean, simulated cloud size statistics are slightly closer to the MT-SAT-1R observation in the maritime continent region (westward of 160E) than in the open ocean region (eastward of 160E). A comparison of temporal variation of cloud area shows that the 3.5 km-mesh simulation captures clear signals of diurnal cycles over the maritime continent, together with amplification associated with the Madden-Julian Oscillation (MJO) event. Morning and afternoon difference of convective activity over a large island within the maritime continent is also simulated by 3.5 km-mesh simulation. When one uses a global cloud resolving model for climate studies, the analysis of cloud size distributions gives another dimension to improve the cloud properties of simulations. It is not only relevant to realistic representations of deep convection, but is also useful for improving the radiation budget of global cloud resolving simulations.
Sensitivity of precipitation diurnal cycle to the horizontal grid spacing was investigated using a mesoscale model without cumulus parameterization. Four numerical experiments with changing horizontal resolution are performed over one month with NCEP reanalysis boundary forcing. The studied range of grid spacing is from 3.5 km to 28 km which has been known as the intermediate scale to simulate mesoscale systems with/without cumulus parameterization. The target area is the Tibetan Plateau where pronounced diurnal cycle of convective systems is observed during the spring season. Lower resolution runs (14 and 28 km grid) show delayed formation and delayed mature stage of the cumulus convection in comparison to satellite observations. On the other hand, higher resolution runs (3.5 and 7 km grid) reproduce the proper development of the clouds after local noon which is consistent with observations. The total hydrometeor content and rainfall rate increase with grid size. Such systematic relationship of resolution dependencies are confirmed even in the monthly mean diurnal cycle, although most of previous studies examined only short periods. These results suggest that finer resolution at less than 7 kilometer is necessary to simulate realistic phase of the precipitation diurnal cycle over the Tibetan Plateau. The mechanism that is responsible for the resolution dependency is discussed. We suggest that the daytime convection which initially occurs due to unstable stratification over the Tibetan Plateau in spring tends to have a horizontal scale smaller than that is resolvable by the coarse resolution runs. The delayed cloud formation induces larger downward shortwave radiation, which increases surface fluxes and results in too strong rainfall in the coarser resolution runs.
Here we present a simple method of improving bulk mixed-phase microphysical schemes to allow for a more realistic representation of partially rimed particles. The new procedure unifies the snow and graupel particles by assigning a single fallspeed to both that is weighted by the mixing ratios, and applying that fallspeed to both sedimentation and accretion processes. This avoids the problem of the species separating out by sedimentation as graupel forms, and the further problem of graupel then accreting snow too quickly because of its high relative fallspeed. Instead the unified graupel/snow moves together and evolves in its relative ratio due to riming, behaving as intermediate or partially rimed particles. Tests of the new method were carried out using the Weather Research and Forecasting (WRF) Single-Moment 6-class (WSM6) microphysics scheme in a high-resolution idealized simulation, and mesoscale heavy precipitation events in the summer and winter over Korea. The effect of the new accretion rates on cloud structure and precipitation was found to be greater than that of the changed sedimentation alone. Verification of these tests showed a much-reduced production of graupel and more snow, influencing the cloud structure and surface precipitation fields. The scheme shows promise in improving precipitation intensity and precipitation type forecasts.
Month-long large-scale forcing data from two field campaigns are used to drive a cloud-resolving model (CRM) and produce ensemble simulations of clouds and precipitation. Observational data are then used to evaluate the model results. To improve the model results, a new parameterization of the Bergeron process is proposed that incorporates the number concentration of ice nuclei (IN). Numerical simulations reveal that atmospheric ensembles are sensitive to IN concentration and ice crystal multiplication. Two- (2D) and three-dimensional (3D) simulations are carried out to address the sensitivity of atmospheric ensembles to model dimensionality. It is found that the ensembles with high IN concentration are more sensitive to dimensionality than those with low IN concentration. Both the analytic solutions of linear dry models and the CRM output show that there are more convective cores with stronger updrafts in 3D simulations than in 2D, which explains the differing sensitivity of the ensembles to dimensionality at different IN concentrations.
The effects of subgrid-scale (SGS) condensation and transport become more important as the grid spacings increase from those typically used in large-eddy simulation (LES) to those typically used in cloud-resolving models (CRMs). Incorporation of these SGS effects can be achieved by a joint probability density function approach that utilizes higher-order moments of thermodynamic and dynamic variables. This study examines how well shallow cumulus and stratocumulus clouds are simulated by two versions of a CRM implemented with low-order (1.5-order) and third-order turbulence closures (LOC and TOC). Resolution sensitivities of the closure are studied by refining the grid spacing from control simulation (with standard CRM grids of 4 km) to simulations with much finer meshes in the horizontal. In our simulations cumulus clouds are mostly produced through SGS transport processes while stratocumulus clouds are produced through both SGS and resolved-scale processes in the TOC version of the CRM at standard resolution. In contrast, the LOC version of the CRM requires resolved-scale circulations to produce both cumulus and stratocumulus clouds, as SGS transports within cloud layer remain small in this model. The mean profiles of thermodynamic variables, cloud fraction and liquid water content exhibit significant differences between the two versions of the CRM, with the TOC results agreeing better with the LES than the LOC results. The characteristics, temporal evolution and mean profiles of shallow cumulus and stratocumulus clouds are weakly dependent upon the horizontal grid spacing used in the TOC CRM. However, the ratio of the SGS to resolved-scale fluxes becomes smaller as the horizontal grid spacing decreases. The subcloud-layer fluxes are mostly due to the resolved scales when horizontal grid spacings approach the depth of this layer. The overall results of the TOC simulations suggest that the 1-km grid spacing is a good choice for CRM simulation of shallow cumulus and stratocumulus.
The vertical evolution of microphysics in trade-wind cumuli (Cu) observed from the NCAR C-130 research aircraft during one flight of the RICO (Rain in Cumulus Over the Ocean) study is analyzed. Conditional sampling of > 200 Cu traversed on this flight is used to chose Cu for which the aircraft penetrated single and growing Cu turrets about 250-m below cloud top where maximum LWC is often found and where radar has often observed initial stages of precipitation. The vertical evolution of the sampled set of Cu was assumed to follow Lagrangian behavior. The entrainment rate, entrained parcel scales, mixing mechanisms, and effects on the droplet size distribution are measured and evaluated. A parcel model is applied over the 1100-m maximum Cu height of the traverses to determine the relationship between the observed large number of small droplets and the fewer ultra-giant sea-salt nuclei (UGN) in order to assess the role of these nuclei in evolving the size spectrum and in causing a growing “drizzle tail”. New insight on these topics is obtained by using the PVM (Particle Volume Monitor) probe to measure incloud microphysics with 10-cm resolution. The results include the following: Entrainment causes primarily dilution of the drops without significant size changes, thus either extreme inhomogeneous mixing or more likely homogeneous mixing resulting from mixing with cool and humid entrained air take place. The entrained parcels are surprisingly small following lognormal behavior and decaying rapidly upon entering the Cu, as a result super-adiabatic drops are not evident. The entrained parcels are consistent with the Bragg-scattering “mantle echo” often observed by radar in small Cu. The FSSP (Forward Scattering Spectrometer Probe) droplet spectra are nearly constant with height. These “self-preserving” spectra are a result of an approximate balance between dilution by entrainment of droplets originating at cloud base, droplet activation on entrained CCN (cloud condensation nuclei), and detrainment and coalescence losses. Sea-salt nuclei follow Woodcock’s wind dependence, and are shown with the parcel model to play an important role in forming the observed drizzle that increases with cloud height. Accretion is the dominant coalescence mechanism near cloud top in these Cu.
This study developed a general grid transformation method for a horizontal grid system on a sphere. The method incorporates the currently used Schmidt transformation method, as well as a new technique that includes intuitive interpretation of the Schmidt transformation. To apply the method, we developed an estimation function that considers both isotropy and homogeneity, and the transformation function uses a governing differential equation to ensure that the function takes the minimum value. Since the proposed transformation method avoids the too-fine grid at the center of the target region, which arises due to the Schmidt transformation method, the new method is superior in terms of computational efficiency. We applied the new transformation to an icosahedral grid. To investigate the stretching effect by this method, we conducted an advection test case using the standard experiment for the shallow water model (Williamson et al. 1992). The error growth rate was minimized over the target region where the fine grid area was distributed. The transition zone between the target region and the coarser grid region exhibited smooth advection, so no spurious error occurred.
We developed new microphysical schemes that have five and six classes of water substances based on the method of Lin et al. (1983). In the new schemes, ice cloud is simply generated by a saturation-adjustment process. Furthermore, the effect of the wetness of graupel is omitted in the six-class scheme to reduce calculation costs. Because of these simplifications, the newly developed scheme is much less computationally costly than are the commonly used Lin-type schemes. For the validation and comparison of the proposed schemes, squall-line simulations were conducted using the NICAM dynamical core on a stretched icosahedral grid. In this test case, the squall line that was simulated by the schemes with graupel processes developed more quickly than that simulated by the schemes without graupel processes. Despite the simplicity of the processes in the new six-class scheme, its physical performance is similar and its computational performance is higher than those of the established Lin-type scheme.
The sensitivity of simulations of shallow cumulus convection to their microphysical representation is explored, with a focus on the parameter space spanned by two common bulk schemes (Seifert and Beheng, SB, versus Khairoutdinov and Kogan, KK). Large-eddy simulation, simple models, and a priori analysis of the underlying microphysical equations are used as the basis for our study. The simulations are initialized using data derived from the Rain in Cumulus Over the Ocean (RICO) field study. Simulated clouds depths range between two and three kilometers. Microphysical sensitivities can largely be rationalized based on the behavior of simpler models. In particular a parcel model consisting of auto-conversion and accretion acting on a parcel condensing water at a fixed rate provides useful insight into the behavior of the microphysical schemes in the full simulation. To a first approximation the number concentration simply selects the cloud depth at which rain begins to develop, with different schemes predicting different cloud depths. Because of the interaction of auto-conversion and accretion the dependence of this cloud depth on cloud-droplet number concentration is considerably reduced from what would be deduced by an examination of auto conversion alone—suggesting a somewhat diminished role for the regulation of rain by the atmospheric aerosol. Dynamic feedbacks, such as the tendency for non-precipitating clouds to deepen more readily, can further dampen (and even reverse) the expected sensitivity of rain-rate on droplet number concentrations. Our analysis suggests that the commonly assumed exponential distribution for rain drops can strongly distort the sedimentation process in two moment microphysical schemes and that processes such as self-collection and drop break-up can not be neglected for shallow cumulus convection.
Realistic large-scale diurnal marches of precipitation in various regions in the global tropics over both land and ocean were successfully simulated in an atmospheric general circulation model (AGCM) developed in collaboration by the Center for Climate System Research, the National Institute for Environmental Studies, and the Frontier Research Center for Global Climate (CCSR/NIES/FRCGC), with a resolution of T106, and was forced with prescribed sea surface temperatures. This is an outstanding performance of the diurnal cycle simulated in the AGCM with similar resolution. Comparison analyses with the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and Precipitation Radar (PR) data revealed that an implementation of the relative humidity (RH) threshold to the prognostic Arakawa-Schubert (AS) cumulus parameterization significantly improved the diurnal variation of precipitation especially in its large-scale marches. It was suggested that the cloud-layer RH threshold lead to a tighter coupling between free-tropospheric gravity waves and cumulus convection compared to the original prognositic AS, with which convection is more obedient to the boundary-layer warming.
High-resolution global simulations over zonally symmetric aqua planets are examined using Fourier analysis in the zonal direction. We highlight the tropics, where the large-scale weather consists of convectively-coupled waves so that explicit convection is an especially topical novelty. Squared differences between pairs of runs grow from initially tiny values to saturation at twice the climatological variance. For wavelengths shorter than 103 km, differences saturate within about a day. For tropical long waves, the time to saturation indicates predictability for at least 2 weeks. This time scale is similar in middle latitude flow, which interacts with tropical waves in the 3D model, but it is also similar in 2D pseudo-equatorial vertical plane simulations of pure convectively coupled gravity waves. As a result, no simple conclusions can be drawn about whether tropical predictability is limited more by tropical chaos or by tropical-extratropical interactions. Difference growth appears to fill out the saturation energy spectrum in a “vertical” (up-magnitude) rather than “horizontal” (up-scale) manner. Up-scale growth thus occurs as a continuing amplification of large scales after small scales saturate, which begs the question of what sets the shape of the saturation (climatological) power spectra. Wind spectra are nearly power-law with a logarithmic slope of about -5/3 in the free troposphere, remarkably so in the 2D runs and clearly distinct from slope -2 (a null hypothesis of spectrally white wind divergence). A common interpretation of -5/3 slope - as indicative of a cascade, a steady conservative transfer of energy from source to sink scales by interactions that are local in log-wavelength space - is hard to apply to these moist tropical waves.
This study investigates the diurnal variation of convection over western Kalimantan (Borneo) Island and the adjacent seas through analysis of results from surface meteorological observations, satellite observations of convection and precipitation, and numerical experiments with a mesoscale model. Precipitation measurements from the TRMM satellite indicated that rainfall occurs mostly in the afternoon and evening along the coast of the island, and at night and in the early morning over the plains in the central regions of the island and over the sea adjacent to the coast. Results from surface meteorological observations showed that intense shortwave radiation is observed on the western coast at Siantan on most days, even on days with heavy rain. Consequently, sea breezes are evident in the afternoon, whereas land breezes prevail in the early morning. Occasionally, a strong offshore flow is observed by the QuikSCAT satellite over the sea adjacent to the western coast of the island in the early morning, with calm surface winds on the coastal land. Climate modeling studies for a 1-month period during the region’s rainy season successfully replicated the main features of the observed regional distribution, diurnal variation of surface winds and convection over the western coast of the island and the adjacent seas. The results showed that convections develop across a wide area along the coast of the island at similar times in the late afternoon with the penetration of the sea breeze. The convections cause a rapid drop in temperature in the lower atmosphere, which produces a temperature contrast between the land and the sea, with lower temperatures occurring over the island. In response to the temperature differences, a strong offshore flow occurs over the sea adjacent to the western coast during nighttime, which creates an intensive wind convergence at the low-levels, and initiates convection offshore near the coast at late night and early morning. The results of this study suggest that the strong offshore flow derived principally from convections that develop earlier in the afternoon and evening over the island plays an important role in the formation of the nocturnal rainfall over the sea in the vicinity of western Kalimantan Island.
The cloud vertical structure at two remote tropical oceanic convective locations is characterized using both ship-based (35 GHz) and space-based (94 GHz) cloud radar data. The field experiment data depicted the full diurnal cycle and detail of the vertical structures, while the CloudSat sampling was more extensive, but did not coincide with the field experiments. One location is the monsoonal Bay of Bengal, sampled as part of the Joint Air-Sea Monsoon Interaction Experiment (JASMINE), and the other location is the eastern Pacific intertropical convergence zone, sampled during the Eastern Pacific Investigation of Climate (EPIC) experiment. Both ship- and space-based radar datasets find more high cloudiness (9-14 km altitude with an 11-12 km maximum) over the Bay of Bengal than over the eastern tropical Pacific, postulated to reflect the advection of cirrus by upper-level easterly winds. Low-level cloudiness, with some extending into what may be cumulus congestus, is more characteristic of the eastern tropical Pacific than the Bay of Bengal. CloudSat cloud fractions over the eastern Pacific were only ˜ one-third those over the Bay of Bengal, in contrast to comparable cloud fractions within the field experiment datasets. A thin melting-level cloud population was also more apparent in the JASMINE data than in the EPIC field experiment data, but the larger CloudSat dataset showed the opposite. These differences discourage regional generalities based on a few weeks of point sampling. Pre-monsoon-onset conditions included a slight afternoon low cloud amount maximum combined with typically one warm rain event per night. High thin cirrus was common (cloud fraction of ˜35% with cloud optical depths of 2 or less and ice water paths typically < 40 g m-2). After monsoon onset, the ship-based cloud radar documented examples of coherent relationships among cloud structure, precipitation, and the larger-scale wind field. CloudSat did not detect much precipitating low cloud during its pre-monsoon time period, but did sample the upper-level clouds at approximately the maximum and minimum of their diurnal range. An intriguing finding is that CloudSat perceived a greater daytime occurrence of the highest clouds with reflectivitives < 10 dBZ at both locations, a finding replicated with a year’s data covering the full, ocean-only, tropical belt. We speculate the daytime high cloud may reflect remains of outflow from previous convection. The comparison also highlights differences in the Rayleigh versus Mie responses of the two radars.
Timeaveraged zonalmean equatorial winds in an aquaplanet experiment were investigated using 30day simulation data from a global 7km mesh nonhydrostatic experiment with explicit moist physics. The simulated mean winds included equatorial easterly (westerly) in the lower (upper) troposphere and Hadley cells with a single upward branch on the equator. The mean zonal wind in the equatorial region, where moist convection was abundant, was investigated in detail. Each term of the momentum equation for the mean zonal wind was diagnosed. The advection terms were the major components that maintained the simulated mean wind. The advection terms were divided into those associated with mean (zonal and temporal) and deviation winds. In the equatorial region (1°N-1°S), the advection due to deviation winds led to westerly acceleration, which was balanced by the vertical transport of lowlevel easterly through mean upward motion. To identify the primary source of the westerly acceleration, deviation winds were decomposed into several classes by zonal scales, and momentum flux divergence was calculated for each class. The role of diabatic heating was also examined in a similar way. It was found that the momentum transport associated with planetaryscale disturbances was the primary source in the upper troposphere, of which moist Kelvin wave modes accounted for a significant part. In the middle to lower troposphere, synopticscale and mesoscale components were responsible for the westerly acceleration. It was also found that the westerly acceleration in the upper (middle to lower) troposphere by deviation winds mainly occurred on cloudfree (convective) grid points.