Abstract

The relationship between upper-tropospheric ice cloud properties and the Hadley circulation intensity is examined through parameter sensitivity studies of global cloud-system-resolving simulations with explicit cloud convection. Experiments under a perpetual July condition were performed by changing parameters in the boundary layer and cloud microphysics schemes, with a mesh size of approximately 14 km. One additional experiment with a mesh size of approximately 7 km was also conducted. These experiments produced a variety of upper-cloud coverage and outgoing longwave radiation (OLR) distributions. The authors found that, as the upper-cloud coverage increased, the total precipitation decreased and the intensity of the Hadley circulation weakened because of energy balance constraints that radiative cooling are balanced by adiabatic warming. Interestingly, the ice water path was not correlated with the upper ice-loud coverage or OLR, indicating that the spatial coverage of upper ice clouds, rather than the ice water content, was the key factor in the radiation budget.

Abstract

This study investigated the resolution dependence of diurnal variation in tropical convective systems represented by a global nonhydrostatic model without cumulus parameterization. This paper describes the detailed characteristics of diurnal variation in surface precipitation based on three-dimensional data, with the aim of explicitly clarifying the mechanism that underlies the variation. The study particularly focused on the evolution in the size of the precipitation area for deep convective systems with an analysis of the vertical structure of thermodynamic fields. This analysis compares the results of simulations with horizontal grid sizes of 14 and 7 km (R14 and R7, respectively). Over land, the phase delay of diurnal variations in R7 is about 3 h less than that in R14. R7 produces a pronounced diurnal variation in the size distributions of precipitating area(s), especially for areas with a radius of 0–100 km; this characteristic is not found for R14. Such areas actively evolve between noon and evening, leading to the smooth development of larger-scale precipitating areas having a radius of 100–150 km. The maximum surface precipitation in R7 over land occurs at around 2000 local time throughout the tropics, approximately 2 h prior to the development of nighttime deep convection. Deep convective regimes are important as agents of vertical heat transport in the tropics. The present results suggest that precipitating areas with a radius <100 km make a strong contribution to the total amount of precipitation and to mass transport.

Abstract

Statistical characteristics of surface meteorology are examined in terms of frequency spectra. According to a recent work using hourly data over 50 yr in the Antarctic, the frequency spectra have a characteristic shape proportional to two different powers of the frequency in the frequency ranges lower and higher than a transition frequency of (several days)⁻¹. To confirm the universality of the characteristic spectra, hourly data—including surface temperature, sea level pressure, and zonal and meridional winds—collected over 45 yr at 138 stations in Japan were analyzed. Similar spectral shapes are obtained for any physical quantities at all stations. The spectral slopes clearly depend on the latitude, particularly for sea level pressure, which in the high-frequency range are steeper at higher latitudes. Next, the analysis was extended using realistic simulation data over one month with a nonhydrostatic model to examine the global characteristics of the spectra in the high-frequency range. The model spectra accord well with the observations in Japan. The spectral slopes are largely dependent on the latitude—that is, shallow in the low latitudes, and steep in the middle and high latitudes for all the physical quantities. The latitudinal change of the spectral slope is severe around 30°, which may be due to the dynamical transition from nongeostrophy to geostrophy. The longitudinal variations are also observed according to the geography. The variance is large in the storm-track region for surface pressure, on the continents for temperature and over the ocean for winds.

Abstract

This study examines the impact of an alteration of a cloud microphysics scheme on the representation of longwave cloud radiative forcing (LWCRF) and its impact on the atmosphere in global cloud-system-resolving simulations. A new double-moment bulk cloud microphysics scheme is used, and the simulated results are compared with those of a previous study. It is demonstrated that improvements within the new cloud microphysics scheme have the potential to substantially improve climate simulations. The new cloud microphysics scheme represents a realistic spatial distribution of the cloud fraction and LWCRF, particularly near the tropopause. The improvement in the cirrus cloud-top height by the new cloud microphysics scheme substantially reduces the warm bias in atmospheric temperature from the previous simulation via LWCRF by the cirrus clouds. The conversion rate of cloud ice to snow and gravitational sedimentation of cloud ice are the most important parameters for determining the strength of the radiative heating near the tropopause and its impact on atmospheric temperature.

Abstract

Super Cyclone Pam (2015) formed in the central tropical Pacific under conditions that included El Niño Modoki and the passage of a convectively enhanced phase of the Madden–Julian oscillation (MJO) in the western Pacific. This study examines the influence that sea surface temperature anomalies (SSTAs) have on the MJO and low-frequency large-scale circulation, and establishes how they modulated the genesis of Pam. Two series of numerical experiments were conducted by using a nonhydrostatic global atmospheric model with observed (OBSSST) and climatological (CLMSST) SSTs. The results suggested that low-frequency westerly winds at 850 hPa (U850) were intensified in the central tropical Pacific due to the observed SSTA. The amplitude of the MJO simulated in OBSSST was larger than in CLMSST. In addition, the experiments initialized 26 February–2 March exhibited that the phase of the MJO in OBSSST was ahead of that in CLMSST, and that the genesis location in OBSSST was ~10° to the east of that in CLMSST. An analysis of large-scale fields indicated that a positive U850 maintained by SSTAs and intensification of U850 by the MJO modified distribution of large-scale cyclonic vorticity and precipitable water. These changes in large-scale fields modified the location and timing of intensification of the disturbance that become Pam and resulted in Pam’s genesis location being 10° farther east with slight impact on its genesis probability. Additional experiments showed that SSTAs in the central tropical Pacific were the dominant cause of modifications to large-scale fields, the MJO, and Pam’s genesis location.

Abstract

In this study, the single-moment 6-class bulk cloud microphysics scheme used in the operational numerical weather prediction system at the Japan Meteorological Agency was improved using the observations of the Global Precipitation Measurement (GPM) core satellite as reference values. The original cloud microphysics scheme has the following biases: underestimation of cloud ice compared to the brightness temperature of the GPM Microwave Imager (GMI) and underestimation of the lower-troposphere rain compared to the reflectivity of GPM Dual-frequency Precipitation Radar (DPR). Furthermore, validation of the dual-frequency rate of reflectivity revealed that the dominant particles in the solid phase of simulation were graupel and deviated from DPR observation. The causes of these issues were investigated using a single-column kinematic model. The underestimation of cloud ice was caused by a high ice-to-snow conversion rate, and the underestimation of precipitation in the lower layers was caused by an excessive number of small-diameter rain particles. The parameterization of microphysics scheme is improved to eliminate the biases in the single-column model. In the forecast obtained using the improved scheme, the underestimation of cloud ice and rain is reduced. Consequently, the prediction errors of hydrometeors were reduced against the GPM satellite observations, and the atmospheric profiles and precipitation forecasts were improved.

Abstract

A new evaluation method for the thermodynamic phases of clouds in cloud-system-resolving models is presented using CALIPSO observations and a satellite simulator. This method determines the thermodynamic phases using the depolarization ratio and a cloud extinction proxy. For the evaluation, we introduced empirical parameterization of the depolarization ratio of ice and water clouds using temperatures of a reanalysis dataset and total attenuated backscatters of CALIPSO. We evaluated the mixed-phase clouds simulated in a cloud-system-resolving model over the Southern Ocean using single-moment and double-moment bulk cloud microphysics schemes, referred to as NSW6 and NDW6, respectively. The NDW6 simulations reproduce supercooled water clouds near the boundary layer that are consistent with the observations. Conversely, the NSW6 simulations failed to reproduce such supercooled water clouds. Consistencies between the cloud classes diagnosed by the evaluation method and the simulated hydrometeor categories were examined. NDW6 shows diagnosed water and ice classes that are consistent with the simulated categories, whereas the ice category simulated with NSW6 is diagnosed as liquid water by the present method due to the large extinction from the ice cloud layers. Additional analyses indicated that ice clouds with a small effective radius and large ice water content in NSW6 lead to erroneous values for the fraction of the diagnosed liquid water. It is shown that the uncertainty in the cloud classification method depends on the details of the cloud microphysics schemes. It is important to understand the causes of inconsistencies in order to properly understand the cloud classification applied to model evaluations as well as retrievals.

Abstract

The Nonhydrostatic Icosahedral Atmospheric Model (NICAM), a global cloud-system-resolving model, successfully simulated the life cycle of Tropical Storm Isobel that formed over the Timor Sea in the austral summer of 2006. The multiscale interactions in the life cycle of the simulated storm were analyzed in this study. The large-scale aspects that affected Isobel’s life cycle are documented in this paper and the corresponding mesoscale processes are documented in a companion paper.

The life cycle of Isobel was largely controlled by a Madden–Julian oscillation (MJO) event and the associated westerly wind burst (WWB). The MJO was found to have both positive and negative effects on the tropical cyclone intensity depending on the location of the storm relative to the WWB center associated with the MJO. The large-scale low-level convergence and high convective available potential energy (CAPE) downwind of the WWB center provided a favorable region to the cyclogenesis and intensification, whereas the strong large-scale stretching deformation field upwind of the WWB center may weaken the storm by exciting wavenumber-2 asymmetries in the eyewall and leading to the eyewall breakdown.

Five stages are identified for the life cycle of the simulated Isobel: the initial eddy, intensifying, temporary weakening, reintensifying, and decaying stages. The initial eddy stage was featured by small-scale/mesoscale convective cyclonic vortices developed in the zonally elongated rainband organized in the preconditioned environment characterized by the WWB over the Java Sea associated with the onset of an MJO event over the East Indian Ocean. As the MJO propagated eastward and the cyclonic eddies moved southward into an environment with weak vertical shear and strong low-level cyclonic vorticity, a typical tropical cyclone structure developed over the Java Sea, namely the genesis of Isobel. Isobel experienced an eyewall breakdown and a temporary weakening when it was located upwind of the WWB center as the MJO propagated southeastward and reintensified as its eyewall reformed as a result of the axisymmetrization of an inward spiraling outer rainband that originally formed downwind of the WWB center. Finally Isobel decayed as it approached the northwest coast of Australia.

Abstract

The life cycle of Tropical Storm Isobel was simulated reasonably well in the Nonhydrostatic Icosahedral Atmospheric Model (NICAM), a global cloud-system-resolving model. The evolution of the large-scale circulation and the storm-scale structure change was discussed in . Both the mesoscale and system-scale processes in the life cycle of the simulated Isobel are documented in this paper. In the preconditioned favorable environment over the Java Sea, mesoscale convective vortices (model MCVs) developed in the mesoscale convective systems (MCSs) and convective towers with cyclonic potential vorticity (PV) anomalies throughout the troposphere [model vortical hot towers (VHTs)] appeared in the model MCVs. Multiple model VHTs strengthened cyclonic PV in the interior of the model MCV and led to the formation of an upright monolithic PV core at the center of the concentric MCV (primary vortex enhancement). As the monolithic PV core with a warm core developed near the circulation center, the intensification and the increase in horizontal size of the cyclonic PV were enhanced through the system-scale intensification (SSI) process (the secondary vortex enhancement), leading to the genesis of Isobel over the Timor Sea. The SSI process can be well explained by the balanced dynamics.

After its genesis, the subsequent evolution of the simulated Isobel was controlled by both the external influence and the internal dynamics. Under the unfavorable environmental conditions, the development of asymmetric structure reduced the axisymmetric diabatic heating in the inner core and the SSI process became ineffective and the storm weakened. Later on, as the eyewall reformed as a result of the axisymmetrization of an inward-propagating outer spiral rainband, the SSI process became effective again, leading to the reintensification of Isobel. Therefore, the large-scale environmental flow provided the precondition for the genesis of Isobel and the triggering mechanism for subsequent storm-scale structure change as discussed in . The system-scale and mesoscale processes, such as the evolution of MCVs and merging VHTs, were responsible for the genesis, while the eyewall processes were critical to the storm intensity change through the SSI process.

Abstract

The degree of gradient wind balance was investigated in a number of tropical cyclones (TCs) simulated under realistic environments. The results of global-scale numerical simulations without cumulus parameterization were used, with a horizontal mesh size of 7 km. On average, azimuthally averaged maximum tangential velocities at 850 (925) hPa in the simulated TCs were 0.72% (1.95%) faster than gradient wind–balanced tangential velocity (GWV) during quasi-steady periods. Of the simulated TCs, 75% satisfied the gradient wind balance at the radius of maximum wind speed (RMW) at 850 and at 925 hPa to within about 4.0%. These results were qualitatively similar to those obtained during the intensification phase. In contrast, averages of the maximum and minimum deviations from the GWV, in all the azimuths at the RMW, achieved up to 40% of the maximum tangential velocity. Azimuthally averaged tangential velocities exceeded the GWV (i.e., supergradient) inside the RMW in the lower troposphere, whereas the velocities were close to or slightly slower than GWV (i.e., subgradient) in the other regions. The tangential velocities at 925 hPa were faster (slower) in the right-hand (left hand) side of the TC motion. When the tangential velocities at the RMW were supergradient, the primary circulation tended to decay rapidly in the vertical direction and slowly in the radial direction, and the eyewall updraft and the RMW were at larger radii. Statistical analyses revealed that the TC with supergradient wind at the RMW at 850 hPa was characterized by stronger intensity, larger RMW, more axisymmetric structure, and an intensity stronger than potential intensity.