Showing papers in "Journal of the Atmospheric Sciences in 1990"

A One-Dimensional Entraining/Detraining Plume Model and Its Application in Convective Parameterization

Abstract: A new one-dimensional cloud model, specifically designed for application in mesoscale convective parameterization schemes (CPSs), is introduced. The model is unique in its representation of environmental entrainment and updraft detrainment rates. In particular, the two-way exchange of mass between clouds and their environment is modulated at each vertical level by a buoyancy sorting mechanism at the interface of clear and cloudy air. The new entrainment/detrainment scheme allows vertical profiles of both updraft moisture detrainment and updraft vertical mass flux to vary in a physically realistic way as a function of the cloud-scale environment. These performance characteristics allow the parameterized vertical distribution of convective heating and drying to be much more responsive to environmental conditions than is possible with a traditional one-dimensional entraining plume model. The sensitivities of the new model to variations in environmental convective available potential energy and verti...

Determination of the Optical Thickness and Effective Particle Radius of Clouds from Reflected Solar Radiation Measurements. Part I: Theory

Abstract: A method is presented for determining the optical thickness and effective particle radius of stratiform cloud layers from reflected solar radiation measurements. A detailed study is presented which shows that the cloud optical thickness (τc) and effective particle radius (re) of water clouds can be determined solely from reflection function measurements at 0.75 and 2.16 μm, provided τc ≳ 4 and re ≳ 6 μm. For optically thin clouds the retrieval becomes ambiguous, resulting in two possible solutions for the effective radius and optical thickness. Adding a third channel near 1.65 μm does not improve the situation noticeably, whereas the addition of a channel near 3.70 μm reduces the ambiguity in deriving the effective radius. The effective radius determined by the above procedure corresponds to the droplet radius at some optical depth within the cloud layer. For clouds having τc ≳ 8, the effective radius determined using the 0.75 and 2.16 μm channels can be regarded as 85%–95% of the radius at cloud...

On the Existence of Storm-Tracks.

Abstract: Given that middle latitude weather systems transport heat in a manner such as to weaken the baroclinicity that is thought to be crucial to their growth, it is perhaps surprising that concentrated regions of such eddy activity, i.e. storm-tracks, are found in the Northern Hemisphere winter. The existence and possible self-maintenance of storm-tracks is investigated using a linear, stationary wave model with storm-track region forcings taken from data averaged over a number of winters. It is found that the direct thermal effect of the eddies does indeed act against the existence of the storm-track. Their vorticity fluxes lead to some reduction of this effect. It is argued that the mean diabatic heating in the storm-track region is an indirect eddy effect. This heating is found to maintain the mean maximum in baroclinicity in the region. Further, the mean low-level flow induced by the eddy effects is such as to enhance the warm western oceanic boundary currents that are crucial to the existence of t...

The Relevance of the Microphysical and Radiative Properties of Cirrus Clouds to Climate and Climatic Feedback

Abstract: This paper examines the effects of the relationship between cirrus cloud ice water content and cloud temperature on climate change A simple mechanistic climate model is used to study the feedback between ice water content and temperature The central question studied in this paper concerns the extent to which both the radiative and microphysical properties of cirrus cloud influence such a feedback To address this question, a parameterization of the albedo and emissivity of clouds is introduced Observations that relate the ice water content to cloud temperature are incorporated in the parameterization to introduce a temperature dependence to both albedo and emittance The cloud properties relevant to the cloud feedback are expressed as functions of particles size re, asymmetry parameter g and cloud temperature and analyses of aircraft measurements, lidar and ground based radiometer data are used to select re and g It was shown that scattering calculations assuming spherical particles with a di

Development Characteristics and Dynamic Structure of Tropical Intraseasonal Convection Anomalies

Abstract: The development and dynamical structure of intraseasonal low-frequency convection anomalies in the equatorial region are investigated using 10 years (1975–85) of outgoing longwave radiation (OLR) and 7 years (1979–85) of 200 and 850 mb wind data. The composite OLR anomalies for 36 cases show a four-stage development process: initiation over equatorial Africa, rapid intensification when passing through the Indian Ocean, mature evolution characterized by a weakening in the maritime continent and redevelopment over the western Pacific, and dissipation near the date line in moderate events or emanation from the equator toward North America and southeastern Pacific in strong events. A noticeable feature in vertical structure is that the 850 mb convergence leads convection and midtropospheric upward motion by about 30 degrees longitude in both developing and mature phases. Equatorial upper- (lower-) level easterly (westerly) anomalies and associated twin anomalous anticyclonic (cyclonic) circulation an...

Tropical-Extratropical Interaction Associated with the 30–60 Day Oscillation and Its Impact on Medium and Extended Range Prediction

Abstract: An observational and modeling study is made of tropical-extratropical interactions on time scales relevant to medium and extended range forecasting. First, an empirical orthogonal function (EOF) analysis is made of outgoing longwave radiation (OLR) in the tropics over seven winters. Having removed the seasonal cycle and interannual variability, the two leading EOFs describe the 30–60 day oscillation. A composite of extratropical 500 mb geopotential height correlated simultaneously with this mode of tropical variability is constructed. In its two phase-quadrature components, this composite has significant projection onto the Pacific/North American teleconnection pattern and onto the North Atlantic oscillation pattern, respectively. The 500 mb height composite is compared with the Simmons, Wallace and Branstator (SWB) mode of barotropic instability, which has similar periodicity and similar spatial structure in both its phase-quadrature components. A simple theoretical analysis shows that the SWB m...

Dynamics of the coupled moist Kelvin-Rossby wave on an equatorial β-plane

Abstract: A simple theoretical analysis on the stability of a resting tropical atmosphere to semigeostrophic perturbations is given using a free atmosphere–boundary layer coupled model on an equatorial β-plane. An unstable mode emerges when sea surface temperature is higher than a critical value. The growing mode is a moist Kelvin wave modified through coupling with a Rossby wave of the lowest meridional index. The modified Rossby modes, however, remain damped even for high SST. The unstable mode selection can be explained in terms of wave energy generation due to the latent heating induced by frictional moisture convergence. The horizontal mode-coupling has profound impacts on wave instability. It favors the amplification of long planetary-scale waves, slows down eastward propagation, and suppresses unrealistically fast growth of the uncoupled moist Kelvin mode by creating substantial meridional flows. These effects make the coupled unstable mode more resemble observed equatorial intraseasonal disturbance...

A Theory for Long-Lived Mesoscale Convective Systems

Abstract: It is proposed that certain long-lived mesoscale convective systems maintain themselves through an interaction between quasi-balanced vertical motions and the diabatic effects of moist convection. Latent heat release, evaporation and melting of precipitation, and thermal radiation are all shown to contribute to the creation of a positive potential vorticity anomaly in the lower troposphere. This anomaly can interact with a sheared environment so as to induce further lifting of low-level air and subsequent release of conditional instability.

A Study of Self-excited Oscillations of the Tropical Ocean–Atmosphere System. Part II: Nonlinear Cases

Abstract: We analyze the linearized version of an analytical model, which combines linear ocean dynamics with a simple version of the Bjerknes hypothesis for El Nino. The ocean is represented by linear shallow water equations on an equatorial beta-plane. It is driven by zonal wind stress, which is assumed to have a fixed spatial form. Stress amplitude is set to be proportional to the thermocline displacement at the eastern boundary. It is shown that, for physically plausible parameter values, the model system can sustain growing Oscillations. Both growth rate and period scale directly with the time that an oceanic Kelvin wave needs to crow the basin. They are quite sensitive to the coupling parameter between thermocline displacement and wind stress, and the zonal location and meridional width of the wind. The most important parameter determining this behavior of the system is the coupling constant. For strong coupling the system exhibits exponential growth without oscillation. As the coupling is decreased ...

On the Conservation and Impermeability Theorems for Potential Vorticity

Abstract: If the partial analogy between the behavior of Rossby-Ertel potential vorticity (PV) and the behavior of chemical tracers is to be correctly used in the general case of diabatic, frictional motion, then certain fundamental differences, as well as similarities, between the behavior of PV and that of chemical tracers must be recognized. These differences stem from the well-known kinematical relationship between PV and isentropic circulation (via Stokes' theorem), which has no counterpart for chemical substances. One way of stating the analogy while recognizing the differences is to say first that PV behaves like the mixing ratio of a peculiar chemical “substance” that has zero source; i.e., is exactly conserved, away from boundaries (conserved not in the material or Lagrangian sense, but in the general sense associated with the idea of an indestructible chemical substance), and second that isentropic surfaces behave exactly as if they were impermeable to this “PV-substance” or “PVS,” even when diab...

Analysis of Turbulence Structure in the Surface Layer with a Modified Similarity Formulation for Near Neutral Conditions

Abstract: Data from a recent detailed surface layer experiment are critically examined in terms of the turbulent kinetic energy budget and the other second order moment budgets formed by the three velocity components and temperature. In moderately unstable and slightly stable conditions nondimensional terms of all the moment budgets studied agree reasonably well with results reported from the Kansas study (after application of a flow distortion correction). In the near-neutral range, where the present experiment contains a large amount of data, results deviate significantly from previous studies in general and, in particular, for ideal, zero-pressure gradient turbulent boundary layers. Several moments, such as u2W, v2w and W2 are not, as expected, constant in the surface layer, but vary logarithmically with height, making instead their non dimensional vertical gradients constant. Some moments scale with the roughness length and others with a length scale containing the large-scale pressure gradient or, wit...

The Intraseasonal (30–50 day) Oscillation of the Australian Summer Monsoon

Abstract: The tropical intraseasonal (30–50 day) oscillation manifests itself in the Australian summer monsoon by a pronounced modulation of the monsoonal westerlies. These 30-50 day fluctuations of the monsoonal westerlies are coherent with rainfall and OLR across northern Australia. The OLR fluctuation originates in the Indian Ocean and systematically propagates eastward at 5 m s−1, consistent with previous studies of the intraseasonal oscillation. The detailed evolution of the intraseasonal oscillation of the monsoon is studied via composites of upper air data in and about the Australian tropics. During the summer periods 1957-87, 91 events were identified at Darwin, Australia. The composite oscillation at Darwin has a very deep baroclinic structure with westerlies extending up to 300 mb. The westerly phase lasts about ten days and lags a similar duration rainfall event by about four days. During the westerly phase, the upper troposphere is warm and the extreme lower troposphere is cool. This structure ...

A Composite Study of Onset of the Australian Summer Monsoon

Abstract: The circulation changes that accompany an onset (defined as the first occurrence of wet 850-mb westerly winds at Darwin, Australia) of the Australian summer monsoon are documented by a composite study for the years 1957-1987. Composites of atmospheric fields at stations in and about the Australian tropics are constructed relative to the onset data at Darwin. It is shown that the composite onset is dominated by a slow eastward migration of a deep-baroclinic convective circulation displaced south of the equator. This propagating anomaly exhibited many features of the so-called 40-50 day oscillation, including an upper level anticyclone that accompanies the convective anomaly.

On the Global Exchange of Mass between the Stratosphere and Troposphere

Abstract: Seasonal mean downward mass fluxes across the 100 mb level in the extratropics of both hemispheres are computed using the meridionally integrated residual vertical circulation as determined from the transformed Eulerian mean equations The eddy momentum and heat flux data required for the calculation are taken from Oort's 15-year climatology Upward mass flux from the troposphere to the stratosphere in the tropics is computed from mass continuity The flux is a maximum during Northern Hemisphere winter and a minimum during Northern Hemisphere summer The computed fluxes imply a 25 year turnover time for the global atmospheric layer above 100 mb

Temporal Changes of the Primary Circulation in Tropical Cyclones.

Abstract: More than 900 radial profiles of in situ aircraft observations collected in 19 Atlantic hurricanes and tropical storms over 13 years confirm that the usual mechanism of tropical cyclone intensification involves contracting maxima of the axisymmetric swirling wind. Radar shows that annuli of convective echoes accompany the wind maxima. These features, called convective rings exist and move inward because latent heat released in the rings leads to descent, adiabatic warming, and rapid isobaric height falls in the area they enclose. The radial change in rate of isobaric height fall is concentrated at the inner edge of the wind maximum, causing the gradient wind to increase there and the maximum to contract. Vigorous convection organized in rings invariably causes well defined, inward moving wind maxima, but when convection is weak, the rings are also weak or even absent. In this case, the swirling wind may be nearly constant with radius and change slowly in time. Hurricanes that have a single, vigor...

Climate change and the middle atmosphere. I - The doubled CO2 climate

Abstract: The effect of doubling the atmospheric content of CO2 on the middle-atmosphere climate is investigated using the GISS global climate model. In the standard experiment, the CO2 concentration is doubled both in the stratosphere and troposphere, and the SSTs are increased to match those of the doubled CO2 run of the GISS model. Results show that the doubling of CO2 leads to higher temperatures in the troposphere, and lower temperatures in the stratosphere, with a net result being a decrease of static stability for the atmosphere as a whole. The middle atmosphere dynamical differences found were on the order of 10-20 percent of the model values for the current climate. These differences, along with the calculated temperature differences of up to about 10 C, may have a significant impact on the chemistry of the future atmosphere, including that of stratospheric ozone, the polar ozone 'hole', and basic atmospheric composition.

The 1985/86 Intraseasonal Oscillation and the Role of the Extratropics

Abstract: An intraseasonal oscillation that occurred in the 1985/86 northern winter is documented in this study. The tropical convection of this event is dominated by the mixture of a standing oscillation over the maritime continent and an eastward moving feature from the Indian Ocean into the central Pacific. The time evolution of the upper tropospheric circulation patterns, instead of propagating eastward along the equator as suggested in the existing composites of the intraseasonal oscillation, is characterized by a series of wave patterns in the Northern Hemisphere and does not complete the cycle around the globe. The familiar moist Kelvin wave explanation for the intraseasonal oscillation receives little support from diagnosis of this event using zonal wind, height field, streamfunction, and potential vorticity. Only in the lower troposphere near the date line is the convincing evidence for its existence found. A scenario for the intraseasonal oscillation, which is suggested by the analysis, includes ...

A scheme for parameterizing ice cloud water content in general circulation models

Abstract: The optical properties of ice clouds are a primary issue for climate and climate change. Evaluating these optical properties in three-dimensional models for studying climate will require a method to calculate the ice water content of such clouds. A procedure is developed to parameterize ice water content as a function of large-scale meteorological characteristics for use in circulation models in which the ice water content is not calculated by means of a three-dimensional prognostic equation for condensed water. The technique identifies large-scale flows in which ice clouds exist and calculates their ice water content by reconstructing the trajectory associated with cloud formation. As the cloud forms, its ice content changes both by deposition of ice from water vapor and by ice removal by sedimentation. The sedimentation process is found to modify significantly the ice water content expected from deposition alone. Ice water contents predicted by the parameterization are compared with aircraft observations collected in the middle latitudes and the tropics, and show reasonable agreement over four orders-of-magnitude of ice water content. A parameterization for the sublimation of ice crystals settling into ice-subsaturated environments is also presented.

Vertical-Velocity Skewness in the Buoyancy-Driven Boundary Layer

Abstract: A number of puzzling features of the skewness of the vertical velocity field, Sw(z), are found in observations and large-eddy simulations (LES) of the buoyancy-driven planetary boundary layer (PBL). For example, observations of Sw(z) in cases where the air is heated from below indicate that Sw(z) > 0 and remains relatively constant for z ≳ 0.3zi, whereas all large-eddy simulations of these cases show a continuing increase of Sw(z) with height. In cases where the air is both heated from below and cooled from above, as in some of the stratus-topped PBL cases, large-eddy simulations show a rather curious feature: Sw is positive in the upper layer and negative in the lower layer. In considering these features, it occurred to us that a theoretical model of what one should expect of the skewness distribution, even in simple situations, did not exist. Hence in the present paper we examine the skewness distributions from direct numerical simulations of several simple archetypes of buoyancy-driven turbule...

Gradient Balance in Tropical Cyclones

Abstract: Analysis of a large inventory of in situ observations from research aircraft shows that the gradient wind approximates the axisymmetric swirling flow in the free atmosphere within 150 km of the centers of Atlantic hurricanes and tropical storms. In the middle and lower troposphere, the rms difference between the azimuthal cream swirling and gradient winds is typically < 1.5 m s−1 with zero bias. This balance prevails only for the azimuthal mean, not locally, nor is balance to be expected in either the surface friction layer or the upper tropospheric outflow layer where the radial flow is comparable with the swirling flow. It is theoretically possible that axisymmetric supergradient flow may occur in response to rapid radial acceleration where the radial flow slows in the friction layer beneath the eyewall or where it converges into intense diabatically forced updrafts. Nevertheless, the observations in the free lower and midtroposphere show that systematic departures of the azimuthal mean vorte...

Cloud-Tracked Winds from Pioneer Venus OCPP Images

Abstract: Analysis of ultraviolet image sequences, obtained from the Pioneer Venus Orbiter Cloud Photopolarimeter and covering five 80-day periods from 1979–1985, provides the first climatological description of the cloud top circulation on Venus. The average zonal winds can be characterized as a 5-day retrograde rotation of the whole cloud-level atmosphere with weak “jets” at middle to high latitudes. Both the midlatitude and equatorial zonal winds vary by about 5–8 m s−1 over time spans of 1–6 years. The average meridional circulation is poleward in both hemispheres up to at least 60° latitude, consistent with the presence of a thermally direct Hadley circulation associated with the clouds. The strength of the Hadley circulation also varies with time. Four wave modes are clearly identified: a diurnal solar tide, a semi-diurnal solar tide, a “4-day equatorial” wave, and a “5-day midlatitude” wave. The semidiurnal tide appears to have an amplitude of about 5 m s−1 and to be approximately constant with time...

Use of Polarization to Characterize Precipitation and Discriminate Large Hail

Abstract: We examine the utility of the correlation coefficient between linear orthogonally polarized echoes for determining precipitation type and gauging hail size. Models and measurements from pure rain coincide in predicting very high correlations (0.98); similar results are obtained with pure hail. Several mechanisms could cause the lowering of correlation but the behavior of the examined data is definitely attributed to a mixture of hydrometeor types. This decrease is an indicator of hail size; it is shown theoretically that in at least two other realistic situations the correlation would decrease with hail size. For the examined case a model of hail shape and orientation during fall is able to reproduce the essential features of polarimetric measurements. It suggests, together with our data and data from other investigators, that substantial negative differential reflectivity (about −1 dB) in a region of high reflectivity factor values is caused by hailstones larger than about 2 cm in diameter.

Synoptic-Scale Disturbances near the Equator

Abstract: A spectral analysis of winds analyzed and initialized at the European Centre for Medium-Range Weather Forecasts reveals an abundance of power in the 850 mb meridional wind along the equator with periods near four days. The power is mostly in the westward propagating component. Using high-pass filtered data it is shown that the waves have westward phase and eastward group propagation relative to the mean wind. The longest wavelengths are found over the Pacific Ocean, while the shortest are found over the convectively variable regions of Indonesia, South America, and Africa. Mean phase speeds at 850 mb are positively correlated with the mean wind on the equator at 500 mb and below, and negatively correlated with the mean wind above that level. The effective advecting zonal wind of the disturbances seems to be the density weighted average of the lower troposphere. The structure of the disturbances bears resemblance to the expected structure of an equatorially trapped mixed Rossby-gravity wave over t...

Planetary-Scale Waves and the Cyclic Nature of Cloud Top Dynamics on Venus

Abstract: Quantitative data on the properties of the planetary scale wave modes at the Venus cloud tops have been derived from eight years of Pioneer Venus UV images. At least four planetary-scale wave modes are established on this basis; it is suggested that all play an important role in the maintenance of the equatorial winds. The evidence obtained suggests that the equatorial dynamics of Venus is cyclic, with an apparent time-scale of 5-10 yr. The long-term evolution and maintenance of the Venus cloud-top superrotation reflects a complex balance among at least four eddy momentum transport equations.

Saturated and Unsaturated Spectra of Gravity Waves and Scale-Dependent Diffusion

Abstract: For a highly idealized condition, the spectrum of saturated and unsaturated gravity waves at each height is calculated directly from the wave equation. A principal feature of this wave equation is the inclusion of wave dissipation, although in an approximate form. In the absence of wave absorption, reflection, radiation, wind shears, resonant wave–wave interactions and other sources and sinks, this dissipation at each height is determined solely by the “turbulent” or chaotic state caused by off-resonant wave–wave interactions and instability of the (broad) wave spectrum at that height. The dissipation is approximately accounted for by a diffusion term. The appropriate diffusivity is self-consistent with the continuum of spectral waves that cause the chaotic state and is argued to be scale dependent. An inverse calculation is also made of what the observed spectra imply for wave dissipation—again assuming that many wave dissipations can be approximately described by a scale-dependent diffusion pro...

A Wind Tunnel and Theoretical Study of the Melting Behavior of Atmospheric Ice Particles. IV: Experiment and Theory for Snow Flakes

Abstract: An experiment in the Mainz vertical Cloud Tunnel is described in which natural and laboratory-made aggregates of snow crystals (snow flakes) were melted under free fall conditions in the vertical air stream of the tunnel, which was allowed to warm up at the rates experienced by falling snow flakes in the atmosphere. The variation of the fall mode, the fall velocity, and the percentage of ice melted, as a function of percentage of distance travelled for 99% melting was recorded by cinematography. The laboratory results were confirmed by the results of a theoretical heat transfer model which we developed for the melting of a snow flake. In this model a snow flake was idealized by an oblate spheroid in which—as confirmed by observation—the melt water did not remain at the periphery of the flake but penetrated inside the flake. The computed distances needed by snowflakes to melt under various atmospheric conditions were found to agree well with recent radar observations.

Observed Changes in Baroclinic Wave Activity during the Life Cycles of Low-Frequency Circulation Anomalies

Abstract: Anomalous baroclinic wave activity during the onset and decay of blocking patterns in the Northern Hemisphere winter is investigated based upon a 30-year record of 500 mb height and sea-level pressure. A slowly-varying “envelope function” is defined at each gridpoint in order to represent the modulation of the amplitude of baroclinic wave activity by the low-frequency circulation anomalies related to blocking. The envelope function fields and the low-frequency circulation fields are composited relative to the onset and decay times of four persistent dipole patterns in low-frequency circulation anomalies located downstream of the climatological-mean stormtracks. One polarity of the anomaly patterns corresponds to dipole or modon-like blocking patterns over the eastern oceans, and the other to strong zonal flow patterns. Each of the eight composites is comprised of more than 50 persistent blocking or zonal flow episodes. The most striking common feature observed during the onset of blocking over th...

Small Error Dynamics and the Predictability of Atmospheric Flows.

Abstract: In this paper, linear small-error theory is applied to the study of weather predictability. A simple baroclinic shear model and a barotropic channel model with a localized jet are used as examples. It is shown that increase in error on synoptic forecast time scales is controlled by rapidly growing perturbations that are not of normal mode form. Unpredictable regimes are not necessarily associated with larger exponential growth rates than are relatively more predictable regimes. Model problems illustrating baroclinic and barotropic dynamics suggest that asymptotic measures of divergence in phase space, while applicable in the limit of infinite time, may not be appropriate over time intervals addressed by present synoptic forecast.

A Thermodynamic Foundation for Modeling the Moist Atmosphere

Abstract: With advances in numerical modeling of the atmosphere, we have experienced that the return to the first principles of physics often enables a model to cope more easily with the complexities of the real atmosphere. The return to the primitive equations of motion from historical balance approximations is an example. This paper proposes a way to return to the “primitive” form of moist themodynamics, in which prediction is made strictly in terms of conservative properties, such as mass and entropy. There is no conservation law that would apply directly to temperature or pressure. These intensive properties, therefore, should be diagnostically determined by thermodynamics, from the predicted conservative properties. The scope of the paper is limited to the thermodynamics of reversible processes. Irreversible processes, which would make a model alive with real weather, are not discussed here, since each of them requires a separate empirical treatment. It is shown, however, that the proposed formulation...

A Diagnosis of the Asymmetry in Top-Down and Bottom-Up Diffusion Using a Lagrangian Stochastic Model

Abstract: A Lagrangian stochastic model of particle trajectories is used to investigate the asymmetry in vertical diffusion from area sources at the bottom and top of an inhomogeneous turbulent boundary layer. Such an asymmetry was discovered in the large-eddy simulations (LES) of the convective boundary layer (CBL) by Wyngaard and Brost (1984) and Moeng and Wyngaard (1984). For inhomogeneous Gaussian turbulence, a diffusion asymmetry results from the vertical asymmetry in the vertical velocity variance about the midplane of the boundary layer. For small turbulence time scales, this is predictable from eddy-diffusion (K) theory. However, for large time scales, K theory is inapplicable as evidenced by countergradient flux regions and K singularities. The fundamental causes of the K model breakdown are the memory (large time scale) and vertical inhomogeneity of the turbulence, which lead to a mean vertical acceleration of particles away from the source and a “drift” velocity. A positive skewness in vertical ...