Diurnal Cycle Of Tropical Convection Research Articles

Katabatic and convective processes drive two preferred peaks in the precipitation diurnal cycle over the Central Himalaya

AbstractThe diurnal cycle of precipitation over the Central Himalaya is governed by a complex interaction between the diurnal cycle of tropical convection and local orographic flow. Understanding this interaction is crucial for model evaluation, where the simulation of such processes is highly sensitive to model resolution and choice of parametrisation schemes. In this study, the mean diurnal cycle is computed using gridded satellite data–Integrated Multi‐satellitE Retrievals for Global Precipitation Measurement (GPM) (IMERG) data and is shown to be bimodal, with one peak in the late afternoon (1700 India Standard Time (IST)) and a stronger one in the early morning (0200 IST). This structure is an artefact of compositing, as individual days are associated with single peaks. The late‐afternoon “convective” peak is shown to be linked to the diurnal cycle of tropical convection, whereas the nocturnal “katabatic” peak is shown to be triggered by nocturnal downslope flow converging with the background monsoon circulation. As such, the katabatic peak is strongly favoured by an active monsoon trough, which provides greater southeasterly moisture flux to the foothills, resulting in increased low‐level moisture flux convergence upon interaction with the katabatic northerlies. When the trough is less active, precipitation is brought to the region by mesoscale convective systems, ranging in scale from tens to thousands of kilometres, resulting in convective peaks. We hypothesise that these convective afternoon peaks are enhanced by anabatic flow. It is shown that the Boreal Summer Intraseasonal Oscillation does not play a significant role in modulating either the timing or the amplitude of the diurnal cycle; however, low‐pressure systems do: either by intensifying the trough (and hence the katabatic peak), or, when further north, by providing deep convection (hence supporting the convective peak). Two reanalyses and a separate 17‐km model with parametrised convection capture both peaks, but overestimate the magnitude of the convective peak and underestimate the magnitude of the katabatic peak.

Subseasonal Weather Prediction in a Global Convection-Permitting Model

AbstractAlthough accurate weather and climate prediction beyond one to two weeks is of great value to society, the skill of such extended prediction is limited in current operational global numerical models, whose coarse horizontal grid spacing necessitates the parameterization of atmospheric processes. Of particular concern is the parameterization of convection and specifically convection in the tropics, which impacts global weather at all time scales through atmospheric teleconnections. Convection-permitting models, which forego convective parameterization by explicitly resolving cumulus-scale motions using fine (1–4 km) horizontal grid spacing, can improve global prediction at extended time scales by more faithfully simulating tropical convection and associated teleconnections. This study demonstrates that convection-permitting resolution in a global numerical model can improve both the statistical features of tropical precipitation and extended predictive skill in the tropics and midlatitudes. Comparing four monthlong global simulations with 3-km grid spacing to coarser-resolution simulations that parameterize convection reveals that convection-permitting simulations improve tropical precipitation rates and the diurnal cycle of tropical convection. The propagation of the Madden–Julian oscillation was better predicted in three of the four 3-km simulations; these three runs also featured more skillful prediction of weekly extratropical circulation anomalies, particularly during week 3 of each forecast. These results, though based on a small sample of four cases, demonstrate that convection-permitting global modeling can benefit extended atmospheric prediction and offers the potential for improved operational subseasonal forecast skill.

Tropical Cyclone Diurnal Cycle Signals in a Hurricane Nature Run

The diurnal cycle of tropical convection and tropical cyclones (TCs) has been previously described in observational-, satellite-, and modeling-based studies. The main objective of this work is to expand on these earlier studies by identifying signals of the TC diurnal cycle (TCDC) in a hurricane nature run, characterize their evolution in time and space, and better understand the processes that cause them. Based on previous studies that identified optimal conditions for the TCDC, a select period of the hurricane nature run is examined when the simulated storm was intense, in a low shear environment, and sufficiently far from land. When analyses are constrained by these conditions, marked radially propagating diurnal signals in radiation, thermodynamics, winds, and precipitation that affect a deep layer of the troposphere become evident in the model. These propagating diurnal signals, or TC diurnal pulses, are a distinguishing characteristic of the TCDC and manifest as a surge in upper-level outflow with underlying radially propagating tropical squall-line-like features. The results of this work support previous studies that examined the TCDC using satellite data and have implications for numerical modeling of TCs and furthering our understanding of how the TCDC forms, evolves, and possibly impacts TC structure and intensity.

Influences of the MJO on the space‐time organization of tropical convection

AbstractThe fact that the Madden‐Julian Oscillation (MJO) is characterized by large‐scale patterns of enhanced tropical rainfall has been widely recognized for decades. However, the precise nature of any two‐way feedback between the MJO and the properties of smaller‐scale organization that makes up its convective envelope is not well understood. Satellite estimates of brightness temperature are used here as a proxy for tropical rainfall, and a variety of diagnostics are applied to determine the degree to which tropical convection is affected either locally or globally by the MJO. To address the multiscale nature of tropical convective organization, the approach ranges from space‐time spectral analysis to an object‐tracking algorithm. In addition to the intensity and distribution of global tropical rainfall, the relationship between the MJO and other tropical processes such as convectively coupled equatorial waves, mesoscale convective systems, and the diurnal cycle of tropical convection is also analyzed. The main findings of this paper are that, aside from the well‐known increase in rainfall activity across scales within the MJO convective envelope, the MJO does not favor any particular scale or type of organization, and there is no clear signature of the MJO in terms of the globally integrated distribution of brightness temperature or rainfall.

Coordinated Observation and Numerical Study on a Diurnal Cycle of Tropical Convection over a Complex Topography in West Java, Indonesia

Coordinated Observation and Numerical Study on a Diurnal Cycle of Tropical Convection over a Complex Topography in West Java, Indonesia

Atmosphere-only GCM (ACCESS1.0) simulations with prescribed land surface temperatures

Abstract. General circulation models (GCMs) are valuable tools for understanding how the global ocean–atmosphere–land surface system interacts and are routinely evaluated relative to observational data sets. Conversely, observational data sets can also be used to constrain GCMs in order to identify systematic errors in their simulated climates. One such example is to prescribe sea surface temperatures (SSTs) such that 70 % of the Earth's surface temperature field is observationally constrained (known as an Atmospheric Model Intercomparison Project, AMIP, simulation). Nevertheless, in such simulations, land surface temperatures are typically allowed to vary freely, and therefore any errors that develop over the land may affect the global circulation. In this study therefore, a method for prescribing the land surface temperatures within a GCM (the Australian Community Climate and Earth System Simulator, ACCESS) is presented. Simulations with this prescribed land surface temperature model produce a mean climate state that is comparable to a simulation with freely varying land temperatures; for example, the diurnal cycle of tropical convection is maintained. The model is then developed further to incorporate a selection of “proof of concept” sensitivity experiments where the land surface temperatures are changed globally and regionally. The resulting changes to the global circulation in these sensitivity experiments are found to be consistent with other idealized model experiments described in the wider scientific literature. Finally, a list of other potential applications is described at the end of the study to highlight the usefulness of such a model to the scientific community.

A multi-scale model for the intraseasonal impact of the diurnal cycle of tropical convection

One of the crucial features of tropical convection is the observed variability on multiple spatiotemporal scales, ranging from cumulus clouds on the daily time scale over a few kilometers to intraseasonal oscillations over planetary scales. The diurnal cycle of tropical convection is a significant process, but its large-scale impact is not well understood. Here, we develop a multi-scale analytic model to assess the intraseasonal impact of planetary-scale inertial oscillations in the diurnal cycle. A self-contained derivation of a multi-scale model governing planetary-scale tropical flows on the daily and intraseasonal time scale is provided below, by following the derivation of systematic multi-scale models for tropical convection. This derivation demonstrates the analytic tractability of the model. The appeal of the multi-scale model developed here is that it provides assessment of eddy flux divergences of momentum and temperature and their intraseasonal impact on the planetary-scale circulation in a transparent fashion. Here, we use it to study the intraseasonal impact of a model for the diurnal cycle heating with two local phase-lagged baroclinic modes with the congestus, deep, stratiform life cycle. The results show that during boreal summer, the eddy flux divergence of temperature dominates in the northern hemisphere, providing significant heating in the middle troposphere of the northern hemisphere with large-scale ascent and cooling with subsidence surrounding this heating center. Due to the analytic tractability of the model, such significant eddy flux divergence of temperature is traced to meridional asymmetry of the diurnal cycle heating. In an ideal zonally symmetric case, the resulting planetary-scale circulation on the intraseasonal time scale during boreal summer is characterized by ascent in the northern hemisphere, southward motion in the upper troposphere, descent around the equator and northward motion in the lower troposphere. The intraseasonal impact of the diurnal cycle on the planetary scale also includes negative potential temperature anomalies in the lower troposphere, which suggests convective triggering in the tropics. Furthermore, a fully coupled model for the intraseasonal impact of the diurnal cycle on the Hadley cell shows that the overturning motion induced by the eddy flux divergences of momentum and temperature from the diurnal cycle can strengthen the upper branch of the winter cell of the Hadley circulation, but weaken the lower branch of the winter cell. The corresponding eddy fluxes from the diurnal cycle are very weak for the equinox case with symmetric meridional profiles, and eddy momentum fluxes are small for all scenarios considered here.

The Tropical Cyclone Diurnal Cycle of Mature Hurricanes

Abstract The diurnal cycle of tropical convection and the tropical cyclone (TC) cirrus canopy has been described extensively in previous studies. However, a complete understanding of the TC diurnal cycle remains elusive and is an area of ongoing research. This work describes a new technique that uses infrared satellite image differencing to examine the evolution of the TC diurnal cycle for all North Atlantic major hurricanes from 2001 to 2010. The imagery reveals cyclical pulses in the infrared cloud field that regularly propagate radially outward from the storm. These diurnal pulses begin forming in the storm’s inner core near the time of sunset each day and continue to move away from the storm overnight, reaching areas several hundreds of kilometers from the circulation center by the following afternoon. A marked warming of the cloud tops occurs behind this propagating feature and there can be pronounced structural changes to a storm as it moves away from the inner core. This suggests that the TC diurnal cycle may be an important element of TC dynamics and may have relevance to TC structure and intensity change. Evidence is also presented showing the existence of statistically significant diurnal signals in TC wind radii and objective Dvorak satellite-based intensity estimates for the 10-yr hurricane dataset that was examined. Findings indicate that TC diurnal pulses are a distinguishing characteristic of the TC diurnal cycle and the repeatability of TC diurnal pulsing in time and space suggests that it may be an unrealized, yet fundamental TC process.

Modelling the diurnal cycle of tropical convection across the ‘grey zone’

AbstractWe present the results of simulations carried out with the Met Office Unified Model at 12, 4 and 1.5 km resolution for a large region centred on West Africa using several different representations of the convection processes. These span a range of resolutions from much coarser than the size of the convection processes to cloud‐system‐resolving and thus encompass the intermediate ‘grey zone’. The diurnal cycle in the extent of convective regions in the models is tested against observations from the Geostationary Earth Radiation Budget instrument on Meteosat‐8. By this measure, the two best‐performing simulations are a 12 km model without convective parametrization, using Smagorinsky‐style subgrid‐scale mixing in all three dimensions, and a 1.5 km simulation with two‐dimensional Smagorinsky mixing. Of these, the 12 km model produces a better match to the magnitude of the total cloud fraction but the 1.5 km one results in better timing for its peak value. The results suggest that the previously reported improvement in the representation of the diurnal cycle of convective organization in the 4 km model compared with the standard 12 km configuration is principally a result of the convection scheme employed rather than the improved resolution per se. The details of this result and implications for high‐resolution model simulations are discussed.

Improving the diurnal cycle of convection in GCMs

AbstractThe Met Office single‐column model is used to examine the development phase of the diurnal cycle of tropical convection over land by comparing against previous results from an idealized cloud‐resolving model study. Changing the deep convective parametrization over land to make the entrainment vary with the height of the lifting condensation level reduces the depth of the convection early in the day. The changes made to improve the early phase of diurnal cycle over land are tested in an atmosphere‐only version of the Met Office Hadley Centre climate model and result in an improvement in the amplitude and timing of the diurnal peak in precipitation over land. Copyright © 2011 British Crown copyright, the Met Office. Published by John Wiley & Sons Ltd.

The Diurnal Cycle of Convection and Atmospheric Tides in an Aquaplanet GCM

Abstract The diurnal cycle of tropical convection and its relationship to the atmospheric tides is investigated using an aquaplanet GCM. The diurnal and semidiurnal harmonics of precipitation are both found to contribute significantly to the total diurnal variability of precipitation in the model, which is broadly consistent with observations of the diurnal cycle of convection over the open ocean. The semidiurnal tide is found to be the dominant forcing for the semidiurnal harmonic of precipitation. In contrast the diurnal tide plays only a small role in forcing the diurnal harmonic of precipitation, which is dominated by the variations in shortwave and longwave heating. In both the diurnal and semidiurnal harmonics, the feedback onto the convection by the humidity tendencies due to the convection is found to be important in determining the phase of the harmonics. Further experiments show that the diurnal cycle of precipitation is sensitive to the choice of closure in the convection scheme. While the surface pressure signal of the simulated atmospheric tides in the model agree well with both theory and observations in their magnitude and phase, sensitivity experiments suggest that the role of the stratospheric ozone in forcing the semidiurnal tide is much reduced compared to theoretical predictions. Furthermore, the influence of the cloud radiative effects seems small. It is suggested that the radiative heating profile in the troposphere, associated primarily with the water vapor distribution, is more important than previously thought for driving the semidiurnal tide. However, this result may be sensitive to the vertical resolution and extent of the model.

Numerical Experiments Examining the Mechanism of Diurnal Variation of Tropical Convection

A regional atmospheric model was used to clarify the mechanisms that drive the diurnal cycle of tropical convection over the western Pacific Ocean. The diurnal cycle of convection was simulated using data from the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE). Mechanisms suggested by previous investigators were confirmed through comparison experiments. The diurnal variation in precipitation was simulated without the effect of horizontal differences in radiative cooling between cloudy and clear regions. Horizontal differences of radiative cooling are not the main factor driving diurnal variation. Direct radiative cooling, and cloud radiative cooling effects destabilize the atmosphere during the night. Numerical experiments that reduced cloud radiative cooling demonstrated the importance of the cloud radiative cooling effect rather than the direct radiative cooling effect. Moreover, in the standard experiment, strong cloud radiative cooling appeared at both upper levels and at the top of the boundary layer. The experiment demonstrated that both the upper level cloud radiative cooling, and the boundary layer cloud radiative cooling, are important in driving the diurnal variation of convection. Boundary-layer cloud radiative cooling destabilizes the low layer atmosphere and promotes the development of shallow convection to deep convection. Diurnal variation of water vapor also plays a role in driving diurnal variation of convection. Moisture is transported upward from the boundary layer at night when radiative cooling of the boundary-layer cloud is intensified.

The diurnal cycle of tropical convection

The diurnal cycle of tropical convection is investigated with global cloud imagery constructed from 11μm radiance measurements taken aboard six satellites. Four harmonics of the diurnal cycle are resolved in the cloud imagery with about 50 km horizontal resolution. To isolate deep convective activity from other processes which cause diurnal fluctuations in longwave radiance, an index of deep convective activity is constructed by thresholding to brightness temperatures less than 230 K. Significant diurnal amplitude of deep convection is found only over tropical landmasses. Over the tropical oceans the diurnal cycle is weak and is barely discernible from the background red spectrum of convective variance. Nonetheless, oceanic convection exhibits a systematic diurnal fluctuation with maximum intensity in the early morning. Nocturnal subsidence along the cloud‐free equator is postulated to play a role in forcing diurnal variation in the intertropical convergence zones. Other mechanisms are also implied to contribute as a similar early morning maximum in deep convection is seen even where no adjoining cloud‐free regions occur. The diurnal cycle of deep convection is found to be organized on planetary scales predominantly in nonmigrating modes (i.e., modes with phase speeds not equal to that of the Sun). The nonmigrating modes are postulated to be produced by the nonuniform distribution of convective centers. The nonmigrating, zonally symmetric component is found to be especially large. While the role of this mode in forcing diurnal variations of the tropospheric circulation (e.g., the Hadley cell) is questionable, its role in modulating the ionospheric electric potential is well established. Discrepancies with the traditional notion that cloud‐to‐ground lightning is the dominant mechanism by which the Earth is negatively charged are discussed. In particular, the probable role of nonlightning‐producing clouds (e.g., the stratiform portion of convective complexes) in diurnally modulating the negative charge on Earth is discussed.