Cycle Of Solar Heating Research Articles

A Multicloud Model for Coastal Convection

Coastal convection is often organized into multiple mesoscale systems that propagate in either direction across the coastline (i.e., landward and oceanward). These systems interact non-trivially with synoptic and intraseasonal disturbances such as convectively coupled waves and the Madden–Julian oscillation. Despite numerous theoretical and observational efforts to understand coastal convection, global climate models still fail to represent it adequately, mainly because of limitations in spatial resolution and shortcomings in the underlying cumulus parameterization schemes. Here, we use a simplified climate model of intermediate complexity to simulate coastal convection under the influence of the diurnal cycle of solar heating. Convection is parameterized via a stochastic multicloud model (SMCM), which mimics the subgrid dynamics of organized convection due to interactions (through the environment) between the cloud types that characterize organized tropical convection. Numerical results demonstrate that the model is able to capture the key modes of coastal convection variability, such as the diurnal cycle of convection and the accompanying sea and land breeze reversals, the slowly propagating mesoscale convective systems that move from land to ocean and vice-versa, and numerous moisture-coupled gravity wave modes. The physical features of the simulated modes, such as their propagation speeds, the timing of rainfall peaks, the penetration of the sea and land breezes, and how they are affected by the latitudinal variation in the Coriolis force, are generally consistent with existing theoretical and observational studies.

A Century of Southern California Coastal Ocean Temperature Measurements

AbstractScripps Institution of Oceanography Pier has been the site of daily temperature measurements since the original structure was built in 1916. The time series is the longest continuous record of temperature on the Pacific Rim and is frequently cited as an indicator of ocean temperature trends in the region. The over 100‐year long record of daily surface temperature and slightly shorter bottom temperature data are presented here, with adjustment for bias owing to changes in sampling time of day. Data were generally collected near 8:00 a.m. in the early years, when water temperatures are cooler, and gradually shifted later in the day, when temperatures are warmer. In addition, cold surges from irregular internal tidal bores complicated attempts to estimate daily heating‐cooling cycles solely as a function of time of day. To isolate the effect of the solar heating cycle, adjustments were calculated based on seasonally variable diurnal solar heating using a surface ocean heating model and locally measured insolation data. Although the observed warming trends in nearshore temperatures are reduced by about 0.2 °C per century in the adjusted data, they still retain a significant increase of 1.24 °C per century at the surface and 1.67 °C per century in bottom waters.

Obtaining turbulence statistics of thermally driven anabatic flow by sonic-hot-film combo anemometer

Achieving a better understanding of atmospheric boundary layer flows, and of slope flows in particular, is of paramount importance for the research of climate processes and production of accurate weather forecasts. We addressed the need for high resolution statistics that describe the turbulence of thermally driven anabatic (upslope) flows by the implementation of a novel sonic-hot-film anemometer, the combo probe. A field experiment was staged to obtain continuous 8-day measurements of a thermally driven anabatic flow diurnal cycle on a moderate slope by a single combo probe mounted atop a 2 m high mast. Variations of the mean and fluctuating upslope velocity field components and temperature exhibit a strong correlation of the developing flow with the diurnal solar heating cycle. The provided detailed analysis of turbulence statistics includes characteristic length scales and spectra of velocity fluctuations almost up to the Kolmogorov scale. Identification of spectral shape similarity led to the introduction of a suitable normalization and comparison of the results to a theoretical model. Additionally, empirical fits of several parameters are produced and discussed with respect to variations of thermal forcing that were derived in terms of bulk temperature differences and buoyancy fluxes up the slope. The main outcomes are spectra resolved down to small scales and turbulence statistics made available for numerical simulations and future studies with slow instruments.

The rainfall factor in lightning-ignited wildfires in Catalonia

Wildland fires originated by lightning in Catalonia (NE Spain) are systematically examined through the use of lightning and precipitation data. The region of study, with a Mediterranean climate, is prone to summer wildfires. Despite being mainly anthropogenic, lightning-ignited fires (10%) are relevant as they can trigger large fires. Given that the lightning efficiency in Catalonia is of 1/1400 fires per lightning, the ability to identify potential ignition candidates among the whole lightning population would be of great value to forest protection agencies. Literature reveals that lightning characteristics such as polarity or multiplicity has proven to be of limited value as predictors. Therefore, another approach is necessary to set a probability of wildfire ignition to each lightning. In this regard, the aim of this study was to explore the relationship between lightning-ignited wildfires and precipitation, as lightning-ignited wildfires in the region are often attributed to ‘dry’ lightning (lightning accompanied with little or no precipitation). Results revealed that 25% of the lightning strokes related to wildfire ignitions had no associated precipitation at all, 40% had less than 2mm of precipitation and 90% had less than 10mm. Results also revealed that holdover fires (ignitions with delayed arrivals) are rare in the region. Finally, results suggest that there is no apparent link between the amount of precipitation and the holdover duration, indicating that the survival phase of lightning-ignitions is mainly driven by the daily cycle of solar heating. All in all, adding information on the precipitation associated to each lightning may help focusing attention on a reduced sample of strokes and provide fire managers with valuable information about potential lightning-caused wildfires.

Is there a stratospheric pacemaker controlling the daily cycle of tropical rainfall?

AbstractRainfall in the tropics exhibits a large, 12 h Sun‐synchronous variation with coherent phase around the globe. A long‐standing, but unproved, hypothesis for this phenomenon is excitation by the prominent 12 h atmospheric tide, which itself is significantly forced remotely by solar heating of the stratospheric ozone layer. We investigated the relative roles of large‐scale tidal forcing and more local effects in accounting for the 12 h variation of tropical rainfall. A model of the atmosphere run with the diurnal cycle of solar heating artificially suppressed below the stratosphere still simulated a strong coherent 12 h rainfall variation (~50% of control run), demonstrating that stratospherically forced atmospheric tide propagates downward to the troposphere and contributes to the organization of large‐scale convection. The results have implications for theories of excitation of tropical atmospheric waves by moist convection, for the evaluation of climate models, and for explaining the recently discovered lunar tidal rainfall cycle.

Diurnal Variations of Albedo Retrieved from Earth Radiation Budget Experiment Measurements

AbstractFive years of measurements from the Earth Radiation Budget Satellite (ERBS) have been analyzed to define the diurnal cycle of albedo from 55°N to 55°S. The ERBS precesses through all local times every 72 days so as to provide data regarding the diurnal cycles for Earth radiation. Albedo together with insolation at the top of the atmosphere is used to compute the heating of the Earth–atmosphere system; thus its diurnal cycle is important in the energetics of the climate system. A principal component (PC) analysis of the diurnal variation of top-of-atmosphere albedo using these data is presented. The analysis is done separately for ocean and land because of the marked differences of cloud behavior over ocean and over land. For ocean, 90%–92% of the variance in the diurnal cycle is described by a single component; for land, the first PC accounts for 83%–89% of the variance. Some of the variation is due to the increase of albedo with increasing solar zenith angle, which is taken into account in the ERBS data processing by a directional model, and some is due to the diurnal cycle of cloudiness. The second PC describes 2%–4% of the variance for ocean and 5% for land, and it is primarily due to variations of cloudiness throughout the day, which are asymmetric about noon. These terms show the response of the atmosphere to the cycle of solar heating. The third PC for ocean is a two-peaked curve, and the associated map shows high values in cloudy regions.

Simple Multicloud Models for the Diurnal Cycle of Tropical Precipitation. Part I: Formulation and the Case of the Tropical Oceans

Abstract The variation of tropical precipitation due to the diurnal cycle of solar heating is examined here in the context of two simple models for tropical convection. The models utilize three cloud types—congestus, deep, and stratiform—that are believed to characterize organized tropical convection and are based on the two first baroclinic modes of vertical structure plus a boundary layer mode. The two models differ mainly in the way they treat the boundary layer dynamics. The first one is purely thermodynamical and is reduced to a single equation for the equivalent potential temperature θe connecting the boundary layer to the upper troposphere through downdrafts and to the surface through evaporation while the second uses full bulk boundary layer (FBBL) dynamics with a careful separation between sensible and latent heat fluxes and parameterization of nonprecipitating shallow cumulus. It turns out that in the case of the precipitation over the ocean where the Bowen ratio is small, both models yield a qualitatively similar solution, characterized by an overnight initiation and early morning peak in precipitation consistent with observations. The modeled diurnal cycle of precipitation over the ocean is divided into four cyclic phases: 1) a CAPE (re)generation phase characterized by the enhancement of the boundary layer θe and moisture fluxes during midday and early afternoon that is followed by 2) a (re)moistening phase dominated by congestus heating during the late afternoon and moistening from downdrafts (due to detrainment of shallow cumulus, specifically in the FBBL model) and radiative cooling that lasts until midnight. 3) Deep convection is initiated around midnight when the midtroposphere is sufficiently moist and cool and (re)establishes the precipitation level near its radiative convective equilibrium (1 K day−1) and then 4) peaks with sunrise at 0600 LST to yield a precipitation maximum of roughly 2 K day−1 at around 0900 LST that dries the troposphere and consumes CAPE and closes the cycle.

Simple Multicloud Models for the Diurnal Cycle of Tropical Precipitation. Part II: The Continental Regime

Abstract The variation of precipitation over land due to the diurnal cycle of solar heating is examined here in the context of a simple multicloud model for tropical convection with bulk atmospheric boundary layer (ABL) dynamics. The model utilizes three cloud types (congestus, deep, and stratiform) that are believed to characterize organized tropical convection based on the first two baroclinic modes of vertical structure in the free troposphere, coupled to the ABL through full bulk boundary layer (FBBL) dynamics, that allow a careful separation between sensible and latent heat surface fluxes. In a land parameter regime, characterized by a strong inversion profile, a large Bowen ratio of 0.4, and active mixing of sensible heat due to cumulus entrainment and downdraft fluxes at the top of the ABL, the model supports a stable 1-day periodic solution that is characterized by a pronounced (7 K day−1) afternoon peak in precipitation consistent with observations of tropical precipitation over continental regions. The current study suggests a division of the diurnal cycle of precipitation over land into a cycle of five phases: 1) an overnight phase of a radiative–convective equilibrium (RCE) state between 2000 and 0600 LST; 2) an early morning CAPE buildup accompanied by a sudden rise in precipitation that quickly dries the middle troposphere occurs between 0600 and roughly 1000 LST; 3) a moistening phase between roughly 1000 and 1600 LST; 4) a phase of maximum precipitation between 1600 and 1800 LST that dries the middle troposphere and quickly consumes CAPE; and 5) a rapid remoistening phase that restores the moisture level to sustain the overnight RCE precipitation and connects to phase 1 in the cycle. Sensitivity tests in the model confirm that the late afternoon precipitation maximum over land depends crucially on a strong inversion, the large Bowen ratio, and the active mixing of sensible heat due to cumulus entrainment and downdraft fluxes at the top of the ABL.

Diurnal variation of outgoing longwave radiation associated with high cloud and UTH changes from Meteosat-5 measurements

Diurnal variations of outgoing longwave radiation (OLR) are examined in conjunction with diurnal variations of high cloud and upper tropospheric humidity (UTH) over the Indian Ocean and surrounding land areas using Meteosat-5 measurements. Most land areas exhibit a significant diurnal variation of OLR with the largest amplitude over the Arabian Peninsula, whereas the diurnal variation of OLR is much weaker over the Indian Ocean. While diurnal maxima of OLR are found in the early afternoon over many regions of the analysis domain following the diurnal cycle of solar heating, convectively active regions of both land and ocean where high cloud and UTH exhibit distinct diurnal variations show OLR maxima before local noon. These results indicate that high cloud development in the afternoon induces a shift in local time of OLR maxima over convective regions. In agreement with earlier studies it is shown that UTH diurnal variations are less important in regard to their impact on the OLR variations.

Orographic Precipitation in the Tropics: Experiments in Dominica

Abstract The “natural laboratory” of mountainous Dominica (15°N) in the trade wind belt is used to study the physics of tropical orographic precipitation in its purest form, unforced by weather disturbances or by the diurnal cycle of solar heating. A cross-island line of rain gauges and 5-min radar scans from Guadeloupe reveal a large annual precipitation at high elevation (7 m yr−1) and a large orographic enhancement factor (2 to 8) caused primarily by repetitive convective triggering over the windward slope. The triggering is caused by terrain-forced lifting of the conditionally unstable trade wind cloud layer. Ambient humidity fluctuations associated with open-ocean convection may play a key role. The convection transports moisture upward and causes frequent brief showers on the hilltops. The drying ratio of the full air column from precipitation is less than 1% whereas the surface air dries by about 17% from the east coast to the mountain top. On the lee side, a plunging trade wind inversion and reduced instability destroys convective clouds and creates an oceanic rain shadow.

Convective Storm Initiation in a Moist Tropical Environment

Abstract Radar and satellite data from the Tropical Rainfall Measuring Mission–Large-Scale Biosphere–Atmosphere (TRMM–LBA) project have been examined to determine causes for convective storm initiation in the southwest Amazon region. The locations and times of storm initiation were based on the National Center for Atmospheric Research (NCAR) S-band dual-polarization Doppler radar (S-Pol). Both the radar and the Geostationary Operational Environmental Satellite-8 (GOES-8) visible data were used to identify cold pools produced by convective precipitation. These data along with high-resolution topographic data were used to determine possible convective storm triggering mechanisms. The terrain elevation varied from 100 to 600 m. Tropical forests cover the area with numerous clear-cut areas used for cattle grazing and farming. This paper presents the results from 5 February 1999. A total of 315 storms were initiated within 130 km of the S-Pol radar. This day was classified as a weak monsoon regime where convection developed in response to the diurnal cycle of solar heating. Scattered shallow cumulus during the morning developed into deep convection by early afternoon. Storm initiation began about 1100 LST and peaked around 1500–1600 LST. The causes of storm initiation were classified into four categories. The most common initiation mechanism was caused by forced lifting by a gust front (GF; 36%). Forcing by terrain (>300 m) without any other triggering mechanism accounted for 21% of the initiations and colliding GFs accounted for 16%. For the remaining 27% a triggering mechanism was not identified. Examination of all days during TRMM–LBA showed that this one detailed study day was representative of many days. A conceptual model of storm initiation and evolution is presented. The results of this study should have implications for other locations when synoptic-scale forcing mechanisms are at a minimum. These results should also have implications for very short-period forecasting techniques in any location where terrain, GFs, and colliding boundaries influence storm evolution.

Climate dynamics along the arid northern coast of Chile: The 1997–1998 Dinámica del Clima de la Región de Antofagasta (DICLIMA) experiment

The DICLIMA field experiment was designed to test and quantify the hypothesis of an afternoon enhancement of the coastal subsidence in the extremely arid northern Chile because of solar heating over the west slope of the Andes. Ten‐day campaigns near Antofagasta (23°S) were carried out in January 1997, July 1997, and January 1998. Significant diurnal cycles in temperature, mixing ratio, and wind from about 1000 to 4000 m above sea level were observed. This layer was decoupled from the marine boundary layer circulation below by the subsidence inversion when its base was under the average height of the coastal mountain range. The solar heating cycle over the Andes and associated circulation resulted in a mean afternoon zonal divergence above the subsidence inversion base of about 30 × 10−6 s−1, exceeding by a factor of 5 typical subtropical west coast divergences. The corresponding early morning convergence was particularly intense during the austral winter experiment. In spite of the very strong El Niño conditions that prevailed during the July 1997 and January 1998 experiments, the overwhelming control that radiation exerts on the daily cycles of the atmospheric circulation over the west slope of the Andes seems to guarantee the general validity of the results.

Cloud-to-ground lightning characteristics in the Spanish Basque Country area during the period 1992–1996

This paper summarises the results corresponding to the first study of lightning phenomenology in the Spanish Basque Country area where a maximum value for lightning density in Spain was observed. Data used in this study comes from the DF network deployed by the Spanish Meteorological Service, integrating 14 observation points in Spain. Results regarding lightning density confirms the existence of a relative maximum in cloud-to-ground (CG) counts located over Bilbao, the largest city in the area. This maximum represents an annual average of 3 flashes/km 2 . However, for the rest of the area, a random pattern in CG counts discharge is observed. A high variability in the annual counts can be seen, this number is reaching a factor of 2 if we compare two different years. This variability also affects the number of counts observed in 1 month in different years and for example, counts can change from 0 to more than 10,000 in a single month in different years. We can also report that positive lightning flashes that present an intensity double than that of negatives strokes follow a seasonal variation in their relative percentage that evolve from 39% in winter to 5% in summer, similar to what has been observed by other researchers. Finally, we found that the lightning activity observed in summer and spring relates to the solar heating cycle and consequently, electrical storms occur mainly in the afternoon.

Group method for temperature analysis of thermally stagnant lakes

The transformation group theoretic approach is applied to present an analysis for the problem of time dependent vertical temperature distribution in a stagnant lake during the yearly cycle of solar heating and cooling. The application of a one-parameter group reduces the number of independent variables by one, and consequently the governing partial differential equation with the boundary and initial conditions to an ordinary differential equation with the appropriate corresponding conditions. The obtained differential equation is solved, for some special forms of the water parameters, analytically, whenever possible, and in some other cases numerically using the shooting technique. The temperature distribution across the lake is plotted against the lake depth.

A Numerical Investigation of Cloud Diurnal Variations

Abstract The large-scale diurnal variation of cloud cover is derived from diurnal variations of temperature, density, water content, and static stability in a linearized calculation. Forced by the diurnal cycle of solar heating, the calculated cloud distribution is broadly consistent with observed diurnal variations under maritime nonconvective, maritime convective, and continental convective conditions. The calculated diurnal variation of low-cloud fraction follows primarily from the diurnal variation of temperature, which creates a diurnal variation of saturation vapor pressure. The calculated diurnal amplitude of low-cloud fraction is large under maritime nonconvective conditions, in which a well-mixed boundary layer promotes the transition between cloudy and clear conditions. The amplitude is further enhanced under continental conditions by the diurnal variation of vertical heat transport from the surface. The diurnal variation of high-cloud fraction under continental conditions follows primarily from...

Empirical orthogonal function analysis of sea surface temperature patterns in Delaware Bay

Empirical orthogonal functions (EOFs) are used to study a time-series of IR images from the Delaware Bay region, to determine the dominant patterns of sea surface temperature (SST) variance. Thirty-two months of AVHRR imagery, from February 1992 to September 1994, were used in the study. The SST images were averaged into monthly means for the analysis. EOF analysis decomposed the time series into its component parts. For Delaware Bay, the seasonal solar heating cycle dominated the results, accounting for 95.3% of the total variance. Higher modes in this area are interpreted as the result of spring heating and fall cooling of shallow river and bay waters ahead of offshore waters. The EOF results provide independent satellite derived evidence for lateral variability in the circulation of the estuary, which had previously been found in in-situ studies. The lateral variability in the bay circulation is a result of Delaware Bay's drowned river valley bathymetry.

Large‐eddy simulation of the equatorial ocean boundary layer: Diurnal cycling, eddy viscosity, and horizontal rotation

Oceanic boundary layers (OBL) at the equator are studied using a large‐eddy simulation (LES) model. The model has no equatorial undercurrent, and the vertical component of Earth's rotation is zero Ωz = 0. It is forced with a constant westward zonal wind stress and with a constant surface heat flux, and, in some cases, a diurnal cycle of solar heating. Averaged over day 4, temperature and velocity are vertically mixed deeper for the diurnal cycling case, the surface temperature is cooler by 0.04°C, and the surface velocity is weaker by 0.08 m/s, compared to a constant surface heating case with the same daily averaged surface heat flux. There is no general relationship between eddy viscosity (or diffusivity) and the gradient Richardson number, especially for the case of diurnal cycling, where nonlocal transports can lead to countergradient fluxes during nighttime convection. Sharp gradients of eddy viscosity are found near the critical Richardson number, Ri = 0.25, and in the lower part of the entrainment layer, where Ri ≈ 1.0. The effect of the horizontal component of Earth's rotation Ωy on an entraining equatorial OBL is also investigated. It is found in our entraining cases that with and without Ωy, mean quantities (temperature, velocity) and turbulent fluxes differ no more than a few percent, in accord with the second‐order closure results of Galperin et al. [1989] and Kantha et al. [1989]. The LES results are used to extend the bulk mixed layer theory of Garwood et al. [1985a, b] to the entraining case. With no entrainment, Garwood et al. [1985a, b] show that Ωy effects can be substantial. The effects of Ωy are small when the entrainment heat flux is significant.

A modelling investigation of the distribution of stratification and phytoplankton abundance in the Irish Sea

Thermal stratification and phytoplankton abundance are modelled on a 5 km grid covering the Irish Sea. The water column is approximated by three layers. The top layer is uniformly mixed by wind stirring and the bottom by tidal energy, while linear gradients can occur in the middle layer. The model is forced with hourly meteorological data and mean tidal energies. Primary production is represented by a model with a single nutrient and a single phytoplankton population. The results from the model show good agreement with data collected on a Ministry of Agriculture, Fisheries and Food (MAFF) cruise in May 1992 and with historical data. When advection is included, driven by depth-averaged currents, the surface temperature patterns are improved but bottom temperatures in deep water are raised and high concentrations of chlorophyll are carried offshore from coastal regions. This indicates a limitation of using depth-averaged currents and a need to account for differences in phytoplankton species composition in coastal and offshore waters. Calculations demonstrate the importance of salinity variations to stratification and phytoplankton growth. Smoothing the wind mixing energy has the effect of delaying the onset of the spring bloom in areas where wind mixing is significant. Removing the diurnal cycle of solar heating also delays the spring bloom. The chlorophyll gradient in the middle layer has a large impact on the response of the model to short-term variability in the meteorological forcings.

Regular and irregular baroclinic waves in a martian general circulation model: A role for diurnal forcing?

The development and evolution of baroclinic transient waves are simulated in a high resolution, general circulation model (GCM) of the Martian atmosphere, including simulations both with and without a representation of the diurnal cycle of solar heating. Simulations with diurnally-averaged insolation are found to develop highly regular baroclinic transients during northern hemisphere autumn, winter and spring seasons, characterized by zonal wavenumber 1 or 2 and periods of 2–6 sols (a sol being a Martian day). With a diurnal cycle, however, transients are found to be significantly more irregular throughout the winter season, sporadically changing their dominant wavenumber and frequency, and resemble more closely data from the Viking Lander time series. These results are discussed with reference to some simple conceptual models in nonlinear dynamics, in terms either of a periodically-perturbed, self-exciting oscillator, or of attractor-merging near a crisis in a periodically perturbed bistable system.

Mixing in the equatorial surface layer and thermocline

Twelve days of microstructure measurements at the equator (140°W) in November 1984 showed a surprisingly strong effect of both the daily cycle of solar heating and wind on mixing in the upper ocean. Because of limited variations in atmospheric forcing and currents during the experiment, processes in the daily mixing cycle were similar from day to day. Only the intensity of mixing varied. The lower boundary of the diurnal surface layer separated two distinct mixing regimes, the diurnal surface layer and the thermocline. Within the diurnal surface layer (which extended to 10‐ to 35‐m depth), turbulent kinetic energy dissipation rates ε varied relatively little. Although variations in surface layer depth coincided with the daily change in direction of air‐sea surface buoyancy production of turbulent kinetic energy (or simply, the surface buoyancy flux), ε was significantly greater relative to the buoyancy flux than was expected for a simple convective layer. In the thermocline below the diurnal surface layer, ε was highly intermittent; the day‐night cycle was stronger, and variability was enhanced by turbulent “bursts” of 2–3 hours duration, which may be related to internal wave breaking events. The turbulent heat flux crossing 20‐m depth was almost equal to the surface heat flux less the irradiance penetrating below 20 m. Seventy percent of the surface heat flux was transported vertically to the water below 30 m by turbulent mixing. Only a negligible amount penetrated to the core of the Equatorial Undercurrent. The gradient Richardson number Ri distinguishes between statistically different mixing environments. However, ε cannot be predicted from the value of Ri, since the intensity of mixing depends on the intensity of forcing in a way not specified by the value of Ri alone.