scholarly journals Diurnal Circulation Adjustment and Organized Deep Convection

2018 ◽  
Vol 31 (12) ◽  
pp. 4899-4916 ◽  
Author(s):  
James H. Ruppert ◽  
Cathy Hohenegger
Keyword(s):  
Diurnal Cycle ◽  
Large Scale ◽  
Spatial Scales ◽  
Deep Convection ◽  
Early Morning ◽  
Hadley Cell ◽  
Lapse Rate ◽  
Radiation Mechanism ◽  
Rate Mechanism ◽  
Organized Convection

This study investigates the diurnal cycle of tropical organized deep convection and the feedback in large-scale circulation. By considering gravity wave phase speeds, we find that the circulation adjustment into weak temperature gradient (WTG) balance occurs rapidly (<6 h) relative to diurnal diabatic forcing on the spatial scales typical of organized convection (≤500 km). Convection-permitting numerical simulations of self-aggregation in diurnal radiative–convective equilibrium (RCE) are conducted to explore this further. These simulations depict a pronounced diurnal cycle of circulation linked to organized convection, which indeed maintains WTG balance to first order. A set of sensitivity experiments is conducted to assess what governs the diurnal cycle of organized convection. We find that the “direct radiation–convection interaction” (or lapse-rate) mechanism is of primary importance for diurnal precipitation range, while the “dynamic cloudy–clear differential radiation” mechanism amplifies the range by approximately 30%, and delays the nocturnal precipitation peak by around 5 h. The differential radiation mechanism therefore explains the tendency for tropical heavy rainfall to peak in the early morning, while the lapse-rate mechanism primarily governs diurnal amplitude. The diurnal evolution of circulation can be understood as follows. While nocturnal deep convection invigorated by cloud-top cooling (i.e., the lapse-rate mechanism) leads to strong bottom-heavy circulation at nighttime, the localized (i.e., differential) top-heavy shortwave warming in the convective region invigorates circulation at upper levels in daytime. A diurnal evolution of the circulation therefore arises, from bottom heavy at nighttime to top heavy in daytime, in a qualitatively consistent manner with the observed diurnal pulsing of the Hadley cell driven by the ITCZ.

Diurnal Cycle of Shallow and Deep Convection for a Tropical Land and an Ocean Environment and Its Relationship to Synoptic Wind Regimes

2006 ◽  
Vol 134 (10) ◽  
pp. 2688-2701 ◽  
Author(s):  
L. Gustavo Pereira ◽  
Steven A. Rutledge
Keyword(s):  
Diurnal Cycle ◽  
Large Scale ◽  
Field Experiments ◽  
Deep Convection ◽  
Shallow Convection ◽  
Wind Regime ◽  
Diurnal Variability ◽  
Synoptic Wind ◽  
Wind Regimes ◽  
Rain Rates

Abstract The characteristics of shallow and deep convection during the Tropical Rainfall Measuring Mission/Large-Scale Biosphere–Atmosphere Experiment in Amazonia (TRMM/LBA) and the Eastern Pacific Investigation of Climate Processes in the Coupled Ocean–Atmosphere System (EPIC) are evaluated in this study. Using high-quality radar data collected during these two tropical field experiments, the reflectivity profiles, rain rates, fraction of convective area, and fraction of rainfall volume in each region are examined. This study focuses on the diurnal cycle of shallow and deep convection for the identified wind regimes in both regions. The easterly phase in TRMM/LBA and the northerly wind regime in EPIC were associated with the strongest convection, indicated by larger rain rates, higher reflectivities, and deeper convective cores compared to the westerly phase in TRMM/LBA and the southerly regime in EPIC. The diurnal cycle results indicated that convection initiates in the morning and peaks in the afternoon during TRMM/LBA, whereas in the east Pacific the diurnal cycle of convection is very dependent on the wind regime. Deep convection in the northerly regime peaks around midnight, nearly 6 h before its southerly regime counterpart. Moreover, the northerly regime of EPIC was dominated by convective rainfall, whereas the southerly regime was dominated by stratiform rainfall. The diurnal variability was more pronounced during TRMM/LBA than in EPIC. Shallow convection was associated with 10% and 3% of precipitation during TRMM/LBA and EPIC, respectively.

scholarly journals Memory Properties in Cloud‐Resolving Simulations of the Diurnal Cycle of Deep Convection.

2021 ◽  
Author(s):  
Chimene Laure Daleu
Keyword(s):  
Diurnal Cycle ◽  
Spatial Scales ◽  
Memory Function ◽  
Deep Convection ◽  
Lower Troposphere ◽  
Lag Time ◽  
Second Phase ◽  
Rainfall Events ◽  
Sensitivity Experiments ◽  
Third Phase

&lt;p&gt;A series of high&amp;#8208;resolution three&amp;#8208;dimensional simulations of the diurnal cycle of deep convection over land are performed using the new Met Office NERC cloud&amp;#8208;resolving model. This study features scattered convection. A memory function is defined to identify the effects of previous convection in modifying current convection. It is based on the probability of finding rain at time &lt;em&gt;t&lt;/em&gt;&lt;sub&gt;0&lt;/sub&gt; and at an earlier time &lt;em&gt;t&lt;/em&gt;&lt;sub&gt;0&lt;/sub&gt;&amp;#8722;&amp;#916;&lt;em&gt;t&lt;/em&gt; compared to the expected probability given no memory. The memory is examined as a function of the lag time &amp;#916;&lt;em&gt;t&lt;/em&gt;. It is strongest at gray&amp;#8208;zone scales of 4&amp;#8211;10&amp;#8201;km, there is a change of behavior for spatial scales between 10 and 15&amp;#8201;km, and it is reduced substantially for spatial scales larger than 25&amp;#8201;km. At gray&amp;#8208;zone scales, there is a first phase of the memory function which represents the persistence of convection and it is maintained for about an hour. There is a second phase which represents the suppression of convection in regions which were raining 1 to 3&amp;#8201;hr previously, and subsequently a third phase which represents a secondary enhancement of precipitation. The second and third phases of the memory function develop earlier for weaker forcing. When thermodynamic fluctuations resulting from the previous day are allowed to influence the development of convection on the next day, there are fewer rainfall events with relatively large sizes, which are more intense, and thus decay and recover more slowly, in comparison to the simulations where feedback from previous days is removed. Further sensitivity experiments reveal that convective memory attributed to these thermodynamic fluctuations resides in the lower troposphere.&lt;/p&gt;

scholarly journals The power distribution between symmetric and anti-symmetric components of the tropical wavenumber-frequency spectrum

2021 ◽  
Author(s):  
Ofer Shamir ◽  
Chen Schwartz ◽  
Chaim I. Garfinkel ◽  
Nathan Paldor
Keyword(s):  
Frequency Spectrum ◽  
Power Distribution ◽  
Large Scale ◽  
Spectral Power ◽  
Spatial Scales ◽  
Deep Convection ◽  
Heat Fluxes ◽  
Small Scale ◽  
Ample Evidence ◽  
Parity Distribution

AbstractA yet unexplained feature of the tropical wavenumber-frequency spectrum is its parity distributions, i.e., the distribution of power between the meridionally symmetric and anti-symmetric components of the spectrum. Due to the linearity of the decomposition to symmetric and anti-symmetric components and the Fourier analysis, the total spectral power equals the sum of the power contained in each of these two components. However, the spectral power need not be evenly distributed between the two components. Satellite observations and reanalysis data provide ample evidence that the parity distribution of the tropical wavenumber-frequency spectrum is biased towards its symmetric component. Using an intermediate-complexity model of an idealized moist atmosphere, we find that the parity distribution of the tropical spectrum is nearly insensitive to large-scale forcing, including topography, ocean heat fluxes, and land-sea contrast. On the other hand, we find that a small-scale (stochastic) forcing has the capacity to affect the parity distribution at large spatial scales via an upscale (inverse) turbulent energy cascade. These results are qualitatively explained by considering the effects of triad interactions on the parity distribution. According to the proposed mechanism, any bias in the small-scale forcing, symmetric or anti-symmetric, leads to symmetric bias in the large-scale spectrum regardless of the source of variability responsible for the onset of the asymmetry. As this process is also associated with the generation of large-scale features in the Tropics by small-scale convection, the present study demonstrates that the physical process associated with deep-convection leads to a symmetric bias in the tropical spectrum.

Entangled impacts of large-scale monsoon flows and terrain circulations on the diurnal cycle of rainfall over the Himalayas

2021 ◽  
Keyword(s):  
High Altitude ◽  
Diurnal Cycle ◽  
Bay Of Bengal ◽  
Large Scale ◽  
Early Morning ◽  
Atmospheric Layer ◽  
Rainfall Total ◽  
Monsoon Flow ◽  
Downslope Flow

Abstract The diurnal features of rainfall over the Himalayas have been widely investigated, but their triggers remain unclear. In this work, we divided the Himalayas and surroundings into four regions, including the plains, foothills, slopes, and plateau, and investigated the above issues. The results show that the rainfall total is controlled by large-scale monsoon flows while its meridional distribution is regulated by terrain circulations. The afternoon rainfall peak in the plains and foothills is linked with the intersection of two monsoon flows. The southward-shifting rainfall peak, which occurs from midnight to early morning in the slopes and foothills, is affected by the nighttime downslope flow and the strong Bay of Bengal monsoon flow in the morning. The evening rainfall peak in the plateau and high-altitude slopes is thought to be a result of the atmospheric layer being at its moistest at that time.

Formation of Nocturnal Offshore Rainfall near the West Coast of Sumatra: Land Breeze or Gravity Wave?

2021 ◽  
Vol 149 (3) ◽  
pp. 715-731
Author(s):  
Hedanqiu Bai ◽  
Gumilang Deranadyan ◽  
Courtney Schumacher ◽  
Aaron Funk ◽  
Craig Epifanio ◽  
...  
Keyword(s):  
West Coast ◽  
Large Scale ◽  
Deep Convection ◽  
Propagation Speed ◽  
Early Morning ◽  
Land Breeze ◽  
Maritime Continent ◽  
The West ◽  
Rainfall Events ◽  
Low Level

AbstractAfternoon deep convection over the Maritime Continent islands propagates offshore in the evening to early morning hours, leading to a nocturnal rainfall maximum over the nearby ocean. This work investigates the formation of the seaward precipitation migration off western Sumatra and its intraseasonal and seasonal characteristics using BMKG C-band radar observations from Padang and ERA5 reanalysis. A total of 117 nocturnal offshore rainfall events were identified in 2018, with an average propagation speed of 4.5 m s−1 within 180 km of Sumatra. Most offshore propagation events occur when the Madden–Julian oscillation (MJO) is either weak (real-time multivariate MJO index < 1) or active over the Indian Ocean (phases 1–3), whereas very few occur when the MJO is active over the Maritime Continent and western Pacific Ocean (phases 4–6). The occurrence of offshore rainfall events also varies on the basis of the seasonal evolution of the large-scale circulation associated with the Asian–Australian monsoons, with fewer events during the monsoon seasons of December–February and June–August and more during the transition seasons of March–May and September–November. Low-level convergence, resulting from the interaction of the land breeze and background low-level westerlies, is found to be the primary driver for producing offshore convective rain propagation from the west coast of Sumatra. Stratiform rain propagation speeds are further increased by upper-level easterlies, which explains the faster migration speed of high reflective clouds observed by satellite. However, temperature anomalies associated with daytime convective latent heating over Sumatra indicate that gravity waves may also modulate the offshore environment to be conducive to seaward convection migration.

scholarly journals Large-Scale Dynamical Response to Subgrid-Scale Organization Provided by Cellular Automata

2011 ◽  
Vol 68 (12) ◽  
pp. 3132-3144 ◽  
Author(s):  
Lisa Bengtsson ◽  
Heiner Körnich ◽  
Erland Källén ◽  
Gunilla Svensson
Keyword(s):  
Large Scale ◽  
Spatial Scales ◽  
Deep Convection ◽  
Weather Prediction ◽  
Energy Spectra ◽  
Dynamical Response ◽  
Subgrid Scale ◽  
Physical Processes ◽  
Stochastic Representation ◽  
Equatorial Wave

Abstract Because of the limited resolution of numerical weather prediction (NWP) models, subgrid-scale physical processes are parameterized and represented by gridbox means. However, some physical processes are better represented by a mean and its variance; a typical example is deep convection, with scales varying from individual updrafts to organized mesoscale systems. This study investigates, in an idealized setting, whether a cellular automaton (CA) can be used to enhance subgrid-scale organization by forming clusters representative of the convective scales and thus yield a stochastic representation of subgrid-scale variability. The authors study the transfer of energy from the convective to the larger atmospheric scales through nonlinear wave interactions. This is done using a shallow water (SW) model initialized with equatorial wave modes. By letting a CA act on a finer resolution than that of the SW model, it can be expected to mimic the effect of, for instance, gravity wave propagation on convective organization. Employing the CA scheme permits the reproduction of the observed behavior of slowing down equatorial Kelvin modes in convectively active regions, while random perturbations fail to feed back on the large-scale flow. The analysis of kinetic energy spectra demonstrates that the CA subgrid scheme introduces energy backscatter from the smallest model scales to medium scales. However, the amount of energy backscattered depends almost solely on the memory time scale introduced to the subgrid scheme, whereas any variation in spatial scales generated does not influence the energy spectra markedly.

scholarly journals Boundary Layer Quasi-Equilibrium Limits Convective Intensity Enhancement from the Diurnal Cycle in Surface Heating

2019 ◽  
Vol 77 (1) ◽  
pp. 217-237
Author(s):  
Zachary R. Hansen ◽  
Larissa E. Back ◽  
Peigen Zhou
Keyword(s):  
Boundary Layer ◽  
Diurnal Cycle ◽  
Large Scale ◽  
Deep Convection ◽  
Reanalysis Data ◽  
Surface Fluxes ◽  
Quasi Equilibrium ◽  
The Impact ◽  
Precipitation Enhancement ◽  
High Percentile

Abstract A combination of cloud-permitting model (CPM) simulations, satellite, and reanalysis data are used to test whether the diurnal cycle in surface temperature has a significant impact on the intensity of deep convection as measured by high-percentile updraft velocities, lightning, and CAPE. The land–ocean contrast in lightning activity shows that convective intensity varies between land and ocean independently from convective quantity. Thus, a mechanism that explains the land–ocean contrast must be able to do so even after controlling for precipitation variations. Motivated by the land–ocean contrast, we use idealized CPM simulations to test the impact of the diurnal cycle on high-percentile updrafts. In simulations, updrafts are somewhat enhanced due to large-scale precipitation enhancement by the diurnal cycle. To control for large-scale precipitation, we use statistical sampling techniques. After controlling for precipitation enhancement, the diurnal cycle does not affect convective intensities. To explain why sampled updrafts are not enhanced, we note that CAPE is also not increased, likely due to boundary layer quasi equilibrium (BLQE) occurring over our land area. Analysis of BLQE in terms of net positive and negative mass flux finds that boundary layer entrainment, and even more importantly downdrafts, account for most of the moist static energy (MSE) sink that is balancing surface fluxes. Using ERA-Interim data, we also find qualitative evidence for BLQE over land in the real world, as high percentiles of CAPE are not greater over land than over ocean.

scholarly journals What Controls the Transition from Shallow to Deep Convection?

2009 ◽  
Vol 66 (6) ◽  
pp. 1793-1806 ◽  
Author(s):  
Chien-Ming Wu ◽  
Bjorn Stevens ◽  
Akio Arakawa
Keyword(s):  
Large Scale ◽  
Control Mechanism ◽  
Deep Convection ◽  
Potential Temperature ◽  
Intrinsic Property ◽  
Lapse Rate ◽  
Control Parameters ◽  
System A ◽  
Virtual Potential Temperature ◽  
Shallow Clouds

Abstract In this study, a 2D cloud-system-resolving model (CSRM) is used to assess the control mechanism for the transition from shallow to deep convection in the diurnal cycle over land. By comparing with a 3D CSRM under conditions taken from the Large-Scale Biosphere–Atmosphere field study (in the Amazon), the authors show that the 2D CSRM reproduces the main features evident in previous 3D simulations reasonably well. To extract the essence of the transition from shallow to deep convection, the observed case is idealized to isolate two control parameters, the free troposphere stability and the relative humidity. The emergence of a distinct transition between shallow and deep convection shows that the convective transition is an intrinsic property of the system. A transition time is defined to evaluate the key mechanism of the transition. The authors show that the transition coincides with the time when the lapse rate of the virtual potential temperature of the clouds becomes larger than that of the environment, suggesting that the transition happens when shallow clouds become, on average, buoyant. This suggests that, given the opportunity, convection prefers to be shallow.

scholarly journals Physical processes controlling the diurnal cycle of convective storms in the Western Ghats

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
U. V. Murali Krishna ◽  
Subrata Kumar Das ◽  
Sachin M. Deshpande ◽  
G. Pandithurai
Keyword(s):  
Diurnal Variation ◽  
Diurnal Cycle ◽  
Western Ghats ◽  
Large Scale ◽  
Monsoon Season ◽  
Surface Wind ◽  
Bimodal Distribution ◽  
Early Morning ◽  
Coastal Areas ◽  
Convective Storms

AbstractDiurnal variation of convective storms (CSs) during monsoon season and associated physical mechanisms are significantly important for accurate forecast of short-time and extreme precipitation. The diurnal cycle of CSs is investigated using ground-based X-band radar, Tropical Rainfall Measuring Mission Precipitation Radar, and reanalysis data during the summer monsoon (June–September of 2014) over complex mountain terrain of Western Ghats, India. Diurnally, CSs show a bimodal distribution in the coastal areas, but this bimodality became weak along the upslope regions and on the mountain top. The first occurrence mode of CSs is in the afternoon–evening hours, while the second peak is in the early-morning hours. The diurnal cycle’s intensity varies with location, such that it reaches maximum in the afternoon–evening hours and early morning on the mountain top and coastal areas, respectively. Two possible mechanisms are proposed for the observed diurnal variation in CSs (a) the radiative cooling effect and (b) the surface wind convergence induced by the interaction between land-sea breeze, local topography and large-scale monsoon winds. It is also observed that the CSs developed on the mountain top during afternoon–evening hours are deeper than those along the coast. The higher moisture in the lower- and mid-troposphere, higher instability and strong upward motion facilitate deeper CSs during afternoon–evening hours.

scholarly journals Diurnal Cycle of Summer Precipitation over Subtropical East Asia in CAM5

2013 ◽  
Vol 26 (10) ◽  
pp. 3159-3172 ◽  
Author(s):  
Weihua Yuan ◽  
Rucong Yu ◽  
Minghua Zhang ◽  
Wuyin Lin ◽  
Jian Li ◽  
...  
Keyword(s):  
East Asia ◽  
Diurnal Cycle ◽  
Large Scale ◽  
Early Morning ◽  
Convective Precipitation ◽  
Total Rainfall ◽  
Low Level ◽  
Resolution Model ◽  
Diurnal Rainfall ◽  
Stratiform Rainfall

Abstract The simulations of summertime diurnal cycle of precipitation and low-level winds by the Community Atmosphere Model, version 5, are evaluated over subtropical East Asia. The evaluation reveals the physical cause of the observed diurnal rainfall variation in East Asia and points to the source of model strengths and weaknesses. Two model versions with horizontal resolutions of 2.8° and 0.5° are used. The models can reproduce the diurnal phase of large-scale winds over East Asia, with an enhanced low-level southwesterly in early morning. Correspondingly, models successfully simulated the diurnal variation of stratiform rainfall with a maximum in early morning. However, the simulated convective rainfall occurs at local noontime, earlier than observations and with larger amplitude (normalized by the daily mean). As a result, models simulated a weaker diurnal cycle in total rainfall over the western plain of China due to an out-of-phase cancellation between convective and stratiform rainfalls and a noontime maximum of total rainfall over the eastern plain of China. Over the East China Sea, models simulated the early-morning maximum of convective precipitation and, together with the correct phase of the stratiform rainfall, they captured the diurnal cycle of total precipitation. The superposition of the stratiform and convective rainfalls also explains the observed diurnal cycle in total rainfall in East Asia. Relative to the coarse-resolution model, the high-resolution model simulated slight improvement in diurnal rainfall amplitudes, due to the larger amplitude of stratiform rainfall. The two models, however, suffer from the same major biases in rainfall diurnal cycles due to the convection parameterization.

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