Miniature tropics and bi-diurnal oscillations

2021 ◽  
Author(s):  
Jan O. Haerter ◽  
Gorm Gruner Jensen ◽  
Romain Fiévet
Keyword(s):  
Boundary Conditions ◽  
Surface Temperature ◽  
Spatial Variation ◽  
Diurnal Cycle ◽  
Domain Size ◽  
Convective Systems ◽  
Tropical Precipitation ◽  
Lateral Boundary Conditions ◽  
Mesoscale Convective ◽  
Self Aggregation

<p>Convective self-aggregation is a well-studied atmospheric state, obtained in typically multi-week idealized numerical experiments, where boundary conditions are constant and spatially homogeneous. As radiative convective equilibrium is approached, the atmosphere develops a heavily precipitating moist patch, which is surrounded by subsiding, cloud-free regions. It was recently shown that a homogeneous, but temporally oscillating surface temperature can quickly lead to the emergence of so-called mesoscale convective systems (MCS, diameters of >100 km) - on temporal scales of only a few days. Furthermore, the patterns formed by these MCS remind of checkerboards, and alternate from day to day [1]. </p><p>We here extend this finding further, to add realism to the otherwise preserved idealization: Mimicking a form of “miniature tropics” we retain a laterally periodic domain (Lx, Ly), but impose spatial variation in mean surface temperature along one dimension - reminiscent of a meridional reduction in mean surface temperature, when moving poleward from the equator. By making the wavelength of spatial variation commensurate with domain size, we retail double-periodic lateral boundary conditions. When the diurnal cycle is set to zero, the system quickly organizes to a forcefully aggregated caricature of the actual tropics - with heavy convection near the equator and pronounced subsidence and enhanced long-wave cooling in the subtropics. When the diurnal cycle is increased, bi-diurnal temporal oscillations appear, which lead to a single precipitation peak centered on the equator on one day, but a bimodal meridional pattern with precipitation away from the equator on the next.</p><p>Our findings, obtained for a still idealized numerical experiment, may have implications for “edge intensifications” suggested from observations and numerical modeling of tropical precipitation patterns near the ITCZ [2,3].</p><p>[1] Haerter, J.O., Meyer, B. & Nissen, S.B. Diurnal self-aggregation. <em>npj Clim Atmos Sci</em> <strong>3, </strong>30 (2020). https://doi.org/10.1038/s41612-020-00132-z</p><p>[2] Mapes, B. E., E.-S. Chung, W. M. Hannah, H. Masunaga, A. J. Wimmers and C. S. Velden, 2018: The meandering margin of the meteorological moist Tropics, <em>Geophys. Res. Lett.</em>, <strong>45</strong>, 1177-1184. doi:10.1002/2017GL076440</p><p>[3] Windmiller, J. M., & Hohenegger, C. 2019: Convection on the edge. <em>J. Adv. Model. Earth Syst.</em>, <strong>11</strong>, 3959-3972, 10.1029/2019MS001820</p>

The diurnal cycle can trigger convective self-aggregation 

2021 ◽  
Author(s):  
Gorm Gruner Jensen ◽  
Romain Fiévet ◽  
Jan O. Haerter
Keyword(s):  
Surface Temperature ◽  
Diurnal Cycle ◽  
Large Scale ◽  
Horizontal Resolution ◽  
Cold Pool ◽  
Atmospheric Sciences ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Spatio Temporal ◽  
Self Aggregation

<p>Convective self-aggregation (CSA) is an established modelling paradigm for large-scale thunderstorm clusters, as they form in mesoscale convective systems, the Madden-JulianOscillation or tropical cyclo-genesis [1]. The onset of CSA is characterized by the spontaneous formation of persistently dry patches with suppressed deep convective rainfall. Recently another type of spatio-temporal pattern formation was observed in simulations where the diurnal cycle was mimicked by a sinusoidally varying surface temperature [2]. This diurnal aggregation (DA) is characterized by clusters of intense rain that correlate negatively from one day to the next. </p><p>Here we demonstrate that the diurnal cycle can also act as a trigger of persistently dry patches resembling the early stages of CSA. When the surface temperature is held constant, CSA has been shown to occur within simulations of coarse horizontal model resolution, but not when the resolution was increased [3]. We show that, when a temporally periodic surface temperature forcing is imposed, persistently convection free patches occur even faster when the spatial resolution is increased. The failure to achieve CSA at high horizontal resolution has so far been attributed to the more pronounced cold pool effects at such resolution. In our simulations these cold pools in fact play a key role in promoting CSA. Our results have implications for the origin of persistent convective organization over continents and the sea — and point a path towards achieving such clustering under realistic conditions.</p><p><br>[1]  Christopher S Bretherton, Peter N Blossey, and Marat Khairoutdinov.  An energy-balance analysisof deep convective self-aggregation above uniform SST.Journal of the Atmospheric Sciences, 62(12):4273–4292, 2005.<br>[2]  J. O. Haerter, B. Meyer, and S. B. Nissen.  Diurnal self-aggregation.npj Climate and AtmosphericScience, 3:30, 2020.<br>[3]  Caroline  Muller  and  Sandrine  Bony.   What  favors  convective  aggregation  and  why?GeophysicalResearch Letters, 42(13):5626–5634, 2015.  doi:  https://doi.org/10.1002/2015GL064260.</p>

scholarly journals Diurnal Self-Aggregation

2020 ◽  
Author(s):  
Jan O. Haerter ◽  
Bettina Meyer ◽  
Silas Boye Nissen
Keyword(s):  
Surface Temperature ◽  
Land Surface ◽  
Domain Size ◽  
Sea Surface ◽  
Tropical Ocean ◽  
Convective Systems ◽  
Surface Temperatures ◽  
Mesoscale Convective ◽  
Weak Surface ◽  
Self Aggregation

<p>Convective self-aggregation is a modelling paradigm for thunderstorm organisation over a constant-temperature tropical sea surface. This setup can give rise to cloud clusters over timescales of weeks. In reality, sea surface temperatures do oscillate diurnally, affecting the atmospheric state. Over land, surface temperatures vary more strongly, and rain rate is significantly influenced. Here, we carry out a substantial suite of cloud-resolving numerical experiments, and find that even weak surface temperature oscillations enable qualitatively different dynamics to emerge: the spatial distribution of rainfall is only homogeneous during the first day. Already on the second day, the rain field is firmly structured. In later days, the clustering becomes stronger and alternates from day to day. We show that these features are robust to changes in resolution, domain size, and surface temperature, but can be removed by a reduction of the amplitude of oscillation, suggesting a transition to a clustered state. Maximal clustering occurs at a scale of l<sub>max</sub>≈180 km, a scale we relate to the emergence of mesoscale convective systems. At l<sub>max</sub> rainfall is strongly enhanced and far exceeds the rainfall expected at random. We explain the transition to clustering using simple conceptual modelling. Our results may help clarify how continental extremes build up and how cloud clustering over the tropical ocean could emerge much faster than through conventional self-aggregation alone.</p>

On the diurnal cycle of rainfall and convection over Lake Victoria and its catchment. Part 1: Rainfall and Mesoscale Convective Systems

2021 ◽  
Author(s):  
Sharon E. Nicholson ◽  
Adam T. Hartman ◽  
Douglas A. Klotter
Keyword(s):  
Diurnal Cycle ◽  
Lake Victoria ◽  
Mesoscale Convective Systems ◽  
Convective Systems ◽  
Early Evening ◽  
Lake Effect ◽  
Mesoscale Convective ◽  
Dry Seasons

AbstractThis article examines the diurnal cycle of lake-effect rains over Lake Victoria and of rainfall in the surrounding catchment. The analysis focuses on four months, which represent the two wet seasons (April and November) and the two dry seasons (February and July). Lake-effect rains are strongest in April, weakest in July. In all cases there is a nocturnal rainfall maximum over the lake and a daytime maximum over the catchment, with the transition between rainfall over the lake and over the catchment occurring between 1200 and 1500 LST. During the night the surrounding catchment is mostly dry. Conversely, little to no rain falls over the lake during the afternoon and early evening. In most cases the maximum over the lake occurs at either 0600 or 0900 LST and the maximum over the catchment occurs around 1500 to 1800 LST. The diurnal cycle of Mesoscale Convective Systems (MCSs) parallels that of over-lake rainfall. MCS initiation generally begins over the catchment around 1500 LST and increases at 1800 LST. MCS initiation over the lake begins around 0300 LST and continues until 1200 LST. While some MCSs originate over the highlands to the east of the lake, most originate in situ over the lake. Maximum MCS activity over the lake occurs at 0600 LST and is associated with the systems that initiate in situ.

The Diurnal Cycle of Rainfall and Convective Intensity according to Three Years of TRMM Measurements

2003 ◽  
Vol 16 (10) ◽  
pp. 1456-1475 ◽  
Author(s):  
Stephen W. Nesbitt ◽  
Edward J. Zipser
Keyword(s):  
Diurnal Cycle ◽  
Tropical Rainfall Measuring Mission ◽  
Early Morning ◽  
Convective Systems ◽  
Rain Gauges ◽  
Mesoscale Convective ◽  
Late Evening ◽  
Microwave Imager ◽  
Trmm Microwave Imager ◽  
Rain Rates

Abstract The Tropical Rainfall Measuring Mission (TRMM) satellite measurements from the precipitation radar and TRMM microwave imager have been combined to yield a comprehensive 3-yr database of precipitation features (PFs) throughout the global Tropics (±36° latitude). The PFs retrieved using this algorithm (which number nearly six million Tropicswide) have been sorted by size and intensity ranging from small shallow features greater than 75 km2 in area to large mesoscale convective systems (MCSs) according to their radar and ice scattering characteristics. This study presents a comprehensive analysis of the diurnal cycle of the observed precipitation features' rainfall amount, precipitation feature frequency, rainfall intensity, convective–stratiform rainfall portioning, and remotely sensed convective intensity, sampled Tropicswide from space. The observations are sorted regionally to examine the stark differences in the diurnal cycle of rainfall and convective intensity over land and ocean areas. Over the oceans, the diurnal cycle of rainfall has small amplitude, with the maximum contribution to rainfall coming from MCSs in the early morning. This increased contribution is due to an increased number of MCSs in the nighttime hours, not increasing MCS areas or conditional rain rates, in agreement with previous works. Rainfall from sub-MCS features over the ocean has little appreciable diurnal cycle of rainfall or convective intensity. Land areas have a much larger rainfall cycle than over the ocean, with a marked minimum in the midmorning hours and a maximum in the afternoon, slowly decreasing through midnight. Non-MCS features have a significant peak in afternoon instantaneous conditional rain rates (the mean rain rate in raining pixels), and convective intensities, which differs from previous studies using rain rates derived from hourly rain gauges. This is attributed to enhancement by afternoon heating. MCSs over land have a convective intensity peak in the late afternoon, however all land regions have MCS rainfall peaks that occur in the late evening through midnight due to their longer life cycle. The diurnal cycle of overland MCS rainfall and convective intensity varies significantly among land regions, attributed to MCS sensitivity to the varying environmental conditions in which they occur.

scholarly journals Extreme precipitation in the tropics is closely associated with long-lived convective systems

2020 ◽  
Vol 1 (1) ◽  
Author(s):  
Rémy Roca ◽  
Thomas Fiolleau
Keyword(s):  
Extreme Precipitation ◽  
Heavy Precipitation ◽  
Mesoscale Convective Systems ◽  
Convective Systems ◽  
Tropical Precipitation ◽  
Water Vapor Feedback ◽  
Mesoscale Convective ◽  
The Tropics ◽  
The Relationship ◽  
Precipitation Events

Abstract Water and energy cycles are linked to global warming through the water vapor feedback and heavy precipitation events are expected to intensify as the climate warms. For the mid-latitudes, extreme precipitation theory has been successful in explaining the observations, however, studies of responses in the tropics have diverged. Here we present an analysis of satellite-derived observations of daily accumulated precipitation and of the characteristics of convective systems throughout the tropics to investigate the relationship between the organization of mesoscale convective systems and extreme precipitation in the tropics. We find that 40% of the days with more than 250 mm precipitation over land are associated with convective systems that last more than 24 hours, although those systems only represent 5% of mesoscale convective systems overall. We conclude that long-lived mesoscale convective systems that are well organized contribute disproportionally to extreme tropical precipitation.

scholarly journals Mesoscale Convective Systems over Western Equatorial Africa and Their Relationship to Large-Scale Circulation

2009 ◽  
Vol 137 (4) ◽  
pp. 1272-1294 ◽  
Author(s):  
Brian Jackson ◽  
Sharon E. Nicholson ◽  
Douglas Klotter
Keyword(s):  
Diurnal Cycle ◽  
Rainy Season ◽  
Large Scale ◽  
Critical Role ◽  
Mesoscale Convective Systems ◽  
Convective Systems ◽  
Continental Scale ◽  
Equatorial Africa ◽  
Mesoscale Convective ◽  
The Right

Abstract This study examines mesoscale convective systems (MCSs) over western equatorial Africa using data from the Tropical Rainfall Measuring Mission (TRMM) satellite. This region experiences some of the world’s most intense thunderstorms and highest lightning frequency, but has low rainfall relative to other equatorial regions. The analyses of MCS activity include the frequency of occurrence, diurnal and annual cycles, and associated volumetric and convective rainfall. Also evaluated is the lightning activity associated with the MCSs. Emphasis is placed on the diurnal cycle and on the continental-scale motion fields in this region. The diurnal cycle shows a maximum in MCS count around 1500–1800 LT, a morning minimum, and substantial activity during the night; there is little seasonal variation in the diurnal cycle, suggesting stationary influences such as orography. Our analysis shows four maxima in MCS activity, three of which are related to local geography (two orographic and one over Lake Victoria). The fourth coincides with a midtropospheric convergence maximum in the right entrance quadrant of the African easterly jet of the Southern Hemisphere (AEJ-S). This maximum is substantially stronger in the September–November rainy season, when the jet is well developed, than in the March–May rainy season, when the jet is absent. Lightning frequency and flashes per MCS are also greatest during September–November; maxima occur in the right entrance quadrant of the AEJ-S. The lightning maximum is somewhat south of the MCS maximum and coincides with the low-lying areas of central Africa. Overall, the results of this study suggest that large-scale topography plays a critical role in the spatial and diurnal patterns of convection, lightning, and rainfall in this region. More speculative is the role of the AEJ-S, but this preliminary analysis suggests that it does play a role in the anomalous intensity of convection in western equatorial Africa.

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