Relationships between Rain Characteristics and Environment. Part II: Atmospheric Disturbances Associated with Shallow Convection over the Eastern Tropical Pacific

Abstract Synoptic-scale westward-propagating disturbances over the eastern Pacific (EP) are analyzed in boreal autumn, utilizing spectral analysis, composite analysis, and energy budget analysis. The results are compared with those over the western Pacific (WP). Spectral peaks of total precipitable water (TPW) and vertical velocity at 850 hPa (ω850), and outgoing longwave radiation (OLR) are detected at periods of ~3–7 days over the EP. Meanwhile over the WP, a spectral peak of OLR is pronounced, but peaks of TPW and ω850 are not detected. Composite analysis reveals that disturbances that have a coupled structure, with a vortex at its center near ~9°N and a mixed Rossby–gravity (MRG) wave–type disturbance, frequently exist over the EP. At the same time, the disturbances have a double-deck structure associated with divergence both in the upper and in the middle to lower troposphere. These disturbances are associated with both deep convection and congestus, which generate kinetic energy of the disturbance in the upper and in the lower troposphere, respectively. Examining diabatic heating in relation to the coupled disturbances, deep heating with the peak at the height of ~7.5 km is greatest in the northeastern part of the vortex. The coupled MRG wave–type disturbance provides a relatively deep cross-equatorial southerly flow into the northeastern part of the vortex. It is suggested that deep rain is maintained with the existence of deep convergence produced by the coupled disturbances over the EP, where a very shallow convergence field exists on average.

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Impacts of Shallow Convection on MJO Simulation: A Moist Static Energy and Moisture Budget Analysis

Journal of Climate ◽

10.1175/jcli-d-12-00127.1 ◽

2013 ◽

Vol 26 (8) ◽

pp. 2417-2431 ◽

Cited By ~ 34

Author(s):

Qiongqiong Cai ◽

Guang J. Zhang ◽

Tianjun Zhou

Keyword(s):

Boundary Layer ◽

Deep Convection ◽

Lower Troposphere ◽

Longwave Radiation ◽

Reanalysis Data ◽

Specific Humidity ◽

Moist Static Energy ◽

Shallow Convection ◽

Budget Analysis ◽

Static Energy

Abstract The role of shallow convection in Madden–Julian oscillation (MJO) simulation is examined in terms of the moist static energy (MSE) and moisture budgets. Two experiments are carried out using the NCAR Community Atmosphere Model, version 3.0 (CAM3.0): a “CTL” run and an “NSC” run that is the same as the CTL except with shallow convection disabled below 700 hPa between 20°S and 20°N. Although the major features in the mean state of outgoing longwave radiation, 850-hPa winds, and vertical structure of specific humidity are reasonably reproduced in both simulations, moisture and clouds are more confined to the planetary boundary layer in the NSC run. While the CTL run gives a better simulation of the MJO life cycle when compared with the reanalysis data, the NSC shows a substantially weaker MJO signal. Both the reanalysis data and simulations show a recharge–discharge mechanism in the MSE evolution that is dominated by the moisture anomalies. However, in the NSC the development of MSE and moisture anomalies is weaker and confined to a shallow layer at the developing phases, which may prevent further development of deep convection. By conducting the budget analysis on both the MSE and moisture, it is found that the major biases in the NSC run are largely attributed to the vertical and horizontal advection. Without shallow convection, the lack of gradual deepening of upward motion during the developing stage of MJO prevents the lower troposphere above the boundary layer from being preconditioned for deep convection.

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What Are the Sources of Mechanical Damping in Matsuno–Gill-Type Models?

Journal of Climate ◽

10.1175/2007jcli1546.1 ◽

2008 ◽

Vol 21 (2) ◽

pp. 165-179 ◽

Cited By ~ 19

Author(s):

Jia-Lin Lin ◽

Brian E. Mapes ◽

Weiqing Han

Keyword(s):

Deep Convection ◽

Lower Troposphere ◽

Walker Circulation ◽

Trade Wind ◽

Shallow Convection ◽

Central Pacific ◽

Strong Damping ◽

Equivalent Linear ◽

Mechanical Damping ◽

The Walker Circulation

Abstract The Matsuno–Gill model has been widely used to study the tropical large-scale circulations and atmosphere–ocean interactions. However, a common critique of this model is that it requires a strong equivalent linear mechanical damping to get realistic wind response and it is unclear what could provide such a strong damping above the boundary layer. This study evaluates the sources and strength of equivalent linear mechanical damping in the Walker circulation by calculating the zonal momentum budget using 15 yr (1979–93) of daily global reanalysis data. Two different reanalyses [NCEP–NCAR and 15-yr ECMWF Re-Analysis (ERA-15)] give qualitatively similar results for all major terms, including the budget residual, whose structure is consistent with its interpretation as eddy momentum flux convergence by convective momentum transport (CMT). The Walker circulation is characterized by two distinct regions: a deep convection region over the Indo-Pacific warm pool and a shallow convection region over the eastern Pacific cold tongue. These two regions are separated by a strong upper-tropospheric ridge and a strong lower-tropospheric trough in the central Pacific. The resultant pressure gradient forces on both sides require strong (approximately 5–10 days) damping to balance them because Coriolis force near the equator is too small to provide the balance. In the deep convection region, the damping is provided by CMT and advection together in both the upper and lower troposphere. In the shallow convection region, on the other hand, the damping is provided mainly by advection in the upper troposphere and by CMT in the lower troposphere. In other words, the upper-level tropical easterly jet and the low-level trade wind are both braked by CMT. These results support the use of strong damping in the Matsuno–Gill-type models but suggest that the damping rate is spatially inhomogeneous and the CMT-related damping increases with the strength of convection. Implications for GCM’s simulation of tropical mean climate are discussed.

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Centered Composite Analysis of Variations Associated with the Madden–Julian Oscillation

Journal of Climate ◽

10.1175/jcli3703.1 ◽

2006 ◽

Vol 19 (9) ◽

pp. 1834-1849 ◽

Cited By ~ 4

Author(s):

Bryan C. Weare

Keyword(s):

Surface Pressure ◽

Lower Troposphere ◽

Longwave Radiation ◽

Specific Humidity ◽

Composite Analysis ◽

Madden Julian Oscillation ◽

Mixing Properties ◽

Zonal Winds ◽

The Indian Ocean ◽

Key Events

Abstract Centered composite analysis is described and applied to gain a better understanding of the initial phases of the Madden–Julian oscillation (MJO). Centered composite analysis identifies the dates and central locations of key events. The elements of the composite means are centered on these central locations before averages are calculated. In this way much of the spatial fuzziness, which is inherent in traditional composite analysis, is removed. The results for the MJO, based on MJO-filtered outgoing longwave radiation for the reference data and 40-yr ECMWF Re-Analysis (ERA-40) and NCEP–NCAR reanalysis products for the composites, show highly significant composites of unfiltered data for not only zero lag, but also lags back to 20 days before the target events. These composites identify propagating patterns of surface pressure, upper- and lower-troposphere zonal winds, surface temperature, and 850-hPa specific humidity associated with MJO convective events in the Indian Ocean. The propagation characteristics of important features, especially surface pressure, differ substantially for MJO convective anomalies centered over the Indian or western Pacific Oceans. This suggests that distinctly different mechanisms may be dominant in these two regions, and that many earlier analyses may be mixing properties of the two.

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Role of Vertical Structure of Convective Heating in MJO Simulation in NCAR CAM5.3

Journal of Climate ◽

10.1175/jcli-d-16-0913.1 ◽

2017 ◽

Vol 30 (18) ◽

pp. 7423-7439 ◽

Cited By ~ 12

Author(s):

Guyu Cao ◽

Guang J. Zhang

Keyword(s):

Vertical Structure ◽

Climate Models ◽

Spectral Power ◽

Deep Convection ◽

Lower Troposphere ◽

Global Climate Models ◽

Convective Heating ◽

Shallow Convection ◽

Heating Profile

Abstract Observational studies suggest that the vertical structure of diabatic heating is important to MJO development. In particular, the lack of a top-heavy heating profile was believed to be responsible for poor MJO simulations in global climate models. In this work, the role of the vertical heating profile in MJO simulation is investigated by modifying the convective heating profile to different shapes, from top-heavy heating to bottom cooling, to mimic mesoscale heating in the NCAR Community Atmosphere Model, version 5.3 (CAM5.3). Results suggest that incorporating a mesoscale stratiform heating structure can significantly improve the MJO simulation. By artificially adding stratiform-like heating and cooling in the experiments, many observed features of MJO are reproduced, including clear eastward propagation, a westward-tilted vertical structure of MJO-scale anomalies of dynamic and thermodynamic fields, and strong 20–80-day spectral power. Further analysis shows an abundance of shallow convection ahead of MJO deep convection, confirming the role of shallow convection in preconditioning the atmosphere by moistening the lower troposphere ahead of deep convection during the MJO life cycle. Additional experiments show that lower-level cooling contributes more to improving the MJO simulation. All these features are lacking in the control simulation, suggesting that the mesoscale stratiform heating, especially its lower-level cooling component, is important to MJO simulation.

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A Low-Level Circulation in the Tropics

Journal of the Atmospheric Sciences ◽

10.1175/2007jas2463.1 ◽

2008 ◽

Vol 65 (3) ◽

pp. 1019-1034 ◽

Cited By ~ 25

Author(s):

Ian Folkins ◽

S. Fueglistaler ◽

G. Lesins ◽

T. Mitovski

Keyword(s):

Mass Flux ◽

Large Scale ◽

Deep Convection ◽

Lower Troposphere ◽

Shallow Convection ◽

Vertical Variation ◽

Convective Systems ◽

Clear Sky ◽

Deep Convective Systems ◽

The Tropics

Abstract Deep convective tropical systems are strongly convergent in the midtroposphere. Horizontal wind measurements from a variety of rawinsonde arrays in the equatorial Pacific and Caribbean are used to calculate the mean dynamical divergence profiles of large-scale arrays (≥1000 km in diameter) in actively convecting regions. Somewhat surprisingly, the magnitude of the midtropospheric divergence calculated from these arrays is usually small. In principle, the midlevel convergence of deep convective systems could be balanced on larger scales either by a vertical variation in the radiative mass flux of the background clear sky atmosphere, or by a divergence from shallow cumuli. The vertical variation of the clear sky mass flux in the midtroposphere is small, however, so that the offsetting divergence must be supplied by shallow cumuli. On spatial scales of ∼1000 km, the midlevel convergent inflow toward deep convection appears to be internally compensated, or “screened,” by a divergent outflow from surrounding precipitating shallow convection. Deep convective systems do not induce a large-scale inflow of midlevel air toward actively convecting regions from the rest of the tropics, but instead help generate a secondary low-level circulation, in which the net downward mass flux from mesoscale and convective-scale downdrafts is balanced by a net upward mass flux from precipitating shallow cumuli. The existence of this circulation is consistent with observational evidence showing that deep and shallow convection are spatiotemporally coupled on a wide range of both spatial and temporal scales. One of the mechanisms proposed for coupling shallow convection to deep convection is the tendency for deep convection to cool the lower troposphere. The authors use radiosonde temperature profiles and the Tropical Rainfall Measuring Mission (TRMM) 3B42 gridded rainfall product to argue that the distance over which deep convection cools the lower troposphere is approximately 1000 km.

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Moisture Variation with Cloud Effects during a BSISO over the Eastern Maritime Continent in a Cloud-Permitting-Scale Simulation

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-20-0210.1 ◽

2021 ◽

Author(s):

Yuntao Wei ◽

Zhaoxia Pu

Keyword(s):

Deep Convection ◽

Lower Troposphere ◽

Radiative Heating ◽

Boreal Summer ◽

Water Evaporation ◽

Maritime Continent ◽

Budget Analysis ◽

Moisture Variation ◽

Precipitation Total ◽

Stratiform Clouds

AbstractDespite the great importance of interactions between moisture, clouds, radiation, and convection in the Madden-Julian Oscillation, their role in the boreal summer intraseasonal oscillation (BSISO) has not been well established. This study investigates the moisture variation of a BSISO during its rapid redevelopment over the eastern Maritime Continent through a cloud-permitting-scale numerical simulation. It is found that moisture variation depends closely on the evolution of clouds and precipitation. Total moisture budget analysis reveals that the deepening and strengthening (lessening) of humidity before (after) the BSISO deep convection are attributed largely to zonal advection. In addition, the column moistening/drying is mostly in phase with the humidity and is related to the maintenance of BSISO.An objective cloud-type classification method and a weak temperature gradient approximation are used to further understand the column moistening/drying. Results indicate that elevated stratiform clouds play a significant role in moistening the lower troposphere through cloud water evaporation. Decreases in deep convection condensation and re-evaporation of deep stratiform precipitation induce moistening during the development and after the decay of BSISO deep convection, respectively. Meanwhile, anomalous longwave radiative heating appears first in the lower troposphere during the developing stage of BSISO, further strengthens via the increase of deep stratiform clouds, and eventually deepens with elevated stratiform clouds. Accordingly, anomalous moistening largely in phase with the humidity of BSISO toward its suppressed stage is induced via compensated ascent. Owing to the anomalous decrease in the ratio of vertical moisture and potential temperature gradients, the cloud-radiation effect is further enhanced in the convective phase of BSISO.

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Comparison of observed and modeled cloud-free longwave downward radiation (2010–2016) at the high mountain BSRN Izaña station

Geoscientific Model Development ◽

10.5194/gmd-11-2139-2018 ◽

2018 ◽

Vol 11 (6) ◽

pp. 2139-2152 ◽

Cited By ~ 2

Author(s):

Rosa Delia García ◽

Africa Barreto ◽

Emilio Cuevas ◽

Julian Gröbner ◽

Omaira Elena García ◽

...

Keyword(s):

Longwave Radiation ◽

Precipitable Water ◽

Surface Radiation ◽

Transmission Model ◽

Standard Group ◽

High Mountain ◽

Atmospheric Transmission ◽

Night Time ◽

Moderate Resolution ◽

Downward Radiation

Abstract. A 7-year (2010–2016) comparison study between measured and simulated longwave downward radiation (LDR) under cloud-free conditions was performed at the Izaña Atmospheric Observatory (IZO, Spain). This analysis encompasses a total of 2062 cases distributed approximately evenly between day and night. Results show an excellent agreement between Baseline Surface Radiation Network (BSRN) measurements and simulations with libRadtran V2.0.1 and MODerate resolution atmospheric TRANsmission model (MODTRAN) V6 radiative transfer models (RTMs). Mean bias (simulated − measured) of <  1.1 % and root mean square of the bias (RMS) of <  1 % are within the instrumental error (2 %). These results highlight the good agreement between the two RTMs, proving to be useful tools for the quality control of LDR observations and for detecting temporal drifts in field instruments. The standard deviations of the residuals, associated with the RTM input parameters uncertainties are rather small, 0.47 and 0.49 % for libRadtran and MODTRAN, respectively, at daytime, and 0.49 to 0.51 % at night-time. For precipitable water vapor (PWV)  >  10 mm, the observed night-time difference between models and measurements is +5 W m−2 indicating a scale change of the World Infrared Standard Group of Pyrgeometers (WISG), which serves as reference for atmospheric longwave radiation measurements. Preliminary results suggest a possible impact of dust aerosol on infrared radiation during daytime that might not be correctly parametrized by the models, resulting in a slight underestimation of the modeled LDR, of about −3 W m−2, for relatively high aerosol optical depth (AOD  >  0.20).

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Simulation of the Madden–Julian Oscillation in the NCAR CCM3 Using a Revised Zhang–McFarlane Convection Parameterization Scheme

Journal of Climate ◽

10.1175/jcli3508.1 ◽

2005 ◽

Vol 18 (19) ◽

pp. 4046-4064 ◽

Cited By ~ 115

Author(s):

Guang J. Zhang ◽

Mingquan Mu

Keyword(s):

Relative Humidity ◽

Indian Ocean ◽

Zonal Wind ◽

Lower Troposphere ◽

Parameterization Scheme ◽

Madden Julian Oscillation ◽

Shallow Convection ◽

Time Period ◽

The Indian Ocean ◽

Convection Parameterization

Abstract This study presents the simulation of the Madden–Julian oscillation (MJO) in the NCAR CCM3 using a modified Zhang–McFarlane convection parameterization scheme. It is shown that, with the modified scheme, the intraseasonal (20–80 day) variability in precipitation, zonal wind, and outgoing longwave radiation (OLR) is enhanced substantially compared to the standard CCM3 simulation. Using a composite technique based on the empirical orthogonal function (EOF) analysis, the paper demonstrates that the simulated MJOs are in better agreement with the observations than the standard model in many important aspects. The amplitudes of the MJOs in 850-mb zonal wind, precipitation, and OLR are comparable to those of the observations, and the MJOs show clearly eastward propagation from the Indian Ocean to the Pacific. In contrast, the simulated MJOs in the standard CCM3 simulation are weak and have a tendency to propagate westward in the Indian Ocean. Nevertheless, there remain several deficiencies that are yet to be addressed. The time period of the MJOs is shorter, about 30 days, compared to the observed time period of 40 days. The spatial scale of the precipitation signal is smaller than observed. Examination of convective heating from both deep and shallow convection and its relationship with moisture anomalies indicates that near the mature phase of the MJO, regions of shallow convection developing ahead of the deep convection coincide with regions of positive moisture anomalies in the lower troposphere. This is consistent with the recent observations and theoretical development that shallow convection helps to precondition the atmosphere for MJO by moistening the lower troposphere. Sensitivity tests are performed on the individual changes in the modified convection scheme. They show that both change of closure and use of a relative humidity threshold for the convection trigger play important roles in improving the MJO simulation. Use of the new closure leads to the eastward propagation of the MJO and increases the intensity of the MJO signal in the wind field, while imposing a relative humidity threshold enhances the MJO variability in precipitation.

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Land–atmosphere interactions in the tropics

10.5194/hess-2019-12 ◽

2019 ◽

Author(s):

Pierre Gentine ◽

Adam Massmann ◽

Benjamin R. Lintner ◽

Sayed Hamed Alemohammad ◽

Rong Fu ◽

...

Keyword(s):

Land Surface ◽

Deep Convection ◽

Lower Troposphere ◽

Surface Fluxes ◽

Surface Radiation ◽

Spatial And Temporal Scales ◽

Wide Range ◽

Shallow Clouds ◽

The Tropics ◽

The Impact

Abstract. The continental tropics play a leading role in the terrestrial water and carbon cycles. Land–atmosphere interactions are integral in the regulation of surface energy, water and carbon fluxes across multiple spatial and temporal scales over tropical continents. We review here some of the important characteristics of tropical continental climates and how land–atmosphere interactions regulate them. Along with a wide range of climates, the tropics manifest a diverse array of land–atmosphere interactions. Broadly speaking, in tropical rainforests, light and energy are typically more limiting than precipitation and water supply for photosynthesis and evapotranspiration; whereas in savanna and semi-arid regions water is the critical regulator of surface fluxes and land–atmosphere interactions. We discuss the impact of the land surface, how it affects shallow clouds and how these clouds can feedback to the surface by modulating surface radiation. Some results from recent research suggest that shallow clouds may be especially critical to land–atmosphere interactions as these regulate the energy budget and moisture transport to the lower troposphere, which in turn affects deep convection. On the other hand, the impact of land surface conditions on deep convection appear to occur over larger, non-local, scales and might be critically affected by transitional regions between the climatologically dry and wet tropics.

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How Do Environmental Conditions Influence Vertical Buoyancy Structure and Shallow-to-Deep Convection Transition across Different Climate Regimes?

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-17-0284.1 ◽

2018 ◽

Vol 75 (6) ◽

pp. 1909-1932 ◽

Cited By ~ 6

Author(s):

Yizhou Zhuang ◽

Rong Fu ◽

Hongqing Wang

Keyword(s):

Great Plains ◽

Deep Convection ◽

Convective Available Potential Energy ◽

Central Africa ◽

Research Quality ◽

Free Troposphere ◽

Shallow Convection ◽

Adiabatic Cooling ◽

Thermodynamic Conditions ◽

Atmospheric Radiation Measurement

Abstract We developed an entraining parcel approach that partitions parcel buoyancy into contributions from different processes (e.g., adiabatic cooling, condensation, freezing, and entrainment). Applying this method to research-quality radiosonde profiles provided by the Atmospheric Radiation Measurement (ARM) program at six sites, we evaluated how atmospheric thermodynamic conditions and entrainment influence various physical processes that determine the vertical buoyancy structure across different climate regimes as represented by these sites. The differences of morning buoyancy profiles between the deep convection (DC)/transition cases and shallow convection (SC)/nontransition cases were used to assess preconditions important for shallow-to-deep convection transition. Our results show that for continental sites such as the U.S. Southern Great Plains (SGP) and west-central Africa, surface conditions alone are enough to account for the buoyancy difference between DC and SC cases, although entrainment further enhances the buoyancy difference at SGP. For oceanic sites in the tropical west Pacific, humidity dilution in the lower to middle free troposphere (~1–6 km) and temperature mixing in the middle to upper troposphere (>4 km) have the most important influences on the buoyancy difference between DC and SC cases. For the humid central Amazon region, entrainment in both the boundary layer and the lower free troposphere (~0–4 km) have significant contributions to the buoyancy difference; the upper-tropospheric influence seems unimportant. In addition, the integral of the condensation term, which represents the parcel’s ability to transform available water vapor into heat through condensation, provides a better discrimination between DC and SC cases than the integral of buoyancy or the convective available potential energy (CAPE).

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