scholarly journals Changes in the Interaction between Tropical Convection, Radiation, and the Large-Scale Circulation in a Warming Environment

2012 ◽  
Vol 25 (2) ◽  
pp. 557-571 ◽  
Author(s):  
Derek J. Posselt ◽  
Susan van den Heever ◽  
Graeme Stephens ◽  
Matthew R. Igel
Keyword(s):  
Relative Humidity ◽  
Large Scale ◽  
Deep Convection ◽  
Static Stability ◽  
Upper Troposphere ◽  
Surface Warming ◽  
Environmental Relative Humidity ◽  
Projected Changes ◽  
Aggregate Nature ◽  
Total Column

Abstract This paper explores the response of the tropical hydrologic cycle to surface warming through the lens of large-domain cloud-system-resolving model experiments run in a radiative–convective equilibrium framework. Simulations are run for 55 days and are driven with fixed insolation and constant sea surface temparatures (SSTs) of 298 K, 300 K, and 302 K. In each experiment, convection organizes into coherent regions of large-scale ascent separated by areas with relatively clear air and troposphere-deep descent. Aspects of the simulations correspond to observed features of the tropical climate system, including the transition to large precipitation rates above a critical value of total column water vapor, and an increase in convective intensity with SST amidst weakening of the large-scale overturning circulation. However, the authors also find notable changes to the interaction between convection and the environment as the surface warms. In particular, organized convection in simulations with SSTs of 298 and 300 K is inhibited by the presence of a strong midtropospheric stable layer and dry upper troposphere. As a result, there is a decrease in the vigor of deep convection and an increase in stratiform precipitation fraction with an increase in SST from 298 to 300 K. With an increase in SST to 302 K, moistening of the middletroposphere and increase in lower-tropospheric buoyancy serve to overcome these limitations, leading to an overall increase in convective intensity and larger increase in upper-tropospheric relative humidity. The authors conclude that, while convective intensity increases with SST, the aggregate nature of deep convection is strongly affected by the details of the thermodynamic environment in which it develops. In particular, the positive feedback between increasing SST and a moistening upper troposphere found in the simulations, operates as a nonmonotonic function of SST and is modulated by a complex interaction between deep convection and the environmental relative humidity and static stability profile. The results suggest that projected changes in convection that assume a monotonic dependence on SST may constitute an oversimplification.

scholarly journals The Fast Response of the Tropical Circulation to CO2 Forcing

2018 ◽  
Vol 31 (24) ◽  
pp. 9903-9920 ◽  
Author(s):  
Elina Plesca ◽  
Stefan A. Buehler ◽  
Verena Grützun
Keyword(s):  
Fast Response ◽  
Static Stability ◽  
Independent Effect ◽  
Tropical Circulation ◽  
Total Change ◽  
Upper Troposphere ◽  
Surface Warming ◽  
Considerable Impact ◽  
One Year ◽  
The Impact

Atmosphere-only CMIP5 idealized climate experiments with quadrupling of atmospheric CO2 are analyzed to understand the fast response of the tropical overturning circulation to this forcing and the main mechanism of this response. A new metric for the circulation, based on pressure velocity in the subsidence regions, is defined, taking advantage of the dynamical stability of these regions and their reduced sensitivity to the GCM’s cloud and precipitation parameterization schemes. This definition permits us to decompose the circulation change into a sum of relative changes in subsidence area, static stability, and heating rate. A comparative analysis of aqua- and Earth-like planet experiments reveals the effect of the land–sea contrast on the total change in circulation. On average, under the influence of CO2 increase without surface warming, the atmosphere radiatively cools less, and this drives the 3%–4% slowdown of the tropical circulation. Even in an Earth-like planet setup, the circulation weakening is dominated by the radiatively driven changes in the subsidence regions over the oceans. However, the land–sea differential heating contributes to the vertical pattern of the circulation weakening by driving the vertical expansion of the tropics. It is further found that the surface warming would, independently of the CO2 effect, lead to up to a 12% slowdown in circulation, dominated by the enhancement of the static stability in the upper troposphere. The two mechanisms identified above combine in the coupled experiment with abrupt quadrupling, causing a circulation slowdown (focused in the upper troposphere) of up to 18%. Here, the independent effect of CO2 has a considerable impact only at time scales less than one year, being overtaken quickly by the impact of surface warming.

Transport into the upper troposphere-lower stratosphere over North America

2020 ◽  
Author(s):  
Xinyue Wang ◽  
William Randel ◽  
Yutian Wu
Keyword(s):  
Gulf Of Mexico ◽  
Large Scale ◽  
Lower Stratosphere ◽  
Climate Model ◽  
Deep Convection ◽  
Vertical Diffusion ◽  
Dynamical Mechanism ◽  
Upper Troposphere ◽  
The North ◽  
Budget Analysis

<p>We study fast transport of air from the surface into the North American upper troposphere-lower stratosphere (UTLS) during northern summer with a large ensemble of Boundary Impulse Response (BIR) idealized tracers. Specifically, we implement 90 pulse tracers at the Northern Hemisphere surface and release them during July and August months in the fully coupled Whole Atmosphere Community Climate Model (WACCM) version 5. We focus on the most efficient transport cases above southern U.S. (10°-40°N, 60°-140°W) at 100 hPa with modal ages fall below 10th percentile. We examine transport-related terms, including resolved dynamics computed inside model transport scheme and parameterized processes (vertical diffusion and convective parameterization), to pin down the dominant dynamical mechanism. Our results show during the fastest transport, air parcels enter ULTS directly above the Gulf of Mexico. The budget analysis indicates that strong deep convection over the Gulf of Mexico fast uplift the tracer into 200 hPa, and then is vertically advected into 100 hPa and circulated by the enhanced large-scale anticyclone. </p>

scholarly journals CAPE Variations in the Current Climate and in a Climate Change

1998 ◽  
Vol 11 (8) ◽  
pp. 1997-2015 ◽  
Author(s):  
Bing Ye ◽  
Anthony D. Del Genio ◽  
Kenneth K-W. Lo
Keyword(s):  
Climate Change ◽  
Relative Humidity ◽  
General Circulation ◽  
Large Scale ◽  
Tropical Pacific ◽  
Lapse Rate ◽  
Moist Convection ◽  
Upper Troposphere ◽  
Current Climate ◽  
The Tropical Pacific

Abstract Observed variations of convective available potential energy (CAPE) in the current climate provide one useful test of the performance of cumulus parameterizations used in general circulation models (GCMs). It is found that frequency distributions of tropical Pacific CAPE, as well as the dependence of CAPE on surface wet-bulb potential temperature (Θw) simulated by the Goddard Institute for Space Studies’s GCM, agree well with that observed during the Australian Monsoon Experiment period. CAPE variability in the current climate greatly overestimates climatic changes in basinwide CAPE in the tropical Pacific in response to a 2°C increase in sea surface temperature (SST) in the GCM because of the different physics involved. In the current climate, CAPE variations in space and time are dominated by regional changes in boundary layer temperature and moisture, which in turn are controlled by SST patterns and large-scale motions. Geographical thermodynamic structure variations in the middle and upper troposphere are smaller because of the canceling effects of adiabatic cooling and subsidence warming in the rising and sinking branches of the Walker and Hadley circulations. In a forced equilibrium global climate change, temperature change is fairly well constrained by the change in the moist adiabatic lapse rate and thus the upper troposphere warms to a greater extent than the surface. For this reason, climate change in CAPE is better predicted by assuming that relative humidity remains constant and that the temperature changes according to the moist adiabatic lapse rate change of a parcel with 80% relative humidity lifted from the surface. The moist adiabatic assumption is not symmetrically applicable to a warmer and colder climate: In a warmer regime moist convection determines the tropical temperature structure, but when the climate becomes colder the effect of moist convection diminishes and the large-scale dynamics and radiative processes become relatively important. Although a prediction based on the change in moist adiabat matches the GCM simulation of climate change averaged over the tropical Pacific basin, it does not match the simulation regionally because small changes in the general circulation change the local boundary layer relative humidity by 1%–2%. Thus, the prediction of regional climate change in CAPE is also dependent on subtle changes in the dynamics.

Characterizing Tropical Cirrus Life Cycle, Evolution, and Interaction with Upper-Tropospheric Water Vapor Using Lagrangian Trajectory Analysis of Satellite Observations

2004 ◽  
Vol 17 (23) ◽  
pp. 4541-4563 ◽  
Author(s):  
Zhengzhao Luo ◽  
William B. Rossow
Keyword(s):  
Water Vapor ◽  
Large Scale ◽  
Trajectory Analysis ◽  
Deep Convection ◽  
Infrared Observation ◽  
Upper Troposphere ◽  
Cloud Data ◽  
Lagrangian Trajectory ◽  
Advection Term

Abstract Tropical cirrus evolution and its relation to upper-tropospheric water vapor (UTWV) are examined in the paper by analyzing satellite-derived cloud data, UTWV data from infrared and microwave measurements, and the NCEP–NCAR reanalysis wind field. Building upon the existing International Satellite Cloud Climatology Project (ISCCP) data and the Television and Infrared Observation Satellite (TIROS) Operational Vertical Sounder (TOVS) product, a global (except polar region), 6-hourly cirrus dataset is developed from two infrared radiance measurements at 11 and 12 μm. The UTWV is obtained in both clear and cloudy locations by developing a combined satellite infrared and microwave-based retrieval. The analysis in this study is conducted in a Lagrangian framework. The Lagrangian trajectory analysis shows that the decay of deep convection is immediately followed by the growth of cirrostratus and cirrus, and then the decay of cirrostratus is followed by the continued growth of cirrus. Cirrus properties continuously evolve along the trajectories as they gradually thin out and move to the lower levels. Typical tropical cirrus systems last for 19–30 ± 16 h. This is much longer than cirrus particle lifetimes, suggesting that other processes (e.g., large-scale lifting) replenish the particles to maintain tropical cirrus. Consequently, tropical cirrus can advect over large distances, about 600–1000 km, during their lifetimes. For almost all current GCMs, this distance spans more than one grid box, requiring that the water vapor and cloud water budgets include an advection term. Based on their relationship to convective systems, detrainment cirrus are distinguished from in situ cirrus. It is found that more than half of the tropical cirrus are formed in situ well away from convection. The interaction between cirrus and UTWV is explored by comparing the evolution of the UTWV along composite clear trajectories and trajectories with cirrus. Cirrus are found to be associated with a moister upper troposphere and a slower rate of decrease of UTWV. Moreover, the elevated UTWV has a longer duration than cirrus. The amount of water in cirrus is too small for evaporation of cirrus ice particles to moisten the upper troposphere significantly (but cirrus may be an important water vapor sink). Rather, it is likely that the same transient motions that produce the cirrus also transport water vapor upward to maintain a larger UTWV.

scholarly journals Thermodynamic Environments of Deep Convection in Atlantic Tropical Disturbances

2013 ◽  
Vol 70 (7) ◽  
pp. 1912-1928 ◽  
Author(s):  
Christopher A. Davis ◽  
David A. Ahijevych
Keyword(s):  
Large Scale ◽  
Inner Core ◽  
System Development ◽  
Statistical Tests ◽  
Deep Convection ◽  
Lower Troposphere ◽  
Thermodynamic Characteristics ◽  
Static Stability ◽  
Tropical Cyclogenesis ◽  
Sea Temperature

Abstract Conditional composites of dropsondes deployed into eight tropical Atlantic weather systems during 2010 are analyzed. The samples are conditioned based on cloud-top temperature within 10 km of the dropsonde, the radius from the cyclonic circulation center of the disturbance, and the stage of system development toward tropical cyclogenesis. Statistical tests are performed to identify significant differences between composite profiles. Cold-cloud-region-composite profiles of virtual temperature deviations from a large-scale instantaneous average indicate enhanced static stability prior to genesis within 200 km of the center of circulation, with negative anomalies below 700 hPa and larger warm anomalies above 600 hPa. Moist static energy is enhanced in the middle troposphere in this composite mainly because of an increase in water vapor content. Prior to genesis the buoyancy of lifted parcels within 200 km of the circulation center is sharply reduced compared to the buoyancy of parcels farther from the center. These thermodynamic characteristics support the conceptual model of an altered mass flux profile prior to genesis that strongly favors convergence in the lower troposphere and rapid increase of circulation near the surface. It is also noted that the air–sea temperature difference is greatest in the inner core of the pregenesis composite, which suggests a means to preferentially initiate new convection in the inner core where the rotation is greatest.

Do tropical islands warm or cool the troposphere?

2021 ◽  
Author(s):  
David Leutwyler ◽  
Cathy Hohenegger
Keyword(s):  
Relative Humidity ◽  
Temperature Profile ◽  
Land Surface ◽  
Large Scale ◽  
Local Density ◽  
Cloud Base ◽  
Tropical Islands ◽  
Maritime Continent ◽  
Surface Warming ◽  
Virtual Temperature

<p><span>Tropical islands are commonly seen as hot spots that heat the troposphere, since during the day, they typically become warmer than the ocean. However, at the same time, they also become dryer (in terms of relative humidity), due to the soil's resistance to evaporate. The surface warm and dry anomaly is then propagated upwards into the troposphere by thunderstorms (deep convection) that frequently form over the islands. The vertical propagation of the anomaly happens because the warmer surface over land tends to push the induced diurnal convection towards a warmer moist adiabat. However, the drying of the land surface also pulls the clouds towards a colder moist adiabat, as more initial lifting along the dry adiabat is needed until saturation is reached. In other words, a dryer island leads to a more elevated cloud base, and thus, to convection at a colder moist adiabat. The formation of convective clouds over land results in a local density anomaly that is then communicated to the island's surroundings by gravity waves since the tropical atmosphere cannot sustain strong horizontal density gradients (WTG theory). Together these ideas allow formulating the hypothesis that surface temperature and humidity anomalies emerging over islands project onto the large-scale temperature profile of the troposphere. Who wins in influencing the troposphere, the surface warming or the drying? Or put differently, do islands heat or cool the troposphere?</span></p><p align="left"> </p><p><span>We assess this hypothesis using a six-member ensemble of double-periodic convection-resolving Radiative-Convective Equilibrium (RCE) simulations (1006x1006x74 grid points), containing an archipelago of flat islands obtained from the Maritime Continent. In contrast to previous RCE simulations, the islands are represented by a land-surface scheme and are thus capable of representing not only the daytime anomaly in temperature but also that in relative humidity. We find that during episodes when precipitation occurs more frequently over land, the domain-mean (virtual) temperature in the mid-troposphere becomes colder. We also find that the drying (i.e, cooling) effect becomes pronounced for larger islands, and thus, removing a large island from the simulation also leads to a systematically colder domain-mean (virtual) temperature profile. The results suggest that islands may rather cool than warm the troposphere and that the inability of evaporation over land to keep up with the daytime surface warming is of key relevance for the temperature profile in the Maritime Continent.</span></p>

scholarly journals Anthropogenic aerosols may have increased upper tropospheric humidity in the 20th century

2011 ◽  
Vol 11 (9) ◽  
pp. 4577-4586 ◽  
Author(s):  
M. Bister ◽  
M. Kulmala
Keyword(s):  
Relative Humidity ◽  
20Th Century ◽  
Satellite Data ◽  
Cloud Model ◽  
Deep Convection ◽  
Longwave Radiation ◽  
Anthropogenic Aerosols ◽  
Upper Troposphere ◽  
Warm Rain ◽  
Upper Tropospheric Humidity

Abstract. Recent simulations of deep convection with a spectral microphysics cloud model show that an increase in aerosol concentration can have a significant effect on the nature of convection with more ice precipitation and less warm rain in polluted air. The cloud lifetime and the area covered by cloud anvils of deep convection are also larger for polluted air. Therefore, it is possible that the increase of anthropogenic aerosols in most of the 20th century has increased humidity and perhaps also cloudiness in the mid- to upper troposphere. Satellite data of upper tropospheric relative humidity in 1979–1997 and observed changes in cloudiness support this hypothesis. As changes in upper tropospheric humidity strongly affect longwave radiation, it is possible that anthropogenic aerosols have had a significant warming effect in addition to their other known effects on radiation.

scholarly journals Boundary Layer and Cloud Structure Controls on Tropical Low Cloud Cover Using A-Train Satellite Data and ECMWF Analyses

2011 ◽  
Vol 24 (1) ◽  
pp. 194-215 ◽  
Author(s):  
Terence L. Kubar ◽  
Duane E. Waliser ◽  
J-L. Li
Keyword(s):  
Boundary Layer ◽  
Cross Section ◽  
Large Scale ◽  
Layer Structure ◽  
Deep Convection ◽  
Static Stability ◽  
Boundary Layer Stability ◽  
Middle Point ◽  
Cloud Data ◽  
Low Cloud

Abstract The Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), CloudSat radar, and the Moderate Resolution Imaging Spectroradiometer (MODIS) cloud data on the A-Train constellation complemented with the European Centre for Medium-Range Forecasts (ECMWF) analyses are used to investigate the cloud and boundary layer structure across a 10° wide cross section starting at 5°S near the international date line and extending to 35°N near the California coast from March 2008 to February 2009. The mean large-scale inversion height and low-level cloud tops, which correspond very closely to each other, are very shallow (∼500 m) over cold SSTs and high static stability near California and deepen southwestward (to a maximum of ∼1.5–2.0 km) along the cross section as SSTs rise. Deep convection near the ITCZ occurs at a surface temperature close to 298 K. While the boundary layer relative humidity (RH) is nearly constant where a boundary layer is well defined, it drops sharply near cloud top in stratocumulus regions, corresponding with strong thermal inversions and water vapor decrease, such that the maximum (−∂RH/∂z) marks the boundary layer cloud top very well. The magnitude correlates well with low cloud frequency during March–May (MAM), June–August (JJA), and September–November (SON) (r 2 = 0.85, 0.88, and 0.86, respectively). Also, CALIPSO and MODIS isolated low cloud frequency generally agree quite well, but CloudSat senses only slightly more than one-third of the low clouds as observed by the other sensors, as many clouds are shallower than 1 km and thus cannot be discerned with CloudSat due to contamination from the strong signal from surface clutter. Mean tropospheric ω between 300 and 700 hPa is examined from the ECMWF Year of Tropical Convection (YOTC) analysis dataset, and during JJA and SON, strong rising motion in the middle troposphere is confined to a range of 2-m surface temperatures between 297 and 300 K, consistent with previous studies that show a narrow range of SSTs over which deep ascent occurs. During December–February (DJF), large-scale ascending motion extends to colder SSTs and high boundary layer stability. A slightly different boundary layer stability metric is derived, the difference of moist static energy (MSE) at the middle point of the inversion (or at 700 hPa if no inversion exists) and the surface, referred to as ΔMSE. The utility of ΔMSE is its prediction of isolated uniform low cloud frequency, with very high r 2 values of 0.93 and 0.88, respectively, for the MODIS and joint lidar plus radar product during JJA but significantly lower values during DJF (0.46 and 0.40), with much scatter. To quantify the importance of free tropospheric dynamics in modulating the ΔMSE–low cloud relationships, the frequency as a function of ΔMSE of rising motion profiles (ω < −0.05 Pa s−1) is added to the observed low cloud frequency for a maximum hypothetical low cloud frequency. Doing this greatly reduces the interseasonal differences and holds promise for using ΔMSE for parameterization schemes and examining low cloud feedbacks.

TRMM-Observed Shallow versus Deep Convection in the Eastern Pacific Related to Large-Scale Circulations in Reanalysis Datasets

2014 ◽  
Vol 27 (14) ◽  
pp. 5575-5592 ◽  
Author(s):  
Chie Yokoyama ◽  
Edward J. Zipser ◽  
Chuntao Liu
Keyword(s):  
Large Scale ◽  
Vertical Structure ◽  
Deep Convection ◽  
Hadley Circulation ◽  
Eastern Pacific ◽  
Return Flow ◽  
Shallow Convection ◽  
Upper Troposphere ◽  
Radar Echo ◽  
Circulation Structure

Abstract Over the eastern Pacific, recent studies have shown that a shallow large-scale meridional circulation with its return flow just above the boundary layer coexists with a deep Hadley circulation. This study examines how the vertical structure of large-scale circulations is related to satellite-observed individual precipitation properties over the eastern Pacific in boreal autumn. Three reanalysis datasets are used to describe differences in their behavior. The results are compared among reanalyses and three distinctly different convection periods, which are defined according to their radar echo depths. Shallow and deep circulations are shown to often coexist for each of the three periods, resulting in the multicell circulation structure. Deep (shallow) circulations preferentially appear in the mostly deep (shallow) convection period of radar echo depths. Thus, depth of convection basically corresponds to which circulation branch is dominant. This anticipated relationship between the circulation structure and depths of convection is common in all three reanalyses. Notable differences among reanalyses are found in the mid- to upper troposphere in either the time-mean state or the composite analysis based on the convection periods. Reanalyses have large variations in characteristics associated with deep circulations such as the upper-tropospheric divergence and outflows and the midlevel inflows, which are consistent with their different profiles of latent heating in the mid- to upper troposphere. On the other hand, discrepancies in shallow circulations and shallow convection are also found, but they are not as large as those in deep ones.

The Role of Convective Moistening in the Madden–Julian Oscillation

2009 ◽  
Vol 66 (11) ◽  
pp. 3297-3312 ◽  
Author(s):  
Katherine Thayer-Calder ◽  
David A. Randall
Keyword(s):  
Large Scale ◽  
Deep Convection ◽  
Lower Troposphere ◽  
High Humidity ◽  
Madden Julian Oscillation ◽  
Near Surface ◽  
Community Atmosphere Model ◽  
Cloud Resolving Model ◽  
Environmental Relative Humidity ◽  
High Humidity Environment

Abstract This study compares two models that differ primarily in their cloud parameterizations and produce extremely different simulations of the Madden–Julian oscillation (MJO). The Community Atmosphere Model (CAM) version 3.0 from NCAR uses the Zhang–McFarlane scheme for deep convection and does not produce an MJO. The “superparameterized” version of the CAM (SP-CAM) replaces the cloud parameterizations with a two-dimensional cloud-resolving model (CRM) in each grid column and produces an extremely vigorous MJO. This analysis shows that the CAM is unable to produce high-humidity regions in the mid- to lower troposphere because of a lack of coupling between parameterized convection and environmental relative humidity. The SP-CAM produces an overly moist column due in part to excessive near-surface winds and evaporation during strong convective events. In the real tropics and the SP-CAM, convection within a high-humidity environment produces intense latent heating, which excites the large-scale circulation that is the signature of the MJO. The authors suggest that a model must realistically represent convective processes that moisten the entire tropical troposphere in order to produce a simulation of the MJO.

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