Convectively Coupled Kelvin Waves: From Linear Theory to Global Models

Abstract The authors analyze composite structures of tropical convectively coupled Kelvin waves (CCKWs) in terms of the theory of Raymond and Fuchs using radiosonde data, 3D analysis and reanalysis model output, and annual integrations with the ECMWF model on the full planet and on an aquaplanet. Precipitation anomalies are estimated using the NOAA interpolated OLR and TRMM 3B42 datasets, as well as using model OLR and rainfall diagnostics. Derived variables from these datasets are used to examine assumptions of the theory. Large-scale characteristics of wave phenomena are robust in all datasets and models where Kelvin wave variance is large. Indices from the theory representing column moisture and convective inhibition are also robust. The results suggest that the CCKW is highly dependent on convective inhibition, while column moisture does not play an important role.

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Tropical temperature variability and Kelvin-wave activity in the UTLS from GPS RO measurements

Atmospheric Chemistry and Physics ◽

10.5194/acp-17-793-2017 ◽

2017 ◽

Vol 17 (2) ◽

pp. 793-806 ◽

Cited By ~ 9

Author(s):

Barbara Scherllin-Pirscher ◽

William J. Randel ◽

Joowan Kim

Keyword(s):

Large Scale ◽

Deep Convection ◽

Wave Activity ◽

Kelvin Waves ◽

Temperature Variability ◽

Stationary Waves ◽

Kelvin Wave ◽

Quasi Biennial Oscillation ◽

Tropical Tropopause ◽

Tropopause Region

Abstract. Tropical temperature variability over 10–30 km and associated Kelvin-wave activity are investigated using GPS radio occultation (RO) data from January 2002 to December 2014. RO data are a powerful tool for quantifying tropical temperature oscillations with short vertical wavelengths due to their high vertical resolution and high accuracy and precision. Gridded temperatures from GPS RO show the strongest variability in the tropical tropopause region (on average 3 K2). Large-scale zonal variability is dominated by transient sub-seasonal waves (2 K2), and about half of sub-seasonal variance is explained by eastward-traveling Kelvin waves with periods of 4 to 30 days (1 K2). Quasi-stationary waves associated with the annual cycle and interannual variability contribute about a third (1 K2) to total resolved zonal variance. Sub-seasonal waves, including Kelvin waves, are highly transient in time. Above 20 km, Kelvin waves are strongly modulated by the quasi-biennial oscillation (QBO) in stratospheric zonal winds, with enhanced wave activity during the westerly shear phase of the QBO. In the tropical tropopause region, however, peaks of Kelvin-wave activity are irregularly distributed in time. Several peaks coincide with maxima of zonal variance in tropospheric deep convection, but other episodes are not evidently related. Further investigations of convective forcing and atmospheric background conditions are needed to better understand variability near the tropopause.

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Synoptic-Scale Structure and the Character of Coastally Trapped Wind Reversals

Monthly Weather Review ◽

10.1175/mwr3267.1 ◽

2007 ◽

Vol 135 (1) ◽

pp. 60-81 ◽

Cited By ~ 11

Author(s):

Wendell A. Nuss

Keyword(s):

Kelvin Waves ◽

Direct Link ◽

Kelvin Wave ◽

Synoptic Scale ◽

Coastal Dynamics ◽

Synoptic Forcing ◽

Different Types ◽

Scale Characteristics ◽

Offshore Flow ◽

The U.S

Abstract Coastally trapped wind reversals that occur along the U.S. West Coast have been described in numerous other studies. The synoptic-scale environment and the forcing of a coastally trapped Kelvin wave are highly linked in the development of these wind reversals. However, not all wind reversals appear to behave like propagating Kelvin waves and the analysis of coastal buoy observations for three years indicates that different types of disturbances occur. Both propagating disturbances and nonpropagating disturbances occur with similar frequencies. While the synoptic-scale characteristics associated with propagating and nonpropagating wind reversals are sometimes rather subtle, several distinct differences occur that suggest a direct link between the coastal dynamics and the synoptic-scale forcing. Synoptic forcing characterized by persistent low-level offshore flow favors the development of propagating disturbances, while weak, nonsustained offshore flow characterizes nonpropagating disturbances. These differences support the idea that propagating events represent a favorable interaction between the synoptic-scale forcing and the excitation and propagation of Kelvin waves, whereas nonpropagating events either represent a less favorable interaction or are simply trapped ageostrophic flow due to the synoptic scale without a Kelvin wave response. These dynamic differences imply different stratus behavior, which is important to forecasters.

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Linking Equatorial African precipitation to Kelvin wave processes in the CP4-Africa convection-permitting regional climate simulation

10.5194/egusphere-egu21-8419 ◽

2021 ◽

Author(s):

Godwin Ayesiga ◽

Christopher Holloway ◽

Charles Williams ◽

Gui-Ying Yang ◽

Rachel Stratton ◽

...

Keyword(s):

Observational Studies ◽

Model Simulation ◽

Wave Activity ◽

Kelvin Waves ◽

Specific Humidity ◽

Climate Simulation ◽

Kelvin Wave ◽

Low Level ◽

Equatorial Africa ◽

Precipitation Anomalies

Synoptic timescale forecasts over Equatorial Africa are important for averting weather-and climate-related disasters and the resulting agricultural losses. Observational studies have shown that rainfall anomalies often propagate eastward across Equatorial Africa, and that there is a linkage between synoptic-scale eastward-propagating precipitation and Convectively Coupled Kelvin Waves (CCKWs) over this region. We explore the mechanisms in which CCKWs modulate the propagation of precipitation from West to East over Equatorial Africa. We examine the first Africa-wide climate simulation from a convection permitting model (CP4A) along with its global driving-model simulation (G25) and evaluate both against observations (TRMM) and ERA-Interim (ERA-I), with a focus on precipitation and Kelvin wave activity.Lagged composites show that both simulations capture the eastward propagating precipitation signal seen in observational studies, though G25 has a weaker signal. Composite analysis on high-amplitude Kelvin waves further shows that both simulations capture the connection between the eastward propagating precipitation anomalies and Kelvin waves. In comparison to TRMM, however, the precipitation signal is weaker in G25, while CP4A is more realistic. As the Kelvin wave activity is also well represented in both simulations when compared to ERA-I, the weak precipitation signal in G25 may be partly associated with the weak coupling between the precipitation and Kelvin waves. We show that CCKWs modulate the eastward propagation of convection and precipitation across Equatorial Africa through at least two related physical processes. Firstly, an enhancement of the low-level westerlies leads to increased low-level convergence; secondly, westerly moisture flux anomalies amplify lower-to-mid-tropospheric specific humidity. Results show that both CP4A and G25 generally simulate the key horizontal features of CCKWs, with anomalous low-level westerlies in phase with positive precipitation anomalies. However, both models show a weakness in capturing the vertical profile of anomalous specific humidity, and the zonal-vertical circulation is too weak in G25 and incoherent in CP4A compared to ERA-I.In both ERA-I and the simulations, Kelvin wave-induced convergence and the westward tilt with height of anomalous zonal winds and specific humidity tends to weaken to the east of the East African highlands. It appears that these highlands impede the coherent eastward propagation of the wave-precipitation coupled structure.

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Organization of Dust Storms and Synoptic Scale Transport of Dust by Kelvin Waves

10.5194/esd-2019-28 ◽

2019 ◽

Author(s):

Ashok Kumar Pokharel ◽

Michael L. Kaplan

Keyword(s):

Dust Storm ◽

Large Scale ◽

Kelvin Waves ◽

Dust Storms ◽

Cold Pool ◽

Kelvin Wave ◽

Synoptic Scale ◽

Dust Transport ◽

Horizontal Length ◽

Bodele Depression

Abstract. Based on the large scale transport of dust driven by the winds parallel to the mountains in the Harmattan, Saudi Arabian, and Bodélé Depression dust storms cases, a detailed study of the generation of Kelvin Waves and its possible role in organizing these dust storms and large scale dust transport was accomplished. For this study, observational and numerical model analyses were done in an in depth manner. For this, MERRA reanalysis datasets, WRF simulated high resolution variables, MODIS/Aqua and Terra images, EUMETSAT images, NAAPS aerosol modelling plots, and MERRA-2 dust scattering AOD modelling plots, surface observations, and rawinsonde soundings were analyzed for each of these three case studies. We found there were meso-β scale (horizontal length scale of 20–200 km) adjustment processes resulting in Kelvin waves only in the Harmattan and the Bodélé Depression cases. The Kelvin wave preceded a cold pool accompanying the air behind the large scale cold front instrumental in the major dust storm. We find that this Kelvin wave organized the major dust storm in a narrow zone parallel to the mountains before it expanded upscale (meso-α to synoptic).

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Tropical temperature variability and Kelvin wave activity in the UTLS from GPS RO measurements

10.5194/acp-2016-576 ◽

2016 ◽

Author(s):

Barbara Scherllin-Pirscher ◽

William J. Randel ◽

Joowan Kim

Keyword(s):

High Frequency ◽

Large Scale ◽

Wave Activity ◽

Kelvin Waves ◽

Temperature Variability ◽

Kelvin Wave ◽

Quasi Biennial Oscillation ◽

Tropical Tropopause ◽

Tropopause Region ◽

High Frequency Waves

Abstract. Tropical temperature variability over 10–30 km and associated Kelvin wave activity is investigated using GPS radio occultation (RO) data from January 2002 to December 2014. RO data are a powerful tool to quantify tropical temperature oscillations with short vertical wavelengths due to their high vertical resolution and high accuracy and precision. Gridded temperatures from GPS RO show strongest variability in the tropical tropopause region (on average 3 K2). Large-scale zonal variability is dominated by transient high-frequency waves (2 K2) and about half of high-frequency variance is explained by eastward traveling Kelvin waves with periods of 7 to 30 days (1 K2). Quasi-stationary waves associated with the annual cycle and inter-annual variability contribute about a third (1 K2) to total resolved zonal variance. High-frequency waves, including Kelvin waves, are highly transient in time. Above 20 km, Kelvin waves are strongly modulated by the quasi-biennial oscillation (QBO) in stratospheric zonal winds, with enhanced wave activity during the westerly shear phase of the QBO. In the tropical tropopause region, however, peaks of Kelvin wave activity are irregularly distributed in time. Several peaks coincide with maxima of zonal variance in tropospheric deep convection, but other episodes are not evidently related. Further investigations of convective forcing and atmospheric background conditions are needed to better understand variability near the tropopause.

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Factors for the Simulation of Convectively Coupled Kelvin Waves

Journal of Climate ◽

10.1175/jcli-d-11-00060.1 ◽

2012 ◽

Vol 25 (10) ◽

pp. 3495-3514 ◽

Cited By ~ 5

Author(s):

Kyong-Hwan Seo ◽

Jin-Ho Choi ◽

Sang-Dae Han

Keyword(s):

Large Scale ◽

Deep Convection ◽

Kelvin Waves ◽

Cumulus Parameterization ◽

Kelvin Wave ◽

Shallow Convection ◽

Cumulus Parameterization Schemes ◽

Environmental Prediction ◽

Major Factors ◽

Heating Profile

Abstract This study investigates the major factors for the realistic simulation of convectively coupled Kelvin waves (CCKWs) using the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS) models. CFS simulations employing relaxed Arakawa–Schubert (RAS; hereafter CTRL) and simplified Arakawa–Schubert (SAS) cumulus parameterization schemes show that the former generates the observed Kelvin wave signature more realistically than the latter does. For example, the space–time spectral signal, eastward propagation, and tilted (and second baroclinic mode) vertical structures in convection, temperature, moisture, and circulation anomalies associated with CCKWs in CTRL are more comparable to observations than in the SAS simulation. CTRL and observations demonstrate the characteristic evolution and vertical heating profile associated with CCKWs similar to those seen in mesoscale convective systems in the tropics: shallow convection, followed by deep convection and then stratiform cloudiness, and resulting in a top-heavy diabatic heating profile. Five additional experiments demonstrate that the effects of convective downdrafts, subgrid-scale convective rain evaporation, and large-scale rain evaporation on CCKWs are assessed to be insignificant in CTRL, possibly due to a more humid environment than observation. However, the Kelvin wave signals are reduced by ~40% when shallow convection is disabled. More importantly, the removal of convective detrainment at the cloud top results in the greatest reduction in Kelvin wave activity (by more than 70%). Therefore, the preconditioning of the atmosphere by shallow convection and detrainment of water vapor and condensate from convective updrafts to the environment and subsequent stratiform heating (grid-scale condensational heating)/precipitation processes are the two most crucial factors for the successful simulation of CCKWs.

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A Systematic Comparison of Tropical Waves over Northern Africa. Part II: Dynamics and Thermodynamics

Journal of Climate ◽

10.1175/jcli-d-18-0651.1 ◽

2019 ◽

Vol 32 (9) ◽

pp. 2605-2625 ◽

Cited By ~ 8

Author(s):

Andreas Schlueter ◽

Andreas H. Fink ◽

Peter Knippertz

Keyword(s):

Gravity Waves ◽

Large Scale ◽

Vertical Wind Shear ◽

Longwave Radiation ◽

Kelvin Waves ◽

Radiosonde Data ◽

Dynamical Structure ◽

Tropical Africa ◽

Systematic Comparison ◽

Tropical Waves

AbstractThis study presents the first systematic comparison of the dynamics and thermodynamics associated with all major tropical wave types causing rainfall modulation over northern tropical Africa: the Madden–Julian oscillation (MJO), equatorial Rossby waves (ERs), tropical disturbances (TDs, including African easterly waves), Kelvin waves, mixed Rossby–gravity waves (MRGs), and eastward inertio-gravity waves (EIGs). Reanalysis and radiosonde data were analyzed for the period 1981–2013 based on space–time filtering of outgoing longwave radiation. The identified circulation patterns are largely consistent with theory. The slow modes, MJO and ER, mainly impact precipitable water, whereas the faster TDs, Kelvin waves, and MRGs primarily modulate moisture convergence. Monsoonal inflow intensifies during wet phases of the MJO, ERs, and MRGs, associated with a northward shift of the intertropical discontinuity for MJO and ERs. This study reveals that MRGs over Africa have a distinct dynamical structure that differs significantly from AEWs. During passages of vertically tilted imbalanced wave modes, such as the MJO, TDs, Kelvin waves, and partly MRG waves, increased vertical wind shear and improved conditions for up- and downdrafts facilitate the organization of mesoscale convective systems. The balanced ERs are not tilted, and rainfall is triggered by large-scale moistening and stratiform lifting. The MJO and ERs interact with intraseasonal variations of the Indian monsoon and extratropical Rossby wave trains. The latter causes a trough over the Atlas Mountains associated with a tropical plume and rainfall over the Sahara. The presented results unveil which dynamical processes need to be modeled realistically to represent the coupling between tropical waves and rainfall in northern tropical Africa.

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Organization of dust storms and synoptic-scale transport of dust by Kelvin waves

Earth System Dynamics ◽

10.5194/esd-10-651-2019 ◽

2019 ◽

Vol 10 (4) ◽

pp. 651-666 ◽

Cited By ~ 1

Author(s):

Ashok Kumar Pokharel ◽

Michael L. Kaplan

Keyword(s):

Dust Storm ◽

Large Scale ◽

Reanalysis Data ◽

Kelvin Waves ◽

Dust Storms ◽

Cold Pool ◽

Kelvin Wave ◽

Dust Transport ◽

Aerosol Modeling ◽

Bodele Depression

Abstract. Based on the large-scale transport of dust driven by the winds parallel to the mountains in the Harmattan, Saudi Arabian, and Bodélé Depression dust storm cases, a detailed study of the generation of Kelvin waves and its possible role in organizing these dust storms and large-scale dust transport was accomplished. For this study, observational and numerical model analyses were done in an in depth manner. For this, MERRA reanalysis data sets; WRF-simulated high-resolution variables; MODIS Aqua and Terra images; EUMETSAT images; NAAPS aerosol modeling plots; and MERRA-2 dust scattering aerosol optical depth (AOD) modeling plots, surface observations, and rawinsonde soundings were analyzed for each of these three case studies. We found that there were meso-β-scale (horizontal length scale of 20–200 km) adjustment processes resulting in Kelvin waves only in the Harmattan and the Bodélé Depression cases. The Kelvin wave preceded a cold pool accompanying the air behind the large-scale cold front instrumental in the major dust storm. We find that this Kelvin wave organized the major dust storm in a narrow zone parallel to the mountains before it expanded upscale (meso-α to synoptic).

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Interaction between equatorial stratospheric Kelvin waves and gravity waves in a QBO phase

10.5194/egusphere-egu21-5535 ◽

2021 ◽

Author(s):

Young-Ha Kim ◽

Ulrich Achatz

Keyword(s):

Gravity Waves ◽

Large Scale ◽

Weather Forecasting ◽

Wave Model ◽

Kelvin Waves ◽

Convective System ◽

Subgrid Scale ◽

Kelvin Wave ◽

Action Density ◽

The Tropics

An interaction between Kelvin waves and gravity waves (GWs) in the tropical stratosphere is investigated using the global weather-forecasting model ICON with a horizontal grid spacing of ~160 km. To represent GWs in ICON, the Multi-Scale Gravity Wave Model (MS-GWaM) is used as a subgrid-scale parameterization, which is a prognostic model that explicitly calculates the evolution of GW action density in phase space. The simulation is initialized on a day in the QBO phase of the easterly maximum at ~20 hPa, so that Kelvin waves can propagate vertically throughout the lower stratosphere during the simulation. We show that Kelvin waves with zonal-wind amplitudes of about 10 m s-1 can largely affect the distribution of GW drag, by disturbing the local wind shear. Moreover, due to the zonal asymmetry in the activity of tropospheric convection, which is the source of GWs in the tropics, this effect of Kelvin waves can also influence the zonal mean of GW drag. The effect seems to be large when a strong convective system, from which large-amplitude GWs are generated, propagates eastward in the troposphere together with a phase of stratospheric Kelvin wave aloft. In our case, such an interaction causes a zonal-mean GW drag of ~0.26 m s-1 d-1 at ~20 hPa for a week during an early phase of the easterly-to-westerly transition of the QBO. The result emphasizes the importance of a correct representation of large-scale waves as well as subgrid-scale GWs in QBO simulations.

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Characteristics of Large-Scale Circulation Affecting the Inter-Annual Precipitation Variability in Northern Sumatra Island during Boreal Summer

Atmosphere ◽

10.3390/atmos12020136 ◽

2021 ◽

Vol 12 (2) ◽

pp. 136

Author(s):

Yahya Darmawan ◽

Huang-Hsiung Hsu ◽

Jia-Yuh Yu

Keyword(s):

Large Scale ◽

Precipitation Variability ◽

Specific Humidity ◽

Boreal Summer ◽

Moisture Budget ◽

Large Scale Circulation ◽

Budget Analysis ◽

Sumatra Island ◽

Precipitation Anomalies ◽

The Impact

This study aims to explore the contrasting characteristics of large-scale circulation that led to the precipitation anomalies over the northern parts of Sumatra Island. Further, the impact of varying the Asian–Australian Monsoon (AAM) was investigated for triggering the precipitation variability over the study area. The moisture budget analysis was applied to quantify the most dominant component that induces precipitation variability during the JJA (June, July, and August) period. Then, the composite analysis and statistical approach were applied to confirm the result of the moisture budget. Using the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Anaysis Interim (ERA-Interim) from 1981 to 2016, we identified 9 (nine) dry and 6 (six) wet years based on precipitation anomalies, respectively. The dry years (wet years) anomalies over the study area were mostly supported by downward (upward) vertical velocity anomaly instead of other variables such as specific humidity, horizontal velocity, and evaporation. In the dry years (wet years), there is a strengthening (weakening) of the descent motion, which triggers a reduction (increase) of convection over the study area. The overall downward (upward) motion of westerly (easterly) winds appears to suppress (support) the convection and lead to negative (positive) precipitation anomaly in the whole region but with the largest anomaly over northern parts of Sumatra. The AAM variability proven has a significant role in the precipitation variability over the study area. A teleconnection between the AAM and other global circulations implies the precipitation variability over the northern part of Sumatra Island as a regional phenomenon. The large-scale tropical circulation is possibly related to the PWC modulation (Pacific Walker Circulation).

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