Gradient Of Equivalent Potential Temperature Research Articles

Verification of tropical cyclogenesis forecasts of the Korean Integrated Model for 2020–2021

Abstract Accurate tropical cyclogenesis (TCG) prediction is important because it allows national operational forecasting agencies to issue timely warnings and implement effective disaster prevention measures. In 2020, the Korea Meteorological Administration employed a self-developed operational model called the Korean Integrated Model (KIM). In this study, we verified KIM’s TCG forecast skill over the western North Pacific. Based on 9-day forecasts, TCG in the model was objectively detected and classified as well-predicted, early formation, late formation, miss, or false alarm by comparing their formation times and locations with those of 46 tropical cyclones (TCs) from June to November in 2020–2021 documented by the Joint Typhoon Warning Center. The prediction of large-scale environmental conditions relevant to TCG was also evaluated. The results showed that the probability of KIM detection was comparable to or better than that of previously reported statistics of other numerical weather prediction models. The intra-basin comparison revealed that the probability of detection in the Philippine Sea was the highest, followed by the South China Sea and Central Pacific. The best TCG prediction performance in the Philippine Sea was supported by unbiased forecasts in large-scale environments. The missed and false alarm cases in all three regions had the largest prediction biases in the large-scale lower-tropospheric relative vorticity. Excessive false alarms may be associated with prediction biases in the vertical gradient of equivalent potential temperature within the boundary layer. This study serves as a primary guide for national forecasters and is useful to model developers for further refinement of KIM.

Precipitation and Moisture Transport of the 2021 Shimokita Heavy Precipitation: A Transformed Extratropical Cyclone from Typhoon#9

This study examines the heavy rainfall event that occurred in the Shimokita Peninsula, Japan, on 9–10 August 2021, resulting from an extra-tropical cyclone that developed from Typhoon#9 (EC9). The objective of this study is to elucidate the relationship between moisture transport and heavy rainfall and to verify the role of EC9. The authors created intensive hourly precipitation data over the Aomori Prefecture and analyzed them together with moisture fields. In most locations where the landslide disaster occurred, there were two precipitation peaks: at 9 UTC and 18 UTC on 9 August. The wind shear was strong from the lower to the upper troposphere with easterly winds in the lower troposphere and warm moist air from south for the first peak. A strong horizontal gradient of equivalent potential temperature, a northerly in lower troposphere, and moisture convergence over Shimokita Peninsula indicate the existence of the stationary front for the latter peak (18 UTC). The heavy precipitation and moisture convergence that caused the Shimokita event were identified by the stationary front of EC9 around the latter peak (15 UTC of 9th–06 UTC of 10 August). The precipitation distribution, which has a precipitation peak northeast of the EC center, is a typical typhoon-turned extratropical cyclone (EC) precipitation distribution.

Rapid Increase of Explosive Cyclone Activity over the Midwinter North Pacific in the Late 1980s

Abstract Long-term changes in the activity of explosively developing “bomb” cyclones over the wintertime North Pacific are investigated by using a particular version of a global atmospheric reanalysis dataset into which only conventional observations have been assimilated. Bomb cyclones in January are found to increase rapidly around 1987 in the midlatitude central North Pacific. Some of the increased bomb cyclones formed over the East China Sea and then moved along the southern coast of Japan before developing explosively in the central North Pacific. The enhanced cyclone activity is found to be concomitant with rapid warming and moistening over the subtropical western Pacific and the South and East China Seas under the weakened monsoonal northerlies, leading to the enhancement of the lower-tropospheric Eady growth rate and equivalent potential temperature gradient, setting a condition favorable for cyclone formation in the area upstream of the North Pacific storm track. Along the storm track, poleward moisture transport in the warm sector of a cyclone and associated precipitation along the warm and cold fronts tended to increase and thereby enhance its explosive development. After the transition around 1987, a bomb cyclone has become more likely to develop without a strong upper-level cyclonic vortex propagating from Eurasia than in the earlier period. The increased bomb cyclone activity in January is found to contribute to the diminished midwinter minimum of the North Pacific storm track activity after the mid-1980s.

Artificial intelligence in forecasting central pressure drop and maximum sustained wind speed of cyclonic systems over Arabian Sea: skill comparison with conventional models

An attempt is made in this study to forecast the central pressure drop (PD) and maximum sustained wind speed (MSWS) associated with cyclonic systems at the stage of the highest intensity over Arabian Sea using artificial neural network models. The cyclonic systems include the phases from deep depression to extreme severe cyclones. The sea surface temperature, mid-tropospheric relative humidity, surface to middle tropospheric equivalent potential temperature gradient, inverse of wind shear and vertical wind velocity at 200 hPa level are evaluated as the most suitable predictors through principal component analysis. Various neural network models with different architectures have been trained with the data from 1990 to 2012 to select the best forecast model. The prediction skill of the intelligent models is evaluated by different accuracy measures. The results show that the multi-layer perceptron (MLP) model with five input layers, one hidden layer with four nodes and one output layer is the best model for forecasting PD with minimum prediction error of 0.14 at 36 h lead time, whereas the MLP model with five input layers, one hidden layer with five nodes and one output layer is found to be the best model for forecasting MSWS with minimum prediction error of 0.19 at 48 h (h) lead time. The results are well validated with the observations from 2013 to 2018. The forecast skill of MLP model is compared with multiple linear regression model and existing operational and numerical weather prediction models.

Dynamics of the spatiotemporal morphology of Mei-yu fronts: an initial survey

Mei-yu fronts are often accompanied with prominent diabatic heating due to the development of frontal clouds and rain bands. The direct effect and relative importance of diabatic heating on the spatiotemporal morphology of Mei-yu fronts however remain unclear. Here a new frontogenesis function is derived to isolate the effect of diabatic heating and this function is then applied to the latest high resolution reanalysis product ERA5 to conduct an initial survey of dynamic and thermodynamic processes driving the intensity and structure evolutions of three typical Mei-yu fronts. It is found that the direct effect of latent heating (moisture depletion) is always frontogenetical (frontolytical) in the pre-frontal and frontal zone throughout the lifecycle of the front with latent heating (moisture depletion) in general dominates the front intensification (dissipation). Tilting is another critical process that turns the vertical gradient of equivalent potential temperature into horizontal gradient leading to frontogenesis during the front development stage, and, after the release of convective instability ahead of the front, flattens the front surface leading to frontolysis during the front decaying stage. Therefore titling maintains consistently positive contributions to the evolution of a Mei-yu front and its importance depends highly on the convective intensity near the front. The deformation effect appears frontogenetical but carries a much smaller weight compared to diabatic heating, moisture depletion and tilting. The structure evolutions of the three Mei-yu fronts studied here exhibit two distinct patterns: “bending and breaking”, and “moving and rotating”. Both patterns are dominated by the front propagation (i.e., frontogenesis through air parcels), further highlighting the critical roles played by diabatic heating, moisture depletion and titling in determining the spatiotemporal characteristics of Mei-yu fronts. As these three processes are all closely tied to updrafts, clouds and precipitation near the fronts, improved representations of clouds and convection are needed to accurately depict their feedbacks to frontal evolutions and ultimately achieve better predictions of heavy rainfall and floods associated with Mei-yu fronts.

An Observational Study of the Symmetric Boundary Layer Structure and Tropical Cyclone Intensity

This study analyses Global Positioning System dropsondes to document the axisymmetric tropical cyclone (TC) boundary-layer structure, based on storm intensity. A total of 2608 dropsondes from 42 named TCs in the Atlantic basin from 1998 to 2017 are used in the composite analyses. The results show that the axisymmetric inflow layer depth, the height of maximum tangential wind speed, and the thermodynamic mixed layer depth are all shallower in more intense TCs. The results also show that more intense TCs tend to have a deep layer of the near-saturated air inside the radius of maximum wind speed (RMW). The magnitude of the radial gradient of equivalent potential temperature (θe) near the RMW correlates positively with storm intensity. Above the inflow layer, composite structures of TCs with different intensities all possess a ring of anomalously cool temperatures surrounding the warm-core, with the magnitude of the warm-core anomaly proportional to TC intensity. The boundary layer composites presented here provide a climatology of how axisymmetric TC boundary layer structure changes with intensity.

Synoptic-scale atmospheric circulation anomalies associated with summertime daily precipitation extremes in the middle–lower reaches of the Yangtze River Basin

The mechanisms of short-lived precipitation extremes in boreal summer over the middle–lower reaches of the Yangtze River Basin (MLYRB) during 1961–2014 are explored using gridded observational and reanalysis datasets. Daily precipitation extremes are defined by the 75th and 95th percentiles and identified for selected regions in the MLYRB. Moisture budget analysis is utilized to quantify the major factors responsible for the variability of extreme precipitation events. Then the atmospheric variables are composited according to these extreme events, to illustrate the temporal evolution of the corresponding synoptic-scale structures. The results show that moisture flux convergence plays a dominant role in the variations of extreme rainfall for both percentile events, and the contribution of evaporation is not evident. Moreover, the dynamic component associated with changes in atmospheric circulation makes a larger contribution than the nonlinear component, followed by the thermodynamic component, owing to changes in specific humidity. The moisture transport pathways increase with increasing magnitude of extreme rainfall intensity. Circulation field anomalies move into the MLYRB during the onset of precipitation extremes for both percentile cases, with stronger anomalies occurring for the 95th percentile cases. Furthermore, the water vapor fluxes transported from the Northwest Pacific are pronounced during the 95th percentile events. A cross-section of specific humidity and vertical gradient of equivalent potential temperature further indicates that dynamic properties play a crucial role in the development of extreme precipitation events linked with quasi-stationary frontal activities across the MLYRB during the past few decades. Composites of Eady growth rate and outgoing longwave radiation anomalies reveal that extratropical cyclones passing through the MLYRB produce a large amount of extreme rainfall by synoptic disturbance.

Multi-scale temporospatial variability of the East Asian Meiyu-Baiu fronts: characterization with a suite of new objective indices

The Meiyu-Baiu front (MBF) is the primary weather system modulating the monsoonal rainfall in East Asia, and it exhibits pronounced variability across multiple temporal and spatial scales. A suite of new objective indices based upon daily reanalysis data are developed to characterize such variability, which include an “intensity” index effectively measuring the meridional gradient of equivalent potential temperature across the frontal zone, a “location” index documenting the central latitude and longitude of the front, and a “stationarity” index corresponding to the standard deviation of the daily central latitude. All these indices are characterized by strong interannual to interdecadal variations. Specifically, the MBF has shown a tendency of southwestward shift since the mid-1990s and become more stationary over southern China with reduced intraseasonal meandering. The western North Pacific subtropical high (WNPSH) is the main circulation feature affecting the location of the MBF on interannual timescales. The MBF moves northward (southward) when the WNPSH is anomalously strong (weak) and the surface high over the region of 35°–50°N/100°–120°E is anomalously weak (strong). When the WNPSH is strong, the MBF is often situated northwest to its climatological position and becomes more stationary. On interdecadal timescales, enhanced activity of high latitude blocking over East Asia is often accompanied with a stronger but less stationary MBF while a negative phase of the Arctic Oscillation favors a southward-shifted and more stationary MBF.

Evaluation of the Use of Moist Potential Vorticity and Moist Potential Vorticity Vector in Describing Annual Cycles of Rainfall over Different Regions in Tanzania

The economy of Tanzania heavily depends on agriculture sector which is primarily rain-fed. In this paper, we compute the moist potential vorticity (MPV) and evaluate its usefulness to describe annual cycles of rainfall. We also modify the convective vorticity vector ((CVV) ) which was defined as the cross product of absolute vorticity and the gradient of equivalent potential temperature to moist potential vorticity vector ((MPVV) ). This vector is calculated as a cross product of absolute vorticity and the gradient of moist air entropic potential temperature. The performance of (MPVV) to describe the annual cycles of rainfall over different regions in Tanzania is analyzed. Twenty six (26) years (1976-2001) daily data of air temperature, specific humidity, zonal and meridional components of the wind at 850 and 600hPa from numerical output generated by the Rossby Center regional climate model version four (RCA4) are used for computation of MPV and (MPVV) at 700hPa. The statistical relationship between (MPVV) and MPV against observed rainfall data from 22 synoptic meteorological stations using Pearson correlation coefficient indicate that (MPVV) bears strong and statistically significant correlation coefficient to rainfall than MPV suggesting its potential use as predictor of annual cycles of rainfall over different regions in Tanzania.

Moist Potential Vorticity Vector for Diagnosis of Heavy Rainfall Events in Tanzania

In this paper, we modify the convective vorticity vector (CVV) defined as a cross product of absolute vorticity and gradient of equivalent potential temperature to moist potential vorticity vector (MPVV) defined as a cross product of absolute vorticity () and the gradient of the moist-air entropy potential temperature (). The patterns of (MPVV) are compared with the patterns of heavy rainfall events that occurred over different regions in Tanzania on 20th to 22nd December, 2011 and on 5th to 8th May, 2015. Moreover, the article aimed at assessing the relative contributions of the magnitude, horizontal and vertical components of (MPVV) detecting on the observed patterns of rainfall events. Dynamic and thermodynamic variables: wind speed, temperature, atmospheric pressure and relative humidity from numerical output generated by the Weather Research and Forecasting (WRF) model running at Tanzania Meteorological Agency (TMA) were used to compute MPVV. It is found that MPVV provide accurate tracking of locations received heavy rainfall, suggesting its potential use as a dynamic tracer for heavy rainfall events in Tanzania. Finally it is found that the first and second components of MPVV contribute almost equally in tracing locations received heavy rainfall events. The magnitude of MPVV described the locations received heavy rainfall events better than the components.

Extratropical fronts in the lower troposphere–global perspectives obtained from two automated methods

In this study two commonly used automated methods to detect atmospheric fronts in the lower troposphere are compared in various synoptic situations. The first method is a thermal approach, relying on the gradient of equivalent potential temperature (TH), while the second method is based on temporal changes in the 10 m wind (WND). For a comprehensive objective comparison of the outputs of these methods of frontal identification, both schemes are firstly applied to an idealised strong baroclinic wave simulation in the absence of topography. Then, two case‐studies (one in the Northern Hemisphere (NH) and one in the Southern Hemisphere (SH)) were conducted to contrast fronts detected by the methods. Finally, we obtain global winter and summer frontal occurrence climatologies (derived from ERA‐Interim for 1979–2012) and compare the structure of these. TH is able to identify cold and warm fronts in strong baroclinic cases that are in good agreement with manual analyses. WND is particularly suited for the detection of strongly elongated, meridionally oriented moving fronts, but has very limited ability to identify zonally oriented warm fronts. We note that the areas of the main TH frontal activity are shifted equatorwards compared to the WND patterns and are located upstream of regions of main WND front activity. The number of WND fronts in the NH shows more interseasonal variations than TH fronts, decreasing by more than 50% from winter to summer. In the SH there is a weaker seasonal variation of the number of observed WND fronts, however TH front activity reduces from summer (DJF) to winter (JJA). The main motivation is to give an overview of the performance of these methods, such that researchers can choose the appropriate one for their particular interest.

Effect of Beta Shear on Simulated Tropical Cyclones

Abstract Through cloud-resolving simulations, this study examines the effect of β on the evolution of tropical cyclones (TCs). It is found that the TC simulated on a β plane with variable Coriolis parameter f is weaker in intensity but larger in size and strength than the TC simulated on an f plane with constant f. Such differences result mainly from the effect of the β shear rather than from the variation of f due to the latitudinal change of the TC position, as illustrated in a three-stage conceptual model developed herein. The first stage begins with the establishment of the β shear and the emergence of asymmetries as the TC intensifies. The β shear peaks in value during the second stage that subsequently leads to the formation of an extensive stratiform region outside of the primary eyewall. The evaporative cooling associated with the stratiform precipitation acts to sharpen the low-level equivalent potential temperature gradient into a frontlike zone outside of the eyewall region, which leads to the burst of convection outside of the primary eyewall. The third stage is characterized by a weakening β shear and the corresponding TC vortex axisymmetrization and expansion. The convection on the inner edge of the stratiform region becomes more organized in the azimuthal direction and eventually causes the TC structure to evolve in a manner similar to the secondary eyewall formation and eyewall replacement usually observed in TCs. It is the active convection outside of the primary eyewall that contributes to a relatively weaker but larger TC on the β plane than that on the f plane.

The 26 July 2005 heavy rainfall event over Mumbai: numerical modeling aspects

The performance of the Advanced Regional Prediction System (ARPS) in simulating an extreme rainfall event is evaluated, and subsequently the physical mechanisms leading to its initiation and sustenance are explored. As a case study, the heavy precipitation event that led to 65 cm of rainfall accumulation in a span of around 6 h (1430 LT–2030 LT) over Santacruz (Mumbai, India), on 26 July, 2005, is selected. Three sets of numerical experiments have been conducted. The first set of experiments (EXP1) consisted of a four-member ensemble, and was carried out in an idealized mode with a model grid spacing of 1 km. In spite of the idealized framework, signatures of heavy rainfall were seen in two of the ensemble members. The second set (EXP2) consisted of a five-member ensemble, with a four-level one-way nested integration and grid spacing of 54, 18, 6 and 1 km. The model was able to simulate a realistic spatial structure with the 54, 18, and 6 km grids; however, with the 1 km grid, the simulations were dominated by the prescribed boundary conditions. The third and final set of experiments (EXP3) consisted of a five-member ensemble, with a four-level one-way nesting and grid spacing of 54, 18, 6, and 2 km. The Scaled Lagged Average Forecasting (SLAF) methodology was employed to construct the ensemble members. The model simulations in this case were closer to observations, as compared to EXP2. Specifically, among all experiments, the timing of maximum rainfall, the abrupt increase in rainfall intensities, which was a major feature of this event, and the rainfall intensities simulated in EXP3 (at 6 km resolution) were closest to observations. Analysis of the physical mechanisms causing the initiation and sustenance of the event reveals some interesting aspects. Deep convection was found to be initiated by mid-tropospheric convergence that extended to lower levels during the later stage. In addition, there was a high negative vertical gradient of equivalent potential temperature suggesting strong atmospheric instability prior to and during the occurrence of the event. Finally, the presence of a conducive vertical wind shear in the lower and mid-troposphere is thought to be one of the major factors influencing the longevity of the event.

Dynamics of the West African monsoon

A review is given of the dynamical mechanisms responsible for the monsoon circulation over West Africa. Features of the circulation are first described, including the seasonal displacement of the rain bands, the structure of the heat low over the Sahara, the meridional circulation to the south and the associated zonal jets. Simple theories for the zonal-mean meridional circulation are then presented, using the principles of angular momentum conservation, thermal wind balance and moist convective equilibrium. The application of these theories to the West African monsoon reveals a sensitivity to the low-level meridional gradient of equivalent potential temperature, which helps explain observed variability in the monsoon onset. Processes leading to east-west asymmetries in the circulation are also described, and mechanisms linking West African rainfall anomalies with remote events in the tropics are discussed. These dynamical considerations are then placed in the broader context of the ongoing AMMA research program.

Response of tropical global ocean temperature to the Sun's quasi‐decadal UV radiative forcing of the stratosphere

The quasi‐decadal oscillation (QDO) in the Earth's climate system fluctuated in phase with the 11‐year period signal in solar total irradiance (STI) variability throughout the 20th century. The QDO was associated with global average upper ocean temperature variability dominated by the tropical global average from 20°S to 20°N. Earlier diagnosis of the tropical global average oceanic thermal budget found the anomalous warming tendency driven not by the radiative forcing (∼0.15 W m−2) from the 11‐year period signal in STI but by the larger anomalous quasi‐decadal sensible plus latent heat flux (∼0.5 W m−2) from the overlying troposphere. Now we diagnose the corresponding thermal budget of the tropical global average atmospheric temperature variability, finding it largest in the lower stratosphere (∼0.8°K), decreasing downward into the lower troposphere (∼0.15°K) and the upper ocean (∼0.05°K). These diagnostics find quasi‐decadal temperature variability in the tropical lower stratosphere arising from a thermal balance between anomalous radiative forcing (1.0–1.5 W m−2) by ozone absorption of the 11‐year signal in solar UV irradiance variability (modified by absorption of mean solar IR irradiance by volcanic aerosol variability) and variable longwave radiation and vertical advection. The latter two processes altered the vertical gradient of equivalent potential temperature in the tropical troposphere that allowed mean vertical circulation to drive an anomalous warming tendency in the lower troposphere. The latter matched the amplitude and phase of the downward quasi‐decadal sensible plus latent heat flux anomaly that drove the anomalous warming tendency in the tropical global ocean.

A Poisson regression model approach to predicting tropical cyclogenesis in the Australian/southwest Pacific Ocean region using the SOI and saturated equivalent potential temperature gradient as predictors

This paper explores the potential of the Southern Oscillation index (SOI) in combination with the saturated equivalent potential temperature gradient (EPT) as predictors of tropical cyclogenesis in the Australian/southwest Pacific Ocean. This is undertaken using a series of Poisson regression models of tropical cyclogenesis developed on a 2° latitude × 5° longitude and monthly grid. Links between tropical cyclogenesis and the predictors are investigated, with the most significant models cross‐validated, and the skill of their hindcasts evaluated. The September lead SOI‐only Poisson regression model provided skillful predictions of the temporal variability of tropical cyclogenesis across the entire region, with a root‐mean‐square error 22% better than climatology. The combination SOI and EPT model adds spatial skill and further improves temporal skill. Temporal skill is best in the Eastern subregion (western tropical Pacific) (significant correlations with observations at ∼99% level), while spatial skill is best elsewhere.

A Poisson Regression Model of Tropical Cyclogenesis for the Australian–Southwest Pacific Ocean Region

Abstract This paper seeks to address some of the limitations in previous statistical forecast models of tropical cyclogenesis through the development of a series of Poisson regression models on a 2° latitude × 5° longitude spatial grid and a monthly grid in time. The “Gray” parameters [low-level relative vorticity, vertical wind shear parameter, ocean thermal energy, (saturated) equivalent potential temperature gradient, and middle-troposphere humidity] were analyzed as potential predictors of tropical cyclogenesis for the Australian–southwest Pacific Ocean region. Various predictor lead times of up to 5 months were investigated, with the most significant Poisson regression models being cross validated, and the skill of their hindcasts evaluated. The Poisson regression model incorporating a combination of saturated equivalent potential temperature gradients at various leads was found to be the most skillful in hindcasting the temporal (phase and amplitude) variability of tropical cyclogenesis for the Aust...

Mesoscale convective systems along the Meiyu frontin a numerical model

Two organized mesoscale convective systems (MCSs) developed sequentially along the Meiyu front over the Yangzi-Huai River basin and caused severe flooding over eastern China during 12–13 June 1991. In this paper, the structure and evolution of these MCSs are studied with a high-resolution (18 km) numerical simulation using the Fifth Generation Penn-State/NCAR Mesocale Model (MM5). The model reproduced the successive development of these two MCSs along the Meiyu front. The evolution of these MCSs was recorded clearly on satellite-derived cloud-top black body temperature (T bb ) maps. A mesoscale low-level jet (mLLJ) and a mesoscale upper-level jet (mULJ) were simulated, respectively, to the south and east of each of these two MCSs. Our analyses shows that the mLLJ and mULJ were formed as a responses to the intense convection associated with the MCS. The mLLJs transported warm, moist air with equivalent potential temperature greater than 352 K into the MCSs, and strong low-level convergence can be identified on the left-front end of the mLLJ. This strong convergence was associated with intense upward motion in the MCS with speed up to 80 cm s−1. Much of inflow into the MCSs extends up to the middle and upper troposphere, and ventilated through the mULJ. The development of the MCSs was also associated with substantial increase in potential vorticity (PV). The build up of PV in the lower-level along the Meiyu front was in turn related to a local intensification of the frontal equivalent potential temperature gradient, suggesting a relationship between the MCSs and the local enhancement and cyclogenesis of the front. In a sensitivity experiment without the effect of latent heating, a series of ascent centers with average separation of about 300 km were simulated. This result suggests that the initial formation of the MCSs along the Meiyu front could occur in absence of moist-diabatic process. Since the horizontal velocity gradient across the Meiyu front near the synoptic-scale low-level jet (LLJ) was quite large while the corresponding temperature gradient across the frontal zone was rather weak, we speculate that barotropic process may be responsible for triggering these MCSs along the Meiyu front.

Three-Dimensional Computations of Equivalent Potential Vorticity

Abstract Moore and Lambert showed how a quantity called equivalent potential vorticity (EPV) can provide quantitative values to assess conditional symmetric instability (CSI), also known as slantwise instability. Expanding the EPV equation into three dimensions, the equation becomes a function of the geostrophic wind shell, the horizontal equivalent potential temperature gradient, the absolute geostrophic vorticity, and the vertical equivalent potential temperature gradient, all of which are easily computed from gridded data. The equation reduces further by recognizing that the geographic wind shear is a function of the horizontal equivalent potential temperature gradient in a saturated environment. This reduced equation is difficult to evaluate quantitatively because of its dependence on the local value of the moist adiabatic lapse rate. Nevertheless, it shows that horizontal temperature gradients will always act to promote slantwise convection. EPV is a mixture of upright potential instability and slant...

Numerical Simulations of the Madden and Julian Oscillation

Abstract An integration of a general circulation model, with an ocean covered globe (or “aqua planet”), exhibits disturbances that are similar to observed eastward propagating waves of period 30 to 60 days (which we refer to as the Madden and Julian oscillation). The structure of the disturbances resembles a Kelvin wave, although the speed of propagation is slower than anticipated from theory as applied to a dry atmosphere. However, a simple model of the tropical atmosphere demonstrates that the wave speed is sensitive to moisture effects. This notion is confirmed by two further general circulation model experiments in which the latent beat release is increased; in both cases the intrinsic speed of the wave is reduced in inverse proportion to the vertical gradient of equivalent potential temperature. The time-mean circulation of the basic aqua-planet integration exhibits some unusual features; for example a double Hadley cell, with wending branches displaced some 15° either side of the equate. Dynamical r...