Tropical Ocean Global Atmosphere Research Articles

Contribution of cloud condensate to surface rainfall process

Contribution of cloud condensate to surface rainfall processes is investigated in a life span of tropical convection based on hourly data from a two-dimensional cloud resolving simulation. The model is forced by the large-scale vertical velocity, zonal wind and horizontal advections obtained from tropical ocean global atmosphere coupled ocean-atmosphere response experiment (TOGA COARE). The results show that during the genesis, development, and decay of tropical convection, calculations with water vapor overestimate surface rain rate, and cloud condensate plays an important role in correcting overestimation in surface rain rates. The analysis is carried out in deep convective clouds and anvil clouds during the development of tropical convection. The surface rain rates calculated with water vapor in deep convective clouds and anvil clouds have similar magnitudes, the large surface rain rate appears in deep convective clouds due to the consumption of water hydrometeors whereas the small surface rain rate occurs in anvil clouds because of the gain of ice hydrometeors. Further analysis of the grid data shows that the surface rain rates calculated with water vapor and with cloud condensate are negatively correlated with the correlation coefficient of - 0.85, and the surface rain rate calculated with cloud condensate is mainly contributed to the water hydrometeors in the tropical deep convective regime.

Momentum transport processes in the stratiform regions of mesoscale convective systems over the western Pacific warm pool

AbstractMomentum transport by the stratiform components of mesoscale convective systems (MCSs) during the Tropical Ocean–Global Atmosphere Coupled Ocean–Atmosphere Response Experiment in December 1992 is investigated using a cloud‐resolving model. The mesoscale momentum transport by the stratiform regions of MCSs is examined in two distinct large‐scale flow regimes associated with the intraseasonal oscillation over the western Pacific warm pool. Model simulations for 14 December 1992 characterize the ‘westerly onset’ period, which has relatively weak low‐level westerlies with easterlies above. Simulations for 23–24 December represent the ‘strong westerly’ regime, when westerlies extend from the upper troposphere to the surface, with a jet 2–3 km above the surface. In the westerly onset simulation, the extensive stratiform region of a MCS contained a broad region of descent that transported easterly momentum associated with the mid‐level easterly jet downward. Thus, the stratiform regions acted as a negative feedback to decrease the large‐scale mean westerly momentum developing at low levels. In the strong westerly regime, the mesoscale downward air motion in the stratiform regions of large MCSs transported westerly momentum downward and thus acted as a positive feedback, strengthening the already strong westerly momentum at low levels. Momentum fluxes by the mesoscale stratiform region downdraughts are shown to have a systematic and measurable impact on the large‐scale momentum budget. Copyright © 2006 Royal Meteorological Society.

Effect of cirrus clouds on the diurnal cycle of tropical deep convective clouds

The cirrus clouds tightly connected with tropical deep convective clouds can extend and persist for some hours after the deep convective clouds themselves dissipate. This can result in time lags of the diurnal cycle of deep convective clouds detected from infrared satellite measurements with different brightness temperature thresholds because different amounts of cirrus clouds contaminate the measurement. The diurnal cycles of rain from the Tropical Ocean–Global Atmosphere (TOGA) radar during the Tropical Rainfall Measuring Mission (TRMM) Wet Season Atmospheric Mesoscale Campaign (WETAMC) Large‐Scale Biosphere Atmosphere (LBA) Experiment in Amazonia and the diurnal cycles of deep convective clouds and high cold clouds from the Precipitation Radar (PR), Visible and Infrared Scanner (VIRS) onboard the TRMM satellite over the tropics (30°S–30°N) from November 1998 to April 1999 are investigated to study the influence of cirrus clouds on the observed diurnal cycle of tropical deep convective clouds. A 2‐hour time lag of the diurnal cycle of deep convective clouds from the VIRS with respect to that from the PR is found over land. Over ocean the cirrus clouds generated by deep convective clouds enhance the diurnal cycle of the deep convective clouds from the VIRS, and a time lag similar to that over land also occurs. The influence of cirrus clouds leads the diurnal cycle of the deep convective clouds from the VIRS to depend strongly on the selected IR threshold and to be very different from that of the PR over the maritime continent. Moreover, over ocean and the maritime continent, from late afternoon to midnight the strong increase of the deep convective clouds from the VIRS is mainly due to the developing cirrus clouds near and above the tropical tropopause layer.

Two- and Three-Dimensional Cloud-Resolving Model Simulations of the Mesoscale Enhancement of Surface Heat Fluxes by Precipitating Deep Convection

Abstract Two-dimensional (2D) and three-dimensional (3D) cloud-resolving model (CRM) simulations are conducted to quantify the enhancement of surface sensible and latent heat fluxes by tropical precipitating cloud systems for 20 days (10–30 December 1992) during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The mesoscale enhancement appears to be analogous across both 2D and 3D CRMs, with the enhancement for the sensible heat flux accounting for 17% of the total flux for each model and the enhancement for the latent heat flux representing 18% and 16% of the total flux for 2D and 3D CRMs, respectively. The convection-induced gustiness is mainly responsible for the enhancement observed in each model simulation. The parameterization schemes of the mesoscale enhancement by the gustiness in terms of convective updraft, downdraft, and precipitation, respectively, are examined using each version of the CRM. The scheme utilizing the precipitation was found to yield the most desirable estimations of the mean fluxes with the smallest rms error. The results together with previous findings from other studies suggest that the mesoscale enhancement of surface heat fluxes by the precipitating deep convection is a subgrid process apparent across various CRMs and is imperative to incorporate into general circulation models (GCMs) for improved climate simulation.

Estimation of Oceanic Precipitation Efficiency in Cloud Models

Abstract Precipitation efficiency is estimated based on vertically integrated budgets of water vapor and clouds using hourly data from both two-dimensional (2D) and three-dimensional (3D) cloud-resolving simulations. The 2D cloud-resolving model is forced by the vertical velocity derived from the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The 3D cloud-resolving modeling is based on the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) simulation of Typhoon Nari (in 2001). The analysis of the hourly moisture and cloud budgets of the 2D simulation shows that the total moisture source (surface evaporation and vertically integrated moisture convergence) is converted into hydrometeors through vapor condensation and deposition rates regardless of the area size where the average is taken. This leads to the conclusion that the large-scale and cloud-microphysics precipitation efficiencies are statistically equivalent. Results further show that convergence (divergence) of hydrometeors would make precipitation efficiency larger (smaller). The precipitation efficiency tends to be larger (even >100%) in light rain conditions as a result of hydrometeor convergence from the neighboring atmospheric columns. Analysis of the hourly moisture and cloud budgets of the 3D results from the simulation of a typhoon system with heavy rainfall generally supports that of 2D results from the simulation of the tropical convective system with moderate rainfall intensity.

A Statistical Approach to Ground Radar-Rainfall Estimation

Abstract This paper presents development of a statistical procedure for estimation of ensemble rainfall fields from a combination of ground radar observations and in situ rain gauge measurements. The uncertainty framework characterizes radar-rainfall estimation algorithm limitation accounting for rain gauge sampling uncertainty. The procedure is applied on a multicomponent rainfall estimation algorithm, which utilizes a rain-path attenuation correction technique, a power-law reflectivity-to-rainfall (Z–R) relationship, and a parameter to differentiate between convective (C) and stratiform (S) regimes in the Z–R conversion. Uncertainty is explicitly accounted for by evaluating the algorithm’s parameter set posterior probability density function (known as parameters’ equifinality) on the basis of the Generalized Likelihood Uncertainty Estimation (GLUE) framework. The study is facilitated by NASA’s C-band Doppler radar [named the Tropical Ocean Global Atmosphere (TOGA)] observations and four dense rain gauge clusters available from the Tropical Rainfall Measuring Mission (TRMM)-Large-Scale Biosphere–Atmosphere (LBA) experiment, conducted between January and February of 1999 in Southwest Amazon. Statistics are proposed for jointly evaluating the wideness of radar retrieval uncertainty limits [uncertainty ratio (UR)] and the percentage of observations that fall within those error bounds [exceedance ratio (ER)]. Results show that the parameter range selected in GLUE could characterize the radar-rainfall estimation uncertainty. Combined assessment of UR and ER for a varying range of parameters’ equifinality provides an objective basis for comparing rain retrieval algorithms and determining uncertainty bounds. Ensemble radar-rainfall fields derived on the basis of this procedure can be used to statistically assess satellite rain retrieval algorithms and derive ensemble hydrologic predictions driven by radar-rainfall input (e.g., runoff and soil moisture).

Resolving SSM/I-ship radar rainfall discrepancies from AIP-3

The third algorithm intercomparison project (AIP-3) involved rain estimates from more than 50 satellite rainfall algorithms and ground radar measurements within the Intensive Flux Array (IFA) over the equatorial western Pacific warm pool region during the Tropical Ocean Global Atmosphere coupled Ocean-Atmosphere Response Experiment (TOGA COARE). Early results indicated that there was a systematic bias between rainrates from satellite passive microwave and ground radar measurements. The mean rainrate from radar measurements is about 50% underestimated compared to that from passive microwave-based retrieval algorithms. This paper is designed to analyze rain patterns from the Florida State University rain retrieval algorithm and radar measurements to understand physically the rain discrepancies. Results show that there is a clear range-dependent bias associated with the radar measurements. However, this range-dependent systematical bias is almost eliminated with the corrected radar rainrates. Results suggest that the effects from radar attenuation correction, calibration and beam filling are the major sources of rain discrepancies. This study demonstrates that rain retrievals based on satellite measurements from passive microwave radiometers such as the Special Sensor of Microwave Imager (SSM/I) are reliable, while rain estimates from ground radar measurements are correctable.

Tropical heat/water vapor quasi-equilibrium and cycle as simulated in a 2D cloud-resolving model

The heat/water vapor equilibrium and cycle in the tropical deep convective regime are analyzed using a two-dimensional cloud-resolving simulation. The model is forced by the large-scale vertical velocity, zonal wind and large-scale horizontal advection derived from Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) for a 20-day period. The analysis shows a near perfect balance between vertical thermal advection and latent heat of condensation in the mass-weighted mean heat budget and between vertical moisture advection and precipitation in the precipitable water budget. The local changes of mass-weighted mean temperature and precipitable water are mainly determined by the heat and moisture residuals, respectively. The error analysis shows that the error of the local thermal and moisture changes could be 5–10 times larger than the errors introduced by the condensation and vertical advections. The condensation and associated latent heat determine local moisture loss and thermal gain for strong convection, whereas the vertically advective moistening and cooling cause local moisture gain and thermal loss for weak convection, forming a tropical heat/water recycling mechanism. The enhanced rainfall associated with the convection consumes the instability energy and moisture source, and causes the decay of convection. The suppression of convection allows the restorations of CAPE and moisture source for the favorable conditions of development of convection.

Surface rainfall processes as simulated in a cloud‐resolving model

Surface rain rate can be simply formulated with the sum of moisture and cloud sources/sinks. In this study the moisture sink comprises the local moisture change, moisture convergence (with an imposed vertical velocity), and surface evaporation, whereas the cloud source/sink comprises the local hydrometeor change since the cyclic boundary condition leads to zero hydrometeor convergence. The sources/sinks and their contributions to the surface rain rate are examined based on hourly zonal mean simulation data from a two‐dimensional cloud‐resolving model. The model is forced by the large‐scale vertical velocity, zonal wind, and horizontal advections obtained from Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE). Although variation in the moisture sink largely accounts for much of the variation in the surface rain rate, the cloud source/sink may modify the surface rain rate significantly. The magnitude of the cloud source/sink increases when the zonal mean surface rain rate increases from 0 to 1 mm h−1, and it decreases when the rain rate increases from 1 to 2 mm h−1. The cloud source/sink is further analyzed by breaking it into ice and water hydrometeors. The ice hydrometeors may account for more contributions to the cloud variations than the water hydrometeors, and their growth may lead the surface rain rate by 1–2 hours.

Moisture Tendency Equations in a Tropical Atmosphere

AbstractDirect diagnostic evaluation of the moisture tendency in the moisture equation is very difficult in practice because two poorly measured terms, moisture convergence and precipitation, dominate the equation. Using the near constancy in space and time of the tropical temperature profile, a variety of energy and entropy based equations have been employed to obtain indirect estimates of the moisture budget. In this paper rigorous versions of the energy and entropy equations are developed. The strengths and weakness of these two formulations are complementary, allowing the authors to estimate moistening and drying tendencies with high confidence when the two formulations are used together.This study applies the theory to data from Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The results demonstrate how convection regulates the tropospheric humidity, drying the atmosphere when it becomes too moist, and moistening it when it becomes too dry.

Modeling Diurnal and Intraseasonal Variability of the Ocean Mixed Layer

Abstract The intraseasonal variability of SST associated with the passage of the Madden–Julian oscillation (MJO) is well documented; yet coupled model integrations generally underpredict the magnitude of this SST variability. Observations from the Improved Meteorological Instrument (IMET) mooring in the western Pacific during the intensive observing period (IOP) of the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) showed a large diurnal signal in SST that is modulated by the passage of the MJO. In this study, observations from the IOP of the TOGA COARE and a one-dimensional (1D) ocean mixed layer model incorporating the K-Profile Parameterization (KPP) vertical mixing scheme have been used to investigate the rectification of the intraseasonal variability of SST by the diurnal cycle and the implied impact of the absence of a representation of this process on the modeled intraseasonal variability in coupled GCMs. Analysis of the SST observations has shown that the increase of the daily mean SST by the diurnal cycle of SST accounts for about one-third of the magnitude of intraseasonal variability of SST associated with the Madden–Julian oscillation in the western Pacific warm pool. Experiments from the 1D model forced with fluxes at a range of temporal resolutions and with differing vertical resolution of the model have shown that to capture 90% of the diurnal variability of SST, and hence 95% of the intraseasonal variability of SST, requires a 3-h or better temporal resolution of the fluxes and a vertical grid with an upper-layer thickness of the order of 1 m. In addition to the impact of the representation of the diurnal cycle on the intraseasonal variability of SST, the strength of the mixing across the thermocline was found to be enhanced by the proper representation of the nighttime deep mixing in the ocean, implying a possible impact of the diurnal cycle onto the mean climate of the tropical ocean.

Organization of Oceanic Convection during the Onset of the 1998 East Asian Summer Monsoon

Abstract The organizational modes of convection over the northern South China Sea (SCS) during the onset of the summer monsoon are documented using radar and sounding data from the May–June 1998 South China Sea Monsoon Experiment (SCSMEX). The onset occurred in mid-May with a rapid increase in deep convection over a 10-day period, accompanied by a major shift in the circulation over the east Asian region. Analysis of Bureau of Meteorology Research Centre (BMRC) radar data from Dongsha Island reveals a wide range of organizational modes of convection over the northern SCS. Proximity sounding data indicate that lower- and middle-level vertical wind shears exerted a dominant control over the orientation of convective lines within mesoscale convective systems in this region, as has been found in the Australian monsoon region and the equatorial western Pacific. The results are consistent with the conceptual model of LeMone et al. based on the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE), except two new organizational modes have been identified: shear-parallel bands for strong low-level shear and weak midlevel shear when there is weak instability and the air is dry aloft, and shear-parallel bands for strong shears in both layers when the shear vectors are in the same direction. Midlatitude influences, namely, the passage of troughs over southern China, likely contributed to these two additional modes. The stratiform rain fraction from the convective systems during the monsoon onset period was relatively small (26%) compared to the estimated average of about 40% for the entire Tropics. This small fraction is attributed to the weak instability during the onset period and relatively dry air in the upper troposphere.

A Tendency of Cloud Ratio Associated with the Development of Tropical Water and Ice Clouds

The ratio of column-integrated cloud content between ice and liquid phases (CR) is a good indicator of cloud dynamics (e.g., convective or stratiform) and radiative properties. Clouds are more stratiform when the CR is larger and more convective for smaller CR values. In this study, a tendency equation of the cloud ratio is derived. The tendency of the cloud ratio (∂CR/∂t) is determined by six major cloud micro physical processes: vapor condensation and deposition as cloud sources, rainfall and evaporation of rain as the sinks of water clouds, and melting of graupel and accretion of cloud water by precipitation ice as the major conversion processes between water and ice clouds. Apparently, ∂CR/∂t is related to water cycling processes among vapor and different categories of clouds, and links the amount of water vapor and clouds with temperature through latent heating. An analysis of the tendency of the cloud ratio is carried out using hourly zonal-mean data based on a 2D cloud-resolving simulation with the imposed forcing from the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. During the genesis and decay stages of clouds (a zonal-mean surface rain rate of smaller than 0.3 mm h^(-1)), the tendency of the cloud ratio is mainly controlled by the processes related to the vapor condensation and deposition. During the mature stage of clouds (a zonal-mean surface rain rate of larger than 0.3 mm h^(-1)), the tendency of the cloud ratio is determined by conversion between water and ice clouds through melting of graupel and accretion of cloud water by precipitation ice.

Tropical convective responses to microphysical and radiative processes: a sensitivity study with a 2-D cloud resolving model

Prognostic cloud schemes are increasingly used in weather and climate models in order to better treat cloud-radiation processes. Simplifications are often made in such schemes for computational efficiency, like the scheme being used in the National Centers for Environment Prediction models that excludes some microphysical processes and precipitation-radiation interaction. In this study, sensitivity tests with a 2D cloud resolving model are carried out to examine effects of the excluded microphysical processes and precipitation-radiation interaction on tropical thermodynamics and cloud properties. The model is integrated for 10 days with the imposed vertical velocity derived from the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. The experiment excluding the depositional growth of snow from cloud ice shows anomalous growth of cloud ice and more than 20% increase of fractional cloud cover, indicating that the lack of the depositional snow growth causes unrealistically large mixing ratio of cloud ice. The experiment excluding the precipitation-radiation interaction displays a significant cooling and drying bias. The analysis of heat and moisture budgets shows that the simulation without the interaction produces more stable upper troposphere and more unstable mid and lower troposphere than does the simulation with the interaction. Thus, the suppressed growth of ice clouds in upper troposphere and stronger radiative cooling in mid and lower troposphere are responsible for the cooling bias, and less evaporation of rain associated with the large-scale subsidence induces the drying in mid and lower troposphere.

Clouds and Shortwave Fluxes at Nauru. Part II: Shortwave Flux Closure

Abstract The datasets currently being collected by the Atmospheric Radiation Measurement (ARM) program on the islands of Nauru and Manus represent the longest time series of ground-based cloud measurements in the tropical western Pacific region. In this series of papers, a shortwave flux closure study is presented using observations collected at the Nauru site between June 1999 and May 2000. The first paper presented frequency of occurrence of nonprecipitating clouds detected by the millimeter-wavelength cloud radar (MMCR) at Nauru and statistics of their retrieved microphysical properties. This paper presents estimates of the cloud radiative effect over the study period and results from a closure study in which retrieved cloud properties are input to a radiative transfer model and the modeled surface fluxes are compared to observations. The average surface shortwave cloud radiative forcing is 48.2 W m−2, which is significantly smaller than the cloud radiative forcing estimates found during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) field project. The difference in the estimates during the two periods is due to the variability in cloud amount over Nauru during different phases of the El Niño–Southern Oscillation (ENSO). In the closure study, modeled and observed surface fluxes show large differences at short time scales, due to the temporal and spatial variability of the clouds observed at Nauru. Averaging over 60 min reduces the average root-mean-square difference in total flux to 10% of the observed flux. Modeled total downwelling fluxes are unbiased with respect to the observed fluxes while direct fluxes are underestimated and diffuse fluxes are overestimated. Examination of the differences indicates that cloud amount derived from the ground-based measurements is an overestimate of the radiatively important cloud amount due to the anisotropy of the cloud field at Nauru, interpolation of the radar data, uncertainty in the microwave brightness temperature measurements for thin clouds, and the uncertainty in relating the sixth moment of the droplet size distribution observed by the radar to the more radiatively important moments.

Wind shear effects on cloud‐radiation feedback in the western Pacific warm pool

Upper tropospheric stratiform clouds associated with deep convection are important to global radiation budgets and to cloud‐radiation feedbacks on climate variability and change. Several recent observational studies indicate that vertical wind shear is an important factor affecting stratiform cloud fraction and cloud overlap. This study further examines wind shear effects on cloud properties (including cloud fraction and cloud optical depth) and associated top of atmosphere (TOA) and surface radiative fluxes, using observations from the Tropical Ocean Global Atmosphere program's Coupled Ocean Atmosphere Response Experiment (TOGA COARE) experiment and long‐term satellite measurements. Wind shear affects cloud‐radiative fluxes, through both the cloud fraction and optical thickness, in a strong and systematic way. In typical convecting conditions, shear‐induced additional cloudiness can reduce outgoing longwave radiation (OLR) by 10s of Wm−2, implying longwave radiative changes on the order of 10% of the total latent heating. Such cloud also reflects shortwave radiation, reducing surface downward flux (energy input to the ocean) by 10s of Wm−2. Current climate models lack these effects.

Radiation Budget of the Tropical Intraseasonal Oscillation

This study examines the relationship between precipitation and radiative heating on intraseasonal time scales in the Tropics using collocated top-of-atmosphere (TOA) and surface radiative flux measurements from special field program data [Atmospheric Radiation Measurement (ARM) Program and Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) experiments] as well as long-term TOA flux data [from Earth Radiation Budget Experiment (ERBE) and Advanced Very High Resolution Radiometer (AVHRR) satellite data]. All the different datasets consistently show that the atmosphere-integrated radiative heating is nearly in phase with the precipitation and enhances the net condensation heating by about 10%–15%. The dominant contribution to this radiative warming during wet periods is the reduction of TOA outgoing longwave radiation (OLR), primarily by clouds but with a small contribution by water vapor. This radiative heating is reduced slightly by enhanced surface downwelling longwave radiation, attributable to low cloud bases and reduced atmospheric shortwave absorption attributable to shadowing by high cloud tops. The intraseasonal budget of TOA radiation, reflecting heating of the whole ocean plus atmosphere column, is characterized by shortwave cloud forcing anomalies that are substantially larger than the longwave cloud forcing anomaly. This imbalance is in contrast with the near cancellation between these two terms at the seasonal time scale.

A convective vorticity vector associated with tropical convection: A two‐dimensional cloud‐resolving modeling study

Although dry/moist potential vorticity (( · ∇θe)/ρ) is a useful physical quantity for meteorological analysis, it cannot be applied to the analysis of two‐dimensional (2‐D) simulations. A new vorticity vector ( × ∇θe)/ρ (convective vorticity vector (CVV)) is introduced in this study to analyze 2‐D cloud‐resolving simulation data associated with 2‐D tropical convection. The cloud model is forced by the vertical velocity, zonal wind, horizontal advection, and sea surface temperature obtained from the Tropical Ocean‐Global Atmosphere (TOGA) Coupled Ocean‐Atmosphere Response Experiment (COARE) and is integrated for a selected 10‐day period. The CVV has zonal and vertical components in the 2‐D x‐z frame. Analysis of zonally averaged and mass‐integrated quantities shows that the correlation coefficient between the vertical component of the CVV and the sum of the cloud hydrometeor mixing ratios is 0.81, whereas the correlation coefficient between the zonal component and the sum of the mixing ratios is only 0.18. This indicates that the vertical component of the CVV is closely associated with tropical convection. The tendency equation for the vertical component of the CVV is derived and the zonally averaged and mass‐integrated tendency budgets are analyzed. The tendency of the vertical component of the CVV is determined by the interaction between the vorticity and the zonal gradient of cloud heating. The results demonstrate that the vertical component of the CVV is a cloud‐linked parameter and can be used to study tropical convection.

The Large-Scale Modulation of Subtropical Cyclogenesis in the Central and Eastern Pacific Ocean

Abstract An analysis of the composite large-scale circulation associated with periods of enhanced (active) or diminished (inactive) cyclogenesis in the subtropical central and eastern Pacific Ocean is presented. Composites were constructed using surface and tropospheric analyses from the ECMWF Tropical Ocean Global Atmosphere (TOGA) dataset for 10 Northern Hemisphere cool seasons (1986–96). Active periods of subtropical cyclogenesis were defined to be periods in which two or more cyclones developed in close succession to each other, while inactive periods were defined to be periods of at least 10-days duration during which no cyclones with a subtropical origin were present in the Pacific basin. The analysis revealed that the occurrence of subtropical cyclones in the central and eastern Pacific Ocean is strongly linked to the strength and location of the Asian jet, with active periods characterized by a weaker, zonally retracted Asian jet while inactive periods are characterized by a stronger, zonally elon...

A Synoptic Climatology of the Subtropical Kona Storm

Ten years of surface and upper-air analyses from the ECMWF Tropical Ocean Global Atmosphere (TOGA) dataset were used to construct a synoptic climatology of kona storms in the subtropical central and eastern Pacific Ocean. Within a sample of 115 cyclones that predominantly occurred during the Northern Hemisphere cool season, three distinct types of kona storms were identified: cold-frontal cyclogenesis (CFC) cyclones, coldfrontal cyclogenesis/trade wind easterlies (CT) cyclones, and trade wind easterlies (TWE) cyclones. Of the three types, CFC cyclones were found to be the most common type of kona storm, while CT and TWE cyclones occur much less frequently. The geographical distribution, propagation characteristics, and the monthly and interannual variability in the number of kona storms are presented. Kona storms initially develop across a large portion of the subtropical Pacific, with the greatest concentration of kona storms found within a southwest-to-northeast-oriented band from west of Hawaii to 408N, 1408W. A distinct latitudinal stratification was evident for each type of kona storm, with CFC, CT, and TWE cyclones each more likely to initially develop at successively lower latitudes. The analysis reveals that kona storms can propagate in any direction but exhibit a clear preference to propagate toward the northeast. Use of the multivariate ENSO index indicates that the number of kona storms that develop during each cool season is not correlated to the phase of ENSO. An analysis of the composite structure and evolution of each type of kona storm revealed some common and some unique characteristics. Development of the surface cyclone in all types results from the intrusion of an upper-level disturbance of extratropical origin into the subtropics, although differences in the initial structure and subsequent evolution of the 300-hPa trough were noted for each type of kona storm. The analysis also revealed that relatively weak 300-hPa winds are present throughout the evolution of each type of kona storm and that the composite kona storm tends to be nestled along the southern boundary of a region of higher surface pressure during the mature stage of its evolution. The development of robust ridges in the 300-hPa geopotential and 1000‐500-hPa thickness fields downstream of the composite surface cyclone were noteworthy features that characterized the evolution of all kona storms, the latter feature strongly suggesting that these disturbances are fundamentally baroclinic in nature.