Integrated Moisture Convergence Research Articles

Kelvin Waves during GOAmazon and Their Relationship to Deep Convection

AbstractThe 2014–15 Observations and Modeling of the Green Ocean Amazon (GOAmazon) field campaign over the central Amazon near Manaus, Brazil, occurred in coordination with the larger Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud-Resolving Modeling and to the Global Precipitation Measurement (CHUVA) project across Brazil. These programs provide observations of convection over the central Amazon on diurnal to annual time scales. In this study, we address the question of how Kelvin waves, observed in satellite observations of deep cloud cover over the GOAmazon region during the 2014–15 time period, modulate the growth, type, and organization of convection over the central Amazon. The answer to this question has implications for improved predictability of organized systems over the region and representation of convection and its growth on local to synoptic scales in global models. Our results demonstrate that Kelvin waves are strong modulators of synoptic-scale low- to midlevel free-tropospheric moisture, integrated moisture convergence, and surface heat fluxes. These regional modifications of the environment impact the local diurnal cycle of convection, favoring the development of mesoscale convective systems. As a result, localized rainfall is also strongly modulated, with the majority of rainfall in the GOAmazon region occurring during the passage of these systems.

Relating Convection to GCM Grid-Scale Fields Using Cloud-Resolving Model Simulation of a Squall Line Observed during MC3E Field Experiment

In this study, a WRF (Weather Research and Forecasting) model is used as a cloud-resolving model to simulate a squall line observed on 20 May 2011 in the Southern Great Plains (SGP) of the United States. The model output is then used to examine the relationships between convective precipitation and coarse-grained variables averaged over a range of subdomain sizes equivalent to various global climate model horizontal resolutions. The objective is to determine to what extent convection within the subdomains can be related to these “large-scale” variables, thus that they can potentially serve as closure in convective parameterization. Results show that convective precipitation is well correlated with the vertical velocity at 500 hPa, column integrated moisture convergence and CAPE change due to large-scale advective forcing (dCAPE) for various subdomain sizes, but the correlation decreases with decreasing subdomain size. dCAPE leads convective precipitation for all subdomain sizes examined; however, the lead time decreases with decreasing subdomain size. Moisture convergence leads convective precipitation for subdomain sizes greater than 32 km but has no lead time for smaller subdomain sizes. Mid-tropospheric vertical velocity has no lead time or slightly lags convective precipitation. The lead/lag composite analysis with respect to maximum precipitation time indicates that peaks of large-scale variables increase with decreasing subdomain size. The peaks of 500 hPa vertical velocity and column integrated moisture convergence occur at the same time as maximum precipitation, but maximum dCAPE leads maximum precipitation by twelve minutes.

The sensitivity of southeastern United States climate to varying irrigation vigor

AbstractFour regional climate model runs centered on the Southeast United States (SEUS) assuming a crop growing season of May through October are irrigated at 25% (IRR25), 50% (IRR50), 75% (IRR75), and 100% (IRR100) of the root zone porosity to assess the sensitivity of the SEUS climate to irrigation. A fifth run, assuming no irrigation (CTL), is used as the basis for comparison. Across all IRR runs, it is found that there is a general reduction in seasonal mean precipitation over the irrigated cells relative to CTL. This manifests as an increase in dry (0–1 mm/d) days and reduction in > 1 mm/d rainfall events. A comparative moisture budget reveals that area‐averaged precipitation over the irrigated cells displays a reduction in precipitation and runoff in IRR100 with a weaker reduction in IRR25. This is despite an increase in vertically integrated moisture convergence and local evaporation. We find that irrigation increases the lower atmospheric stability, which in turn reduces the convective rainfall over the irrigated areas. Seasonally averaged temperatures reduce over irrigated areas, with the intensity of the reduction increasing with irrigation vigor. This is largely attributed to a repartitioning of sensible heat flux into latent heat flux. There is also, however, a small increase of heat flow to deeper soil layers. Precipitation ahead of transient cold fronts is also reduced by irrigation as they pass over irrigated cells, owing to the increased stability in the lower troposphere. The intensity of this precipitation reduction becomes more intense as irrigation vigor increases. Lastly, heat waves in the SEUS are reduced in intensity over irrigated cells.

An investigation of long-term changes in rainfall over the equatorial Indian Ocean trough region during northern summer using multisatellite data

During the Indian monsoon season, organized convection in the form of inter-tropical convergence zone (ITCZ) originates from the equatorial trough (ET) region over the equatorial Indian Ocean, propagates northward towards the heated Indian landmass at intraseasonal timescales (30–60 days). In this paper, the long-term changes in rainfall over the ET region during the northern summer season has been investigated for a 34-year (1979–2012) period using gauge-adjusted multisatellite Global Precipitation Climatology Project (GPCP) rainfall data set. Rainfall over this region shows a pronounced seasonality and the eastern ET (EET) receives higher rainfall than the western ET (WET) during the northern summer season. Moreover, the northern summer rainfall over the WET and EET are not significantly correlated with each other. Linear trend analysis of domain-mean seasonal rainfall shows a statistically significant increasing trend of 0.4 mm day−1 decade−1 during the northern summer over the WET, whereas no significant trend is observed over the EET. The long-term changes in the associated variables linked through the moisture budget equation are also examined over both regions of ET for the study period. Even though evaporation over both WET and EET shows statistically significant increasing trend associated with an increase in sea surface temperature and near-surface wind, the vertically integrated moisture convergence shows no significant change over the WET whereas it shows a decrease over the EET during the study period. These might be the possible reasons behind a significant increase in rainfall over the WET with an insignificant change in rainfall over the EET.

Moisture budget analysis of SST-driven decadal Sahel precipitation variability in the twentieth century

It is well known that the Sahel region of Africa is impacted by decadal scale variability in precipitation, driven by global sea surface temperatures. This work demonstrates that the National Center for Atmospheric Research’s Community Atmosphere Model, version 4 is capable of reproducing relationships between Sahelian precipitation variability and Indian and Atlantic Ocean sea surface temperature variations on such timescales. Further analysis then constructs a moisture budget breakdown using model output and shows that the change in precipitation minus evaporation in the region is dominated by column integrated moisture convergence due to the mean flow, with the convergence of mass in the atmospheric column mainly responsible. It is concluded that the oceanic forcing of atmospheric mass convergence and divergence to a first order explains the moisture balance patterns in the region. In particular, the anomalous circulation patterns, including net moisture divergence by the mean and transient flows combined with negative moisture advection, together explain the drying of the Sahel during the second half of the twentieth century. Diagnosis of moisture budget and circulation components within the main rainbelt and along the monsoon margins show that changes to the mass convergence are related to the magnitude of precipitation that falls in the region, while the advection of dry air is associated with the maximum latitudinal extent of precipitation.

Baiu Rainband Termination in Atmospheric and Coupled Atmosphere–Ocean Models

Abstract The baiu rainband is a summer rainband stretching from eastern China through Japan toward the northwestern Pacific. The climatological termination of the baiu rainband is investigated using the Japanese 25-yr Reanalysis (JRA-25), a stand-alone atmospheric general circulation model (GCM) forced with observed sea surface temperature (SST) and an atmosphere–ocean GCM (AOGCM). The baiu rainband over the North Pacific abruptly shifts northward and weakens substantially in early July in the atmospheric GCM (AGCM), too early compared to observations (late July). The midtroposphere westerly jet and its thermal advection explain this meridional shift of the baiu rainband, but the ocean surface evaporation modulates the precipitation intensity. In AGCM, deep convection in the subtropical northwestern Pacific sets in prematurely, displacing the westerly jet northward over the cold ocean surface earlier than in observations. The suppressed surface evaporation over the cold ocean suppresses precipitation even though the midtropospheric warm advection and vertically integrated moisture convergence are similar to those before the westerly jet's northward shift. As a result, the baiu rainband abruptly weakens after the northward shift in JRA-25 and AGCM. In AOGCM, cold SST biases in the subtropics inhibit deep convection, delaying the poleward excursion of the westerly jet. As a result, the upward motion induced by both the strong westerly jet and the rainband persist over the northwestern Pacific through summer in the AOGCM. The results indicate that the westerly jet and the ocean evaporation underneath are important for the baiu rainband, the latter suggesting an oceanic effect on this important phenomenon.

Moisture transport over the Arabian Sea associated with summer rainfall over Pakistan in 1994 and 2002

In this study, we aimed to elucidate the critical role of moisture transport affecting monsoon activity in two contrasting summers over the Arabian Sea during the years 1994, a relatively wet year, and 2002, a relatively dry year. A comprehensive diagnostic evaluation and comparisons of the moisture fields were conducted; we focused on the precipitation and evaporation as well as the moisture transport and its divergence or convergence in the atmosphere. Monthly mean reanalysis data were obtained from the National Centers for Environmental Prediction (NCEP-I and -II). A detailed evaluation of the moisture budgets over Pakistan during these two years was made by calculating the latent energy flux at the surface (E-P) from the divergence of the total moisture transport. Our results confirm the moisture supply over the Arabian Sea to be the major source of rainfall in Pakistan and neighboring regions. In 1994, Pakistan received more rainfall compared to 2002 during the summer monsoon. Moisture flow deepens and strengthens over Arabian Sea during the peak summer monsoon months of July and August. Our analysis shows that vertically integrated moisture transport flux have a significant role in supplying moisture to the convective centers over Pakistan and neighboring regions from the divergent regions of the Arabian Sea and the Bay of Bengal. Moreover, in 1994, a deeper vertically integrated moisture convergence progression occurred over Pakistan compared to that in 2002. Perhaps that deeper convergence resulted in a more intense moisture depression over Pakistan and also caused more rainfall in 1994 during the summer monsoon. Finally, from the water budget analysis, it has been surmised that the water budget was larger in 1994 than in 2002 during the summer monsoon.

Hydrological Modeling of Large Drainage Basins Using a GIS-based Hybrid Atmospheric and Terrestrial Water Balance (HATWAB) Model

A Hydrological model is proposed to study the spatial and temporal variability of the water budget components of large drainage basin systems from atmospheric and terrestrial water balances. In order to understand the water balances that include, surface runoff, actual evapotranspiration and soil moisture, a GIS-based simple water balance model which is referred as Hydrological Model from Hybrid Atmospheric and Terrestrial Water Balances with acronym HATWAB is presented. The spatio-temporal climatology database was created from a network of climate stations from CLIMWAT data base to reconstruct the monthly primary inputs to HATWAB model, rainfall and potential evapotranspiration. The modeling principles and HATWAB model are demonstrated using the Limpopo and Congo basins in Africa. The model was used to simulate water balance components by taking rainfall-runoff processes in the basin including soil-texture controlled moisture in the terrestrial system, and the vertical integrated moisture convergence that accounts for the net water vapor flux from the basins in order to close the hydrologic water budget.

Numerical Simulations of the Impacts of Land-Cover Change on Cold Fronts in South-West Western Australia

The south-west of Western Australia has experienced significant land-cover change as well as a decline in rainfall. Given that most precipitation in the region results from frontal passages, the impact of land-cover change on the dynamics of cold fronts is explored using the Regional Atmospheric Modeling System version 6.0. Frontal simulations are evaluated against high resolution atmospheric soundings, station observations, and gridded rainfall analyses and shown to reproduce the qualitative features of cold fronts. Land-cover change results in a decrease in total frontal precipitation through a decrease in boundary-layer turbulent kinetic energy and vertically integrated moisture convergence, and an increase in wind speed within the lower boundary layer. Such processes contribute to reduced convective rainfall under current vegetation cover.

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.

Summertime Response of the Tropical Atmosphere to the Indian Ocean Dipole Sea Surface Temperature Anomalies

The anomalous response of the tropical atmosphere to the recently discovered Indian Ocean Dipole has been studied in the present article, using an Atmospheric General Circulation Model (AGCM), and the NCEP/NCAR Reanalysis. Our AGCM study shows that the response of the atmosphere to the IOD is of dipole—like pattern in the circulation, and is baroclinic. An anomalous circulation in the zonalvertical plane is induced, with the subsidence over colder pole and the upward motion over the warmer pole, modulating the Walker circulation over the equatorial Indian Ocean, as observed. An anomalous circulation in the meridional-vertical plane from the eastern pole of the dipole towards India, and the Bay of Bengal is simulated, which affects the Indian summer monsoon rainfall. The model atmosphere is heated by the anomalous surplus of the latent heat, the net long-wave radiation, and sensible heat fluxes over the warmer pole of the IOD. This results in an excess of net vertically integrated moisture convergence, and excess latent heat release in the atmospheric column above the warm pole. This energy, together with the anomalous convergence of enthalpy, is converted into anomalous mechanical energy and leads to the divergence of mechanical energy flux. This anomalous divergence of the mechanical energy in the upper tropospheric part of the column causes propagation of the disturbances to the surrounding regions. The mean seasonal circulation during the boreal summer is crucial for maintenance of the anomalous energy distribution and propagation during the IOD event. Transient high frequency variability does not appear to contribute much to the energy conversions.

The Climate Signal in Regional Moisture Fluxes: A Comparison of Three Global Data Assimilation Products

This study assesses the quality of estimates of climate variability in moisture flux and convergence from threeassimilated datasets: two are reanalysis products generated at the Goddard Data Assimilation Office and theNational Centers for Environmental Prediction–National Center for Atmospheric Research, and the third consistsof the operational analyses generated at the European Centre for Medium-Range Weather Forecasts (ECMWF).The regions under study (the United States Great Plains, the Indian monsoon region, and Argentina east of theAndes) are characterized by frequent low-level jets and other interannual low-level wind variations tied to thelarge-scale flow. While the emphasis is on the reanalysis products, the comparison with the operational productis provided to help assess the improvements gained from a fixed analysis system. All three analyses capture the main moisture flux anomalies associated with selected extreme climate (droughtand flood) events during the period 1985–93. The correspondence is strongest over the Great Plains and weakestover the Indian monsoon region reflecting differences in the observational coverage. For the reanalysis products,the uncertainties in the lower tropospheric winds is by far the dominant source of the discrepancies in themoisture flux anomalies in the middle latitude regions. Only in the Indian Monsoon region, where interannualvariability in the low-level winds is comparatively small, does the moisture bias play a substantial role. Incontrast, the comparisons with the operational product show differences in moisture that are comparable to thedifferences in the wind in all three regions. Compared with the fluxes, the anomalous moisture convergences show substantially larger differences amongthe three products. The best agreement occurs over the Great Plains region where all three products showvertically integrated moisture convergence during the floods and divergence during the drought with differencesin magnitude of about 25%. The reanalysis products, in particular, show good agreement in depicting the differentroles of the mean flow and transients during the flood and drought periods. Differences between the threeproducts in the other two regions exceed 100% reflecting differences in the low-level jets and the large-scalecirculation patterns. The operational product tends to have locally larger amplitude convergence fields, whichaverage out in area-mean budgets: this appears to be at least in part due to errors in the surface pressure fieldsand aliasing from the higher resolution of the original ECMWF fields. On average, the reanalysis products show higher coherence with each other than with the operational productin the estimates of interannual variability. This result is less clear in the Indian monsoon region where differencesin the input observations appear to be an important factor. The agreement in the anomalous convergence patternsis, however, still rather poor even over relatively data-dense regions such as the United States Great Plains.These differences are attributed to deficiencies in the assimilating general circulation model’s representations ofthe planetary boundary layer and orography, and a global observing system incapable of resolving the highlyconfined low-level winds associated with the climate anomalies.

Prognostic Evaluation of Assumptions Used by Cumulus Parameterizations

Abstract Using a spectral-type cumulus parameterization that includes moist downdrafts within a three-dimensional mesoscale model, various disparate closure assumptions are systematically tested within the generalized framework of dynamic control, static control, and feedback. Only one assumption at a time is changed and tested using a midlatitude environment of severe convection. A control run is presented, which shows good agreement with observations in many aspects. Results of the sensitivity tests are compared to observations in terms of sea level pressure, rainfall patterns, and domain-averaged bias errors (compared to the control run) of various properties. The dynamic control is the part that determines the modulation of the convection by the environment. It is shown that rate of destabilization, as well as instantaneous stability, work well for the dynamic control. Integrated moisture convergence leads to underprediction of rainfall rates and subsequent degrading of the results in terms of movemen...

Time Series of Total Heating and Moistening over the Gulf of Carpentaria Radiosonde Array during AMEX

Abstract Line integral techniques are used to calculate vertically integrated heat and moisture budgets over the Gulf of Carpentaria during Phase II of the Australian Monsoon Experiment (AMEX). The budget area is an array of six radiosondes in a monsoon environment, and the calculations are performed every 6 hours over a period of 33 days. During convective outbreaks the integrated heating and drying of the large scale by the cumulonimbus activity has a magnitude of the order of 10°C day−1. The heat and moisture sources are dominated by the flux divergence terms, which account for over 90% of the variance. The observed warming is as large as ±1°C day−1 but is diurnally dominated and does not correspond to the latent heat release. The integrated moisture convergence has a high correlation with latent heat release but not with the measured moisture storage. The convective heat source is also highly correlated with middle tropospheric vertical velocity. Mean budgets are presented for each of the four diurnal...