Patrick T. Haertel

Zonal and Vertical Structure of the Madden–Julian Oscillation

Abstract A statistical study of the three-dimensional structure of the Madden–Julian oscillation (MJO) is carried out by projecting dynamical fields from reanalysis and radiosonde data onto space–time filtered outgoing longwave radiation (OLR) data. MJO convection is generally preceded by low-level convergence and upward motion in the lower troposphere, while subsidence, cooling, and drying prevail aloft. This leads to moistening of the boundary layer and the development of shallow convection, followed by a gradual and then more rapid lofting of moisture into the middle troposphere at the onset of deep convection. After the passage of the heaviest rainfall, a westerly wind burst region is accompanied by stratiform precipitation, where lower tropospheric subsidence and drying coincide with continuing upper tropospheric upward motion. The evolution of the heating field leads to a temperature structure that favors the growth of the MJO. The analysis also reveals distinct differences in the vertical structure...

Dynamics of 2-day equatorial waves

Abstract The dynamics of the 2-day wave, a type of convectively coupled disturbance that frequents the equatorial western Pacific, is examined using observations and a linear primitive equation model. A statistical composite of the waves kinematic and thermodynamic structure is presented. It is shown that 1) the waves wind and temperature perturbations can be modeled as linear responses to convective heating and cooling, and 2) the bulk of the waves dynamical and convective structure can be represented with two vertical modes. The observations and model results suggest that the 2-day wave is an n = 1 westward-propagating inertio–gravity wave with a shallow equivalent depth (14 m) that results from the partial cancelation of adiabatic temperature changes due to vertical motion by convective heating and cooling.

Corrected TOGA COARE Sounding Humidity Data: Impact on Diagnosed Properties of Convection and Climate over the Warm Pool

This study reports on the humidity corrections in the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean‐Atmosphere Response Experiment (COARE) upper-air sounding dataset and their impact on diagnosed properties of convection and climate over the warm pool. During COARE, sounding data were collected from 29 sites with Vaisala-manufactured systems and 13 sites with VIZ-manufactured systems. A recent publication has documented the characteristics of the humidity errors at the Vaisala sites and a procedure to correct them. This study extends that work by describing the nature of the VIZ humidity errors and their correction scheme. The corrections, which are largest in lower-tropospheric levels, generally increase the moisture in the Vaisala sondes and decrease it in the VIZ sondes. Use of the corrected humidity data gives a much different perspective on the characteristics of convection during COARE. For example, application of a simple cloud model shows that the peak in convective mass flux shifts from about 88N with the uncorrected data to just south of the equator with corrected data, which agrees better with the diagnosed vertical motion and observed rainfall. Also, with uncorrected data the difference in mean convective available potential energy (CAPE) between Vaisala and VIZ sites is over 700 J kg21; with the correction, both CAPEs are around ;1300 J kg21, which is consistent with a generally uniform warm pool SST field. These results suggest that the intensity and location of convection would differ significantly in model simulations with humidity-corrected data, and that the difficulties which the reanalysis products had in reproducing the observed rainfall during COARE may be due to the sonde humidity biases. The humidity-corrected data appear to have a beneficial impact on budget-derived estimates of rainfall and radiative heating rate, such that revised estimates show better agreement with those from independent sources.

Vertical-Mode Decompositions of 2-Day Waves and the Madden–Julian Oscillation

Abstract Vertical structures of 2-day waves and the Madden–Julian oscillation (MJO) are projected onto vertical normal modes for a quiescent tropical troposphere. Three modes capture the gross tropospheric structure of 2-day waves, while only two modes are needed to represent most of the baroclinic structure of the MJO. Deep circulations that project onto the first baroclinic mode are associated with deep cumulonimbus and stratiform rainfall. Shallow circulations that project onto higher wavenumber modes are associated with precipitating shallow cumulus and congestus and stratiform rainfall. For both disturbances the horizontal divergence contributed by shallow modes is an important factor in the column-integrated moist enthalpy budget. These modes converge moist static energy for a time prior to when deep circulations export moist static energy. These results highlight the importance of properly representing the effects of shallow cumulus, congestus, and stratiform precipitation in theories of convective...

An Analysis of Convectively Coupled Kelvin Waves in 20 WCRP CMIP3 Global Coupled Climate Models

Abstract Output from 20 coupled global climate models is analyzed to determine whether convectively coupled Kelvin waves exist in the models, and, if so, how their horizontal and vertical structures compare to observations. Model data are obtained from the World Climate Research Program’s (WCRP’s) Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel dataset. Ten of the 20 models contain spectral peaks in precipitation in the Kelvin wave band, and, of these 10, only 5 contain wave activity distributions and three-dimensional wave structures that resemble the observations. Thus, the majority (75%) of the global climate models surveyed do not accurately represent convectively coupled Kelvin waves, one of the primary sources of submonthly zonally propagating variability in the tropics. The primary feature common to the five successful models is the convective parameterization. Three of the five models use the Tiedtke–Nordeng convective scheme, while the other two utilize the Pan and Randall scheme...

The Linear Dynamics of Squall Line Mesohighs and Wake Lows

Abstract Two surface pressure features that commonly accompany squall lines, the mesohigh and the wake low, may be explained at least in part as a linear response to the low-level cooling associated with stratiform precipitation. This response is numerically simulated for two- and three-dimensional, moving and stationary low-level coolings characteristic of squall line stratiform regions. When the cooling is defined to move and to have three-dimensional structure both a mesohigh and a mesolow develop, and their structures and evolutions resemble those of squall line mesohighs and wake lows. When an upper boundary is introduced directly above the cooling the response approaches a steady state in which a mesohigh–mesolow couplet is centered on the cooling. The simulations and the steady-state analysis presented here help to explain observed characteristics of squall line mesohighs and wake lows, including their life cycles and positioning relative to precipitation structures.

Some Simple Simulations of Thunderstorm Outflows

Three idealized simulations of thunderstorm outflows are presented. Each outflow is a response to an instantaneous low-level cooling. The vertical structures of the coolings differ as do the environments in which the outflows form, and consequently the dynamics of the outflows differ. One outflow is a gravity current, another is a gravity wave, and the third comprises both a gravity current and a gravity wave. The horizontal transport of mass is important for the advance of the gravity-current outflow, but not for the gravity wave outflow, and it is suggested that this is the defining dynamical distinction between the two outflows. The simulations are compared to observations and it is suggested that some outflows previously characterized as gravity currents may better fit the gravity wave or gravity current/wave archetypes. It is also noted that the gravity wave component of an outflow may be generated directly by low-level cooling in addition to the commonly suggested mechanism of the interaction of a gravity current with a stable layer.

Could a Pile of Slippery Sacks Behave Like an Ocean

Abstract A method for simulating fluid motions that shows promise for application to the oceans is explored. Incompressible inviscid fluids with free surfaces are represented as piles of slippery sacks. A system of ordinary differential equations governs the motions of the sacks, and this system is solved numerically in order to simulate a nonlinear deformation, internal and external gravity waves, and Rossby waves. The simulations are compared to analytic and finite-difference solutions, and the former converge to the latter as the sizes of the sacks are decreased. The slippery-sack method appears to be well suited to ocean modeling for the following reasons: 1) it perfectly conserves a fluids distributions of density and tracers; 2) unlike existing isopycnic models the slippery-sack method is capable of representing a continuum of fluid densities and vertically resolving neutral regions; 3) the inclusion of continuous topography adds no numerical complexity to the slippery-sack method; 4) the slippery-...

The Ventilated Ocean

AbstractAdiabatic theories of ocean circulation and density structure have a long tradition, from the concept of the ventilated thermocline to the notion that deep ocean ventilation is controlled by westerly winds over the Southern Ocean. This study explores these ideas using a recently developed Lagrangian ocean model (LOM), which simulates ocean motions by computing trajectories of water parcels. A unique feature of the LOM is its capacity to model ocean circulations in the adiabatic limit, in which water parcels exactly conserve their densities when they are not in contact with the ocean surface. The authors take advantage of this property of the LOM and consider the circulation and stratification that develop in an ocean with a fully adiabatic interior (with both isopycnal and diapycnal diffusivities set to zero). The ocean basin in the study mimics that of the Atlantic Ocean and includes a circumpolar channel. The model is forced by zonal wind stress and a density restoring at the surface.Despite the...

Developing High-Quality Field Program Sounding Datasets

Enormous resources of time, effort, and finances are expended in collecting field program rawinsonde (sonde) datasets. Correcting the data and performing quality control (QC) in a timely fashion after the field phase of an experiment are important for facilitating scientific research while interest is still high and funding is available. However, a variety of issues (different sonde types, ground station software, data formats, quality control issues, sonde errors, etc.) often makes working with these datasets difficult and time consuming. Our experience working with sounding data for several field programs has led to the design of a general procedure for creating user-friendly, bias-reduced, QCed sonde datasets. This paper describes the steps in this procedure, gives examples for the various processing stages, and provides access to software tools to aide in this process.