Abstract

We investigate the temporal and spatial characteristics of unstable normal modes in a horizontally sheared flow on a sphere using the shallow water equations. Both inertial and barotropic instabilities are identified in cases where the appropriate necessary conditions are satisfied.

A primary focus is determining what conditions favor asymmetric modes of inertial instability rather than symmetric modes. With the Bickley jet profile, the region of instability [f(f + &xi) ≤ 0] is confined to the anticyclonic side of the jet in a limited region. We find that symmetric instability is preferred only for modes of very small vertical wide, for which the pressure gradient force is secondary. Relatively small dissipation is needed to stabilize these modes. With deeper vertical scales, asymmetric instabilities are preferred in which the zonal scale of the instability is comparable to the width of the unstable region.

This study extends previous results for linear shear on an equatorial beta plane to the midlatitude jet case. Our results suggest that deep atmospheric circulations in spatially confined regions of negative potential vorticity may develop as asymmetric rather than symmetric instabilities.

Abstract

Observations from two enhanced sounding arrays during the May–June 1998 South China Sea Monsoon Experiment (SCSMEX) are used to determine and contrast the properties of convection over the northern and southern South China Sea (SCS). A regression analysis between SST data and monthly rainfall indicates that the ENSO signal exerted a strong influence on the rainfall distribution over the SCS during SCSMEX. This resulted in wetter-than-normal conditions along the south China coast and northern SCS, and generally drier-than-average conditions elsewhere, particularly over the Philippine Islands.

The monsoon onset as determined by a shift in the low-level winds from easterly to southwesterly over the SCS occurred around mid-May. Over the southern enhanced sounding array (SESA), the onset was characterized by a rainy period associated with the passage of a convectively coupled Kelvin wave. This was followed by a weeklong break and then several episodic rain events with increasingly higher rain rates. Rainfall over the northern enhanced sounding array (NESA), which was largely out of phase with SESA rainfall events, occurred primarily during two 10-day periods separated by a weeklong break. Convective characteristics over the SESA, deduced primarily from heat and moisture budget profiles, indicate a high stratiform rain fraction consisting of alternating periods with decaying mesoscale systems that organized near the western Borneo coastline and shallower convective clouds. In contrast, NESA-averaged profiles were indicative of deep convection with a relatively small stratiform rain fraction, which was confirmed with radar analyses during the onset convective period.

The diurnal cycle of convection is a dominant feature throughout much of the SCS. Over both budget regions, early morning (0500–0800 LT) convective systems were frequently initiated near the coasts, then gradually dissipated during the course of the day as the midlevel steering currents moved the systems away from the coastline. These decaying convective systems resulted in an early afternoon (1400 LT) rainfall peak over both sonde arrays.

Abstract

Atmospheric heat and moisture budgets are used to determine rainfall and radiative heating rates over the western Pacific warm pool during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). Results are compared to independent estimates of these quantities from the other sources. Using the COARE bulk flux algorithm to estimate surface evaporation over the intensive flux array (IFA), the IFA moisture budget-derived average rainfall for the 120-day intensive observing period (IOP) is 8.2 mm day⁻¹. This value agrees closely with recent estimates from satellites and the ocean salinity budget. For a smaller area within the IFA containing the rain-mapping domain of the TOGA and Massachusetts Institute of Technology 5-cm radars, the atmospheric budget for the 101-day radar deployment yields 6.8 mm day⁻¹, slightly greater than the independent radar rain rate estimate of 5.4 mm day⁻¹.

Comparison of budget-derived rainfall with National Centers for Environmental Prediction and European Centre for Medium-Range Weather Forecasts reanalyses indicates that the reanalyses produce excessive precipitation in the northern ITCZ (around 10°N) in association with anomalously moist low-level conditions at those latitudes. These anomalous conditions arise from moist-biased VIZ humidity sensors on rawinsondes launched at operational sites there, while outside those latitudes dry-biased Vaisala sensors were almost exclusively used.

Computation of the vertically integrated net radiative heating rate 〈Q _R 〉 as a residual from the heat and moisture budgets reveals a ∼1.5 K day⁻¹ variation on the timescale of the Madden–Julian oscillation. The implied horizontal variation of 〈Q _R 〉 is large enough to have significant impacts on the tropical Walker and Hadley circulations. The IFA–IOP mean 〈Q _R 〉 is −0.41 K day⁻¹. This net cooling rate is less than many previous estimates for the Tropics but is within the range of independent estimates for COARE based on radiation models and observations. This small value may arise from decreased longwave emission to space due to abundant cirrus over the warm pool and, in addition, may reflect some shortwave absorption by cirrus, but not necessarily “anomalous absorption” as has been recently proposed.

Abstract

The kinematic and thermodynamic characteristics of the October and November 2011 Madden–Julian oscillations (MJOs) that occurred over the Indian Ocean during Dynamics of the MJO (DYNAMO) are investigated. Analyses are presented 1) for two primary sounding arrays, where results are independent of model parameterizations, and 2) on larger scales, including the Indian Ocean, using operational and reanalysis data.

Mean precipitation during DYNAMO was characterized by maxima in two east–west bands north and south of the equator. This pattern alternated between two bands during the inactive phase of the MJOs and a single rainfall maximum on the equator during the active phases. Precipitation over the northern sounding array (NSA), where the MJO signal was strongest, was significantly modulated by the MJOs, while the southern array experienced more frequent, briefer episodes of rainfall mostly related to ITCZ convection. Over the NSA the MJOs were characterized by gradual moistening of the low to midtroposphere over approximately 2-week periods. The October MJO featured multiple westward-moving, 2-day disturbances whereas the November MJO principally comprised two prominent Kelvin waves. Patterns of moistening, divergence, and vertical motion suggest a stepwise progression of convection, from shallow cumulus to congestus to deep convection. Tilted thermal anomalies in the upper troposphere–lower stratosphere reveal gravity or Kelvin waves excited by the MJO convective envelopes, which modulate the tropopause and contribute to preactive-phase upper-tropospheric moistening. While there is a number of similarities in the characteristics of the two MJOs, there are sufficient differences to warrant caution in generalizing results from these two events.

Abstract

During the Dynamics of the MJO (DYNAMO) field campaign, radiosonde launches were regularly conducted from three small islands/atolls (Malé and Gan, Maldives, and Diego Garcia, British Indian Ocean Territory) as part of a large-scale sounding network. Comparison of island upsondes with nearby and near-contemporaneous dropsondes over the ocean provides evidence for the magnitude and scope of the islands’ influence on the surrounding atmosphere and on the island upsonde profiles. The island’s impact on the upsonde data is most prominent in the lowest 200 m. Noting that the vertical gradients of temperature, moisture, and winds over the ocean are generally constant in the lowest 0.5 km of dropsondes, a simple procedure was constructed to adjust the upsonde profiles in the lowest few hundred meters to resemble the atmospheric structures over the open ocean. This procedure was applied to the soundings from the three islands mentioned above for the October–December 2011 period of DYNAMO. As a result of this procedure, the adjusted diurnal cycle amplitude of surface temperature is reduced fivefold, resembling that over the ocean, and low-level wind speeds are increased in ~90% of the island soundings. Examination of the impact of these sounding adjustments shows that dynamical and budget fields are primarily affected by adjustments to the wind field, whereas convective parameters are sensitive to the adjustments in thermodynamic fields. Although the impact of the adjustments is generally small (on the order of a few percent), intraseasonal wind regime changes result in some systematic variations in divergence and vertical motion over the sounding arrays.

Abstract

Properties of the atmospheric boundary layer (ABL) over the central Indian Ocean are investigated using sounding data obtained during the Dynamics of the MJO (DYNAMO) field campaign in 2011/12. Observations from Gan Island on Addu Atoll, the R/V Revelle, and Malé in the Maldives are used to determine the frequency of well-mixed layers and their mean thermodynamic and wind profiles. Well-mixed boundary layers or mixed layers were observed 68% of the time from the three sites, ranging from ~100-m depth in recovering convective downdraft wakes to ~925 m in undisturbed conditions, with a mean depth of 508 m. At Revelle, the site most representative of the open ocean, the ABL displayed a distinct signal of modulation by the October and November MJOs, with mixed-layer depths gradually increasing through the suppressed phases as the sea surface temperature (SST) increased leading up to the active phases, followed by frequent ABL stabilization and shallow mixed layers in recovering wakes. A distinct diurnal cycle of mixed-layer depths and properties was observed during the MJO suppressed phases in response to a diurnal cycle of the SST under the mostly light-wind, clear-sky conditions. The daytime growth of the mixed layer contributed to an afternoon maximum in cumulus cloud development and rainfall during the suppressed periods by allowing more boundary layer thermals to reach their condensation levels. The variability of the ABL on time scales ranging from convective to diurnal to monthly poses significant challenges for numerical simulations of the MJO and the tropical circulation in general.

Abstract

During the North American Monsoon Experiment (NAME), an unprecedented surface dataset was collected over the core monsoon region. Observations from 157 surface sites in this region along with twice-daily Quick Scatterometer (QuikSCAT) oceanic winds were quality controlled and processed into a gridded dataset covering the domain (15°–40°N, 90°–120°W) at 1-h, 0.25° resolution for the period from 1 July to 15 August. Using this dataset, the mean, temporal variability, and diurnal characteristics of the monsoon surface flow are documented with detail not previously possible. Being independent of model data over land, these objectively analyzed surface products are compared to similar analyses from a special North American Regional Reanlysis for NAME (NARR_NAME) that was produced for the same period.

Observed surface fields indicate that a robust land–sea breeze circulation is present over most of the Gulf of California (GoC) region in response to the strong diurnal heating of landmasses on both sides of the gulf. Many details of this land–sea breeze circulation are either missing (e.g., the nighttime/early morning land breeze) or poorly represented in the NARR_NAME. Observations from high elevation sites in the Sierra Madre Occidental (SMO) show weak downslope flows (∼0.5 m s⁻¹), near-saturated conditions, and low cloud bases during nighttime hours. These observations are consistent with the notion that high-terrain nocturnal clouds limit radiational cooling and, thus, nocturnal downslope flows as well. Over land, a cool and dry bias is observed in the NARR_NAME surface fields. This dry bias appears to limit the formation of nighttime cloudiness at high elevations, resulting in stronger radiational cooling at night and slope flows in the NARR_NAME that are 2–3 times stronger than observed. In addition, the daytime transition to surface convergence and rising motion over the western slopes of the SMO occurs about 3 h earlier in the NARR_NAME than observed, which indicates the tendency in the reanalyses to initiate the daily convective cycle too early, similar to that observed in operational forecast models over this region.

Following significant rainfall events, increased soil moisture and evapotranspiration due to vegetative green-up result in a smaller diurnal temperature signal over land and weaker slope flows over the SMO. In response to this weaker heating cycle, the magnitude and offshore extent of the land–sea breeze circulation is observed to diminish as the monsoon progresses.

Abstract

The West African summer monsoon features multiple, complex interactions between African easterly waves (AEWs), moist convection, variable land surface properties, dust aerosols, and the diurnal cycle. One aspect of these interactions, the coupling between convection and AEWs, is explored using observations obtained during the 2006 African Monsoon Multidisciplinary Analyses (AMMA) field campaign. During AMMA, a research weather radar operated at Niamey, Niger, where it surveilled 28 squall-line systems characterized by leading convective lines and trailing stratiform regions. Nieto Ferreira et al. found that the squall lines were linked with the passage of AEWs and classified them into two tracks, northerly and southerly, based on the position of the African easterly jet (AEJ). Using AMMA sounding data, we create a composite of northerly squall lines that tracked on the cyclonic shear side of the AEJ. Latent heating within the trailing stratiform regions produced a midtropospheric positive potential vorticity (PV) anomaly centered at the melting level, as commonly observed in such systems. However, a unique aspect of these PV anomalies is that they combined with a 400–500-hPa positive PV anomaly extending southward from the Sahara. The latter feature is a consequence of the deep convective boundary layer over the hot Saharan Desert. Results provide evidence of a coupling and merging of two PV sources—one associated with the Saharan heat low and another with latent heating—that ends up creating a prominent midtropospheric positive PV maximum to the rear of West African squall lines.

Abstract

A linear primitive equation model has been used to test the hypothesis that the vertical structure of observed Caribbean easterly waves is determined by the interaction between convective heating and the environmental wind. The model determines the response to a propagating heat source in a specified basic state. The model allows for the inclusion of diffusion and cumulus momentum transports. The linear perturbations are assumed to have the form of a single Fourier component in the zonal direction. The frequency and zonal wavelength of the disturbance are taken from observations of the three-dimensional structure of a series of Caribbean easterly waves made by Shapiro. The structure of the basic state zonal wind, assumed to be a function of height, is based on observations near the latitude of largest observed wave amplitude. The maximum heating rate is 5 K day⁻¹, centered at about 19°N.

Very good agreement is found between the model-derived vertical structure of the waves and that observed by Shapiro (1986). In particular, the observed 90° westward phase shift between the 200 mb and near-surface troughs, and the westward tilt of the trough axis with height, are reproduced in the model solutions. Although linearization is not strictly valid for the observed wind amplitudes of ∼5 m s⁻¹, the model's linear dynamical framework appears to represent the wave's structure well. The westward phase shift is found to depend on the downward flux of wave energy toward a near-critical layer near the ground. Experiments also suggest that the latitude of the disturbance may be as important a factor in the determination of the westward tilt of the trough axis as is the structure of the basic state zonal wind. An eastward tilt of the trough axis in the lower troposphere, such as that in the classical model of a Caribbean easterly wave, can occur at low latitudes, when the westward phase shift is in a narrow layer near the level of maximum heating. Cumulus momentum transports do not substantially change the structure of the forced wave disturbance. The model solutions are compared with similar experiments of Holton, and are related to results of Stevens, Lindzen and Shapiro.

Abstract

Rawinsonde data used for sounding-array budget computations have random errors, both instrumental errors and errors of representativeness (here called sampling errors). The latter are associated with the fact that radiosondes do not measure large-scale mean winds and state variables, but are contaminated by small-scale variations as well. Data from the western Pacific and the summer monsoon of southeast Asia are used to estimate these random errors, and to propagate them through budget computations to assign error bars to derived quantities.

The statistics of sampling errors in directly measured variables are estimated from station pair analysis, in which variance is partitioned into contributions by resolved and unresolved scales. Resolved scales contribute the portion that is contained in averages of adjacent sounding stations and/or adjacent launch times (6-h intervals), while the rest of the total variance is defined as unresolved. Magnitudes of unresolved variability for typical rawinsonde-array spacings are ∼0.5 K for temperature; ∼5% for relative humidity at low levels, rising to nearly 15% in the middle-upper troposphere; and ∼2 m s⁻¹ for winds, rising to 3 m s⁻¹ in the upper troposphere. These are much larger than random instrumental errors, as estimated from pairs of simultaneous rawinsondes launched very close together. Vertical correlation scales of unresolved variability are 100–200 hPa. Up to 50% of the variance of humidity is unresolved, while for zonal wind the unresolved portion is only a few percent. Spatial and temporal sampling errors become about equal for 6-hourly rawinsondes ∼200 km apart.

The effects of sampling errors on budget computations are estimated by a perturbed-observation ensemble approach. All computations are repeated 20 times, with random realizations of unresolved variability added to the rawinsonde data entering the analysis. The ensemble standard deviation serves as an estimate of sampling error, which naturally decreases as the results are averaged over larger areas and longer time periods. For example, rainfall estimates on ∼500 km scales have sampling errors of ∼5 mm day⁻¹ in daily means, and ∼1 mm day⁻¹ in monthly means. The ensemble spread of 120-day time integrations of the vertically averaged moist enthalpy equation with rawinsonde-array-derived advective sources exceeds 20 K, implying that sampling error could be responsible for substantial biases in column models forced with such source terms.