Brian E. Mapes

Tropical Intraseasonal Variability in 14 IPCC AR4 Climate Models Part I: Convective Signals

Abstract This study evaluates the tropical intraseasonal variability, especially the fidelity of Madden–Julian oscillation (MJO) simulations, in 14 coupled general circulation models (GCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Eight years of daily precipitation from each model’s twentieth-century climate simulation are analyzed and compared with daily satellite-retrieved precipitation. Space–time spectral analysis is used to obtain the variance and phase speed of dominant convectively coupled equatorial waves, including the MJO, Kelvin, equatorial Rossby (ER), mixed Rossby–gravity (MRG), and eastward inertio–gravity (EIG) and westward inertio–gravity (WIG) waves. The variance and propagation of the MJO, defined as the eastward wavenumbers 1–6, 30–70-day mode, are examined in detail. The results show that current state-of-the-art GCMs still have significant problems and display a wide range of skill in simulating the tropical intraseasonal va...

Multiscale variability of deep convection in relation to large-scale circulation in TOGA COARE

Deep convection over the Indo-Pacific oceanic warm pool in the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) occurred in cloud clusters, which grouped together in regions favoring their occurrence. These large groups of cloud clusters produced large-scale regions of satellite-observed cold cloud-top temperature. This paper investigates the manner in which the cloud clusters were organized on time and space scales ranging from the seasonal mean pattern over the whole warm-pool region to the scale of individual cloud clusters and their relationship to the large-scale circulation and sea surface temperature (SST). The dominant convective variability was associated with the intraseasonal oscillation (ISO). A large eastward propagating ensemble of cloud clusters marked the ISOs progress. The meridional structure of the ISO was strongly affected by the seasonal cycle with a southward shift from the Northern Hemisphere in October-November to the Southern Hemisphere in January-February. The SST had an intraseasonal signal in lagged quadrature with the cold cloudiness and rainfall in COARE. The SST increased (decreased) during the convectively suppressed (active) phases of the ISO, Enhanced low-level westerly winds occurred toward the later stages of the enhanced-convection periods of the ISO, though not always centered at the equator. The strongest westerlies tended to be located between two synoptic-scale cyclonic gyres, which were often not symmetric about the equator in the low-level. wind field. This asymmetry in the anomalous equatorial low-level westerlies may have different implications for the oceanic response in the coupled atmosphere-ocean system than those centered on the equator. The cyclonic gyres contained highly concentrated deep convection, and, in four cases, the gyres developed into tropical cyclones. Within the envelope marking the convectively active phase of the ISO, cloud clusters were frequently concentrated into westward-propagating disturbances with a local periodicity of similar to 2 days. These 2-day disturbances have been identified in earlier spectral studies and appear to be related to westward propagating inertio-gravity waves. In COARE, they typically contained numerous cloud clusters, which underwent a distinct diurnal cycle. Most of the cloud clusters embedded in the 2-day disturbances moved westward, though some were stationary, and a few moved eastward. A cloud-duster tracking program identified groups of clusters (lime dusters) that exhibited continuity in time and space. In the most convectively active period of the ISO, the tracking program identified almost the entire ISO cloud ensemble as a long-lasting, trackable superconvective system. This observation indicates the lack of a distinct scale-separation between convection and large-scale disturbances during the most intense convective periods in COARE.

Convective Inhibition, Subgrid-Scale Triggering Energy, and Stratiform Instability in a Toy Tropical Wave Model

A toy model of large-scale deep convection variations is constructed around a radiative‐convective equilibrium climate, with an observed mean sounding as its thermodynamic basic state. Vertical structure is truncated at two modes, excited by convective (one-signed) and stratiform (two-signed) heating processes in tropical deep convection. Separate treatments of deep and shallow convection are justified by observations that deep convection is more variable. Deep convection intensity is assumed to be modulated by convective available potential energy (CAPE), while occurrence frequency is modulated by the ratio of convective inhibition (CIN) to ‘‘triggering energy’’ K, a scalar representing the intensity of subgrid-scale fluctuations. Deep convective downdrafts cool and dry the boundary layer but also increase K. Variations of K make the relationship between convection and thermodynamic variables (CAPE, CIN, ue) nonunique and amplify the deep convective response to temperature waves of small (;18C) amplitude. For a parameter set in which CAPE variations control convection, moist convective damping destroys all variability. When CIN/K variations have dominant importance (the ‘‘inhibition-controlled’’ regime), a mechanism termed ‘‘stratiform instability’’ generates large-scale waves. This mechanism involves lower-tropospheric cooling by stratiform precipitation, which preferentially occurs where the already cool lower troposphere favors deep convection, via smaller CIN. Stratiform instability has two subregimes, based on the relative importance of the two opposite effects of downdrafts: When boundary layer ue reduction (a local negative feedback) is stronger, small-scale waves with frequency based on the boundary layer recovery time are preferred. When the K-generation effect (positive feedback) is stronger, very large scales (low wavenumbers of the domain) develop. A mixture of these scales occurs for parameter choices based on observations. Model waves resemble observed waves, with a phase speed ; 20 ms 21 (near the dry wave speed of the second internal mode), and a ‘‘cold boomerang’’ vertical temperature structure. Although K exhibits ‘‘quasi-equilibrium’’ with other convection variables (correlations . 0.99), replacing the prognostic K equation with diagnostic equations based on these relationships can put the model into wildly different regimes, if small time lags indicative of causality are distorted. The response of model convection to climatological spatial anomalies of ue (proxy for SST) and K (proxy for orographic and coastal triggering) is considered. Higher SST tends broadly to favor convection under either CAPE-controlled or inhibition-controlled regimes, but there are dynamical embellishments in the inhibition-controlled regime. The Kelvin wave seems to be the preferred structure when the model is run on a uniform equatorial b plane.

Gregarious Tropical Convection

Abstract A beat source with a vertical profile like that of observed tropical mesoscale convective systems (MCSs) is shown to cause, through inviscid gravity wave dynamics, upward displacement at low levels in a mesoscale region surrounding the heating. Typical values are ∼10%–30% area contraction at the surface everywhere within 270 km of the heating 6 h after it starts. As a result, conditions near an existing MCS (but beyond the area of MCS outflow) become more favorable for the development of additional convection. This theory predicts that cloud clusters should be gregarious. Infrared satellite imagery confirms that almost half of the cold cloudiness observed in a month over the oceanic warm pool region was contributed by just 14 objectively defined multiday “supercluters”.

Diabatic Divergence Profiles in Western Pacific Mesoscale Convective Systems

Abstract Heating in the atmosphere can be expressed as diabatic divergence δd, which is nearly equal to the actual horizontal divergence δ in tropical convection. High-quality δ profile measurements from airborne Doppler radar “purls” in ten mesoscale convective systems (MCS) observed during TOGA-COARE are examined, and the mean profile is compared with rawinsonde array measurements. Young convective features have strong near-surface convergence, while older cells with better-developed downdrafts and stratiform precipitation areas have their peak convergence aloft. In the mean, then, surface flow is only weakly convergent or oven divergent, so that the main convergence into MCSs is deep and peaked aloft, with a sharp “melting convergence” at 0°C. Divergence prevails above ∼10 km altitude but was undersampled by the radar. Unusual but well-sampled observations in the purl dataset include: a persistent, widespread δ profile feature in one well-sampled MCS (a cyclone rainband); oscillatory “reverberations” c...

Stratiform Precipitation, Vertical Heating Profiles, and the Madden-Julian Oscillation

The observed profile of heating through the troposphere in the Madden‐Julian oscillation (MJO) is found to be very top heavy: more so than seasonal-mean heating and systematically more so than all of the seven models for which intraseasonal heating anomaly profiles have been published. Consistently, the Tropical Rainfall Measuring Mission (TRMM) precipitation radar shows that stratiform precipitation (known to heat the upper troposphere and cool the lower troposphere) contributes more to intraseasonal rainfall variations than it does to seasonal-mean rainfall. Stratiform rainfall anomalies lag convective rainfall anomalies by a few days. Reasons for this lag apparently include increased wind shear and middle‐upper tropospheric humidity, which also lag convective (and total) rainfall by a few days. A distinct rearward tilt is seen in anomalous heating time‐height sections, in both the strong December 1992 MJO event observed by the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE) and a composite MJO constructed from multiyear datasets. Interpretation is aided by a formal partitioning of the COARE heating section into convective, stratiform, and radiative components. The tilted structure after the maximum surface rainfall appears to be largely contributed by latent and radiative heating in enhanced stratiform anvils. However, the tilt of anomalous heating ahead of maximum rainfall is seen within the convective component, suggesting a change from shallower to deeper convective heating as the wet phase of the MJO approached the longitude of the observations.

Diurnal Patterns of Rainfall in Northwestern South America. Part III: Diurnal Gravity Waves and Nocturnal Convection Offshore

Abstract Afternoon/evening near-coastal convection over land is easily understood as a response to solar heating of the land, turbulent transfer of heat and moisture to the boundary layer, and lifting of air by vigorous sea-breeze fronts. Subtler processes apparently underlie the late night and morning convection that is prevalent over coastal waters throughout the Tropics. Sensitivity tests using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5), and further diagnoses of the control run described in Part II, are used to explore these processes. Prior studies have speculated that “land-breeze” circulations, analogous but opposite to the sea breeze, drive offshore convection at night. However, nighttime radiative cooling of land and the associated thermal breezes are much weaker than the corresponding daytime processes, especially under humid tropical skies. Analysis of model mean soundings reveals that modest (fractions of a degree Celsius) temperature changes near the 800-hPa ...

Diurnal Patterns of Rainfall in Northwestern South America. Part I: Observations and Context

One of the rainiest areas on earth, the Panama Bight and Pacific (western) littoral of Colombia, is the focal point for a regional modeling study utilizing the fifth-generation Pennsylvania State University‐NCAR Mesoscale Model (MM5) with nested grids. In this first of three parts, the observed climatology of the region is presented. The seasonal march of rainfall has a northwest‐southeast axis, with western Colombia near the center, receiving rain throughout the year. This study focuses on the August‐September season. The diurnal cycle of rainfall over land exhibits an afternoon maximum over most of South and Central America, typically composed of relatively small convective cloud systems. Over some large valleys in the Andes, and over Lake Maracaibo, a nocturnal maximum of rainfall is observed. A strong night/morning maximum of rainfall prevails over the coastal ocean, propagating offshore and westward with time. This offshore convection often takes the form of mesoscale convective systems with sizes comparable to the region’s coastal concavities and other geographical features. The 10-day period of these model studies (28 August‐7 September 1998) is shown to be a period of unusually active weather, but with a time-mean rainfall pattern similar to longer-term climatology. It is concluded that the rain-producing processes during this time period are likely to be typical of those that shape the seasonal climatology.

Influence of cloud‐radiation interaction on simulating tropical intraseasonal oscillation with an atmospheric general circulation model

The influence of cloud-radiation interaction in simulating the tropical intraseasonal oscillation (ISO) is examined using an aqua planet general circulation model (GCM). Two types of simulation are conducted: one with prescribed zonal mean radiation and the other with fully interactive clouds and radiation. In contrast to the fixed radiation case, where the ISO is simulated reasonably well, the cloud-radiation interaction significantly contaminates the eastward propagation of the ISO by producing small-scale disturbances moving westward with the easterly basic winds. The small-scale disturbances are persistently excited by a strong positive feedback through interaction between cumulus-anvil clouds and radiation. The longwave interaction is shown to play a bigger role in contaminating the ISO than the shortwave interaction does. The anvil clouds reduce the longwave cooling significantly in the lower troposphere while releasing latent heating in the upper troposphere. To moderate the strong cloud-radiation feedback, the large-scale condensation scheme in the GCM is modified by reducing the autoconversion timescale, needed for cloud condensates to grow up to rain drops. In addition, upper air ice cloud contents are reduced to change the cloud albedo. These modifications make a more realistic simulation of the ISO similar to the observed.

Subseasonal Variability Associated with Asian Summer Monsoon Simulated by 14 IPCC AR4 Coupled GCMs

Abstract This study evaluates the subseasonal variability associated with the Asian summer monsoon in 14 coupled general circulation models (GCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Eight years of each model’s twentieth-century climate simulation are analyzed. The authors focus on the three major components of Asian summer monsoon: the Indian summer monsoon (ISM), the western North Pacific summer monsoon (WNPSM), and the East Asian summer monsoon (EASM), together with the two dominant subseasonal modes: the eastward- and northward-propagating boreal summer intraseasonal oscillation (BSIO) and the westward-propagating 12–24-day mode. The results show that current state-of-the-art GCMs still have difficulties and display a wide range of skill in simulating the subseasonal variability associated with Asian summer monsoon. During boreal summer (May–October), most of the models produce reasonable seasonal-mean precipitation over the ISM region,...