Larissa E. Back

Relationships between Water Vapor Path and Precipitation over the Tropical Oceans

The relationship between water vapor path W and surface precipitation rate P over tropical oceanic regions is analyzed using 4 yr of gridded daily SSM/I satellite microwave radiometer data. A tight monthly mean relationship P (mm day21) 5 exp[11.4(r 2 0.522)] for all tropical ocean regions and seasons is found between P and a column-relative humidity r obtained by dividing W by the corresponding saturation water vapor path. A similar relation, albeit with more scatter, also holds at daily time scales, and can be interpreted as a moisture adjustment time scale of 12 h for convective rainfall to affect humidity anomalies on 300-km space scales. Cross-spectral analysis shows statistically significant covariability of actual and r-predicted precipitation at all frequencies, with negligible phase lag. The correlation of actual and r-predicted precipitation exceeds 0.5 on intraseasonal and longer time scales. The SSM/I retrievals of W and P are found to be skillful even at daily time scales when compared with in situ radiosonde and radar-derived area-averaged precipitation data from Kwajalein Island, but the microwave estimates of daily P scatter considerably about the radar estimates (which are considered to be more reliable). Using the radar-derived precipitation in combination with microwave-derived W yields a daily r‐P relationship at Kwajalein similar to that derived solely from microwave measurements, but with somewhat less P associated with the highest values of r. This emphasizes that the absolute calibration of the r‐P relationship is somewhat dependent on the datasets used to derive r and especially P. Nevertheless, the results provide a useful constraint on conceptual models and parameterizations of tropical deep convection.

On the Relationship between SST Gradients, Boundary Layer Winds, and Convergence over the Tropical Oceans

A linear mixed layer model that skillfully reproduces observed surface winds and convergence over the tropicaloceans is used to examine the relative influenceof boundarylayer and free-tropospheric processes on the distribution of climatological surface winds and convergence. The semiempirical model assumes a subcloud-layer momentum force balance between pressure gradients, Coriolis acceleration, linearized friction, and downward momentum mixing, and it utilizes boundary conditions from the 40-yr ECMWF Re-Analysis (ERA-40). Observed pressure gradients are linearly decomposed into boundary layer (defined as the region below 850 hPa) and free-tropospheric components, and the surface winds and convergence associated with these components are computed. Results show that surface zonal winds are predominantly associated with a combination of free-tropospheric pressure gradients and downward momentum mixing, whereas the distribution of convergence is primarily due to boundary layer temperature gradients, which are closely related to SST gradients. The authors conclude that the climatological distribution of boundary layer convergence is primarily a function of the pattern of SST gradients and is better regarded as a cause rather than a consequence of deep convection.

Intensification of precipitation extremes with warming in a cloud resolving model

AbstractA cloud-resolving model is used to investigate the effect of warming on high percentiles of precipitation (precipitation extremes) in the idealized setting of radiative-convective equilibrium. While this idealized setting does not allow for several factors that influence precipitation in the tropics, it does allow for an evaluation of the response of precipitation extremes to warming in simulations with resolved rather than parameterized convection. The methodology developed should also be applicable to less idealized simulations.Modeled precipitation extremes are found to increase in magnitude in response to an increase in sea surface temperature. A dry static energy budget is used to relate the changes in precipitation extremes to changes in atmospheric temperature, vertical velocity, and precipitation efficiency. To first order, the changes in precipitation extremes are captured by changes in the mean temperature structure of the atmosphere. Changes in vertical velocities play a secondary role ...

The Relationship between Wind Speed and Precipitation in the Pacific ITCZ

Abstract The relationship between wind speed and precipitation in the Pacific ITCZ is analyzed using 4 yr of daily Special Sensor Microwave Imager (SSM/I) and Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) satellite passive microwave retrievals averaged over 2.5° boxes. Throughout the ITCZ, at high-column relative humidities (conditions under which deep convection is likely to occur), faster winds are associated with substantially more precipitation, explaining a small, but highly statistically significant fraction of daily rainfall variability. The slope of this relationship varies geographically and rapidly increases as the atmosphere becomes moister. Analysis of other data sources, including vector mean winds computed from QuikSCAT and area-averaged radar-derived precipitation estimates from Kwajalein Island, shows that the wind speed–precipitation correlation is robust. This relation provides a test of large-scale forecast models and insight into conceptual models of deep convection...

A Simple Model of Climatological Rainfall and Vertical Motion Patterns over the Tropical Oceans

Abstract A simple model is developed that predicts climatological rainfall, vertical motion, and diabatic heating profiles over the tropical oceans given the sea surface temperature (SST), using statistical relationships deduced from the 40-yr ECMWF Re-Analysis (ERA-40). The model allows for two modes of variability in the vertical motion profiles: a shallow mode responsible for all “boundary layer” convergence between 850 hPa and the surface, and a deep mode with no boundary layer convergence. The model is based on the argument expressed in the authors’ companion paper that boundary layer convergence can be usefully viewed as a forcing on deep convection, not just a result thereof. The shallow mode is either specified from satellite observations or modeled using a simple mixed-layer model that has SST as well as 850-hPa geopotential height, winds, and temperature as boundary conditions. The deep-mode amplitude is empirically shown to be proportional to a simple measure of conditional instability in conve...

Column-Integrated Moist Static Energy Budget Analysis on Various Time Scales during TOGA COARE

AbstractMoist static energy (MSE) budgets on different time scales are analyzed in the TOGA COARE data using Lanczos filters to separate variability with different frequencies. Four different time scales (~2-day, ~5-day, ~10-day, and MJO time scales) are chosen based on the power spectrum of the precipitation and previous TOGA COARE studies. The lag regression-slope technique is utilized to depict characteristic patterns of the variability associated with the MSE budgets on the different time scales.This analysis illustrates that the MSE budgets behave in significantly different ways on the different time scales. On shorter time scales, the vertical advection acts as a primary driver of the recharge–discharge mechanism of column MSE. As the time scale gets longer, in contrast, the relative contributions of the other budget terms become greater, and consequently, on the MJO time scale all the budget terms have nearly the same amplitude. Specifically, these results indicate that horizontal advection plays a...

Gross Moist Stability Assessment during TOGA COARE: Various Interpretations of Gross Moist Stability

AbstractDaily averaged TOGA COARE data are analyzed to investigate the convective amplification/decay mechanisms. The gross moist stability (GMS), which represents moist static energy (MSE) export efficiency by large-scale circulations associated with the convection, is studied together with two quantities, called the critical GMS (a ratio of diabatic forcing to the convective intensity) and the drying efficiency [a version of the effective GMS (GMS minus critical GMS)]. The analyses reveal that convection intensifies (decays) via negative (positive) drying efficiency.The authors illustrate that variability of the drying efficiency during the convective amplifying phase is predominantly explained by the vertical MSE advection (or vertical GMS), which imports MSE via bottom-heavy vertical velocity profiles (associated with negative vertical GMS) and eventually starts exporting MSE via top-heavy profiles (associated with positive vertical GMS). The variability of the drying efficiency during the decaying ph...

Estimating Vertical Motion Profile Shape within Tropical Weather States over the Oceans

AbstractThe vertical structure of tropical deep convection strongly influences interactions with larger-scale circulations and climate. This paper focuses on investigating this vertical structure and its relationship with mesoscale tropical weather states. The authors test the hypothesis that latent heating plus turbulent flux convergence varies (in space and time) in association with weather state type.The authors estimate mean-state vertical motion profile shape and latent heating plus turbulent flux convergence for six weather states defined using cloud-top pressure and optical depth properties from the International Satellite Cloud Climatology Project (ISCCP) dataset. Assuming two modes of vertical motion profile variability, these modes are statistically extracted from reanalysis data using a principal component analysis. Using these modes and the relationship between vertical motion, the dry static energy budget, and mass continuity, the authors estimate vertical motion profile shape. In these estim...

Global Hydrological Cycle Response to Rapid and Slow Global Warming

This study analyzes the response of global water vapor to global warmingin a series of fully coupled climate model simulations. The authorsfind that a roughly 7% K 21 rate of increase of water vapor with global surface temperature is robust only for rapid anthropogenic-like climate change. For slower warming that occurred naturally in the past, the Southern Ocean has time to equilibrate, producing a different pattern of surface warming, so that water vapor increases at only 4.2% K 21 . This lower rate of increase of water vapor with warming is not due to relative humidity changes or differences in mean lower-tropospheric temperature. A temperatureofover808Cwould berequiredin theClausius‐Clapeyronrelationship tomatchthe4.2%K 21 rate of increase. Instead, the low rate of increase is due to spatially heterogeneous warming. During slower global warming, there is enhanced warming at southern high latitudes, and hence less warming in the tropics per kelvin of global surface temperatureincrease. This leads to a smaller global watervapor increase, because most of the atmospheric water vapor is in the tropics. A formula is proposed that applies to general warming scenarios. This study also examines the response of global-mean precipitation and the meridional profile of precipitation minus evaporation and compares the latter to thermodynamic scalings. It is found that globalmean precipitation changes are remarkably robust between rapid and slow warming. Thermodynamic scalings for the rapid- and slow-warming zonal-mean precipitation are similar, but the precipitation changes are significantly different, suggesting that circulation changes are important in driving these differences.

Gross Moist Stability Analysis: Assessment of Satellite-Based Products in the GMS Plane

AbstractNew diagnostic applications of the gross moist stability (GMS) are proposed with demonstrations using satellite-based data. The plane of the divergence of column moist static energy (MSE) against the divergence of column dry static energy (DSE), referred to as the GMS plane here, is utilized. In this plane, one can determine whether the convection is in the amplifying phase or in the decaying phase; if a data point lies below (above) a critical line in the GMS plane, the convection is in the amplifying (decaying) phase. The GMS plane behaves as a phase plane in which each convective life cycle can be viewed as an orbiting fluctuation around the critical line, and this property is robust even on the MJO time scale. This phase-plane behavior indicates that values of the GMS can qualitatively predict the subsequent convective evolution. This study demonstrates that GMS analyses possess two different aspects: time-dependent and quasi-time-independent aspects. Transitions of time-dependent GMS can be v...