Matthew E. Peters

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.

Analysis of Atmospheric Energy Transport in ERA-40 and Implications for Simple Models of the Mean Tropical Circulation

Abstract An analysis of atmospheric energy transport in 22 years (1980–2001) of the 40-yr ECMWF Re-Analysis (ERA-40) is presented. In the analyzed budgets, there is a large cancellation between divergences of dry static and latent energy such that the total energy divergence is positive over all tropical oceanic regions except for the east Pacific cold tongue, consistent with previous studies. The west Pacific and Indian Oceans are characterized by a balance between diabatic sources and mean advective energy export, with a small eddy contribution. However, in the central and eastern Pacific convergence zone, total energy convergence by the mean circulation is balanced by submonthly eddies, with a small diabatic source. Decomposing the mean advective tendency into terms due to horizontal and vertical advection shows that the spatial variation in the mean advection is due largely to variations in vertical advection; these variations are further attributed to variations in the vertical profile of the vertica...

A Simplified Model of the Walker Circulation with an Interactive Ocean Mixed Layer and Cloud-Radiative Feedbacks

Abstract Cloud–climate feedbacks between precipitation, radiation, circulation strength, atmospheric temperature and moisture, and ocean temperature are studied with an idealized model of the Walker circulation in a nonrotating atmosphere coupled to an ocean mixed layer. This study has two main purposes: 1) to formulate a conceptual framework that includes the dominant feedbacks between clouds and a large-scale divergent circulation; and 2) to use this framework to investigate the sensitivity of the climate system to these interactions. Two cloud types—high, convective anvils and low, nonprecipitating stratus—are included and coupled to the large-scale dynamics. The atmosphere is coupled to an ocean mixed layer via a consistent surface energy budget. Analytic approximations with a simplified radiation scheme are derived and used to explain numerical results with a more realistic radiation scheme. The model simplicity allows interactions between different parts of the ocean–atmosphere system to be cleanly ...

Convection, Cloud‐Radiative Feedbacks and Thermodynamic Ocean Coupling in Simple Models of the Walker Circulation

The authors consider a set of simple models for the divergent component of the tropical atmospheric circulation, and the associated precipitation field. A number of strong simplifying assumptions are made, leaving deep convection and radiation (both simply parameterized) as the key processes in the models. The first case considered is that of fixed sea surface temperature (SST), with an SST gradient imposed across the domain, in the limit of zero convective time scale or strict quasi-equilibrium (SQE). Steady solutions are found for sufficiently weak cloud-radiative feedback. As the parameter controlling the strength of the cloud-radiative feedback is increased, the region of nonzero precipitation shrinks in size and the precipitation grows stronger there. For cloud-radiative feedback stronger than a particular value, steady solutions cannot be found, and numerically obtained time-dependent solutions blow up. In the next case considered, the slab ocean lower boundary condition is used. In this case the behavior is dramatically different. In the steady solutions, the strength of the radiative feedback now has little effect on the size of the precipitating region. The finite convective time scale changes the stability criterion, and when the steady solutions become unstable, rather than blowing up, well-behaved radiative-convective oscillations set in. The mechanism for these oscillations is essentially local radiative-convective or surface flux convective instability, so they can be studied at a single point. Their frequencies are intraseasonal, and some inferences can be drawn from them about how ocean coupling affects the Madden-Julian oscillation. The simplicity of the models allows a relatively complete understanding of their behavior. The sensitivity of the solutions to the key parameters, especially the convective time scale and the cloud radiative feedback parameter, are discussed in some detail.