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The diurnal cycle: A bridge between weather and climate

7 October 2016
Our complete understanding of climate and its evolution will require mastery of the 24-hour weather cycle.

As passengers on a rotating planet, our lives are fully synchronized with the day–night cycle, and we are intimately familiar with its many effects on the weather. Most of us can instinctively predict the daily rise and fall of temperature, or the development of afternoon thunderstorms on a hot, humid day. The features of weather that vary through the course of the day are collectively referred to as the diurnal cycle.

As fundamental to the climate system as the changing seasons, the diurnal weather cycle is prevalent in most of the world, including over the remote oceans. Yet, more than simply revealing what time of day an umbrella might be necessary, the diurnal cycle may act as a bridge between weather and climate. Recent research reveals that the diurnal cycle can impact climate through a feedback mechanism in which the weather’s daily effects integrate over many days. Understanding such feedbacks is a critical step toward more fully understanding what drives climate and, in turn, toward better predicting its changes.

Consider a hypothetical Earth in which the diurnal cycle is absent: It receives the same amount of heat from the Sun, but the heat is received evenly over a 24-hour period. Would the amount of precipitation differ in this world from ours, on average? Are clouds distributed differently?

An apt tool for working with such questions is a numerical model of the atmosphere, which allows scientists to modify the laws of physics and, with the help of a supercomputer, simulate the impacts of those changes on the climate. In this model, we can remove the diurnal cycle and assess the differences between the resulting climate and a control climate. Or we can stretch or reduce the period of the diurnal cycle by changing when the Sun rises and sets each day. Increasing the length of the day amplifies any time-scale feedbacks from the diurnal cycle, while reducing the length of the day suppresses those feedbacks.

I recently carried out a set of model simulations in which each climate starts with shallow clouds in the same conditions and then evolves over 16 days (see figure 1). The key variable—length of day—was modified across the simulations between 12, 24, and 48 hours, with an additional simulation with no diurnal cycle (NODC). The results show that the climate remains driest without a diurnal cycle and is increasingly more humid the longer the diurnal cycle. (Humidity is measured in precipitable water: the amount of water vapor in the atmosphere that can be converted to precipitation.) Further, the humidity exhibits a clear diurnal cycle, which increases in amplitude the longer the day lasts.

Figure 1. This time series tracks precipitable water (the total water vapor added throughout the vertical dimension of the atmosphere) over 16 simulated days in four simulated climates. The diurnal cycle of solar heating is either removed (NODC) or modified in length (NNH indicates a NN-hour long diurnal cycle). Credit: J.H. Ruppert Jr., Journal of Advances in Modeling Earth Systems, in press.
Figure 1. This time series tracks precipitable water (the total water vapor added throughout the vertical dimension of the atmosphere) over 16 simulated days in four simulated climates. The diurnal cycle of solar heating is either removed (NODC) or modified in length (NNH indicates an NN-hour long diurnal cycle).

Clearly the diurnal cycle—the only modified variable in these simulations—has major impacts on the simulated climate and its evolution. Not only does the diurnal cycle impact the daily variation of humidity, but with a longer diurnal cycle, the atmosphere also grows more humid each day. As a result of greater humidity, deeper clouds and increased rainfall develop over the course of many days (see figure 2). Increased moisture and deeper clouds indicate a diurnal time-scale feedback, whereby the climate and its evolution are different as a result of the diurnal cycle.

Figure 2. A climate that includes a diurnal cycle (top) evolves differently than one without a diurnal cycle: Deeper clouds and greater rainfall develop as a result of more humid conditions. Blue–brown shading indicates moist–dry conditions. Credit: J.H. Ruppert Jr., Journal of Advances in Modeling Earth Systems, in press.
Figure 2. A climate that includes a diurnal cycle (top) evolves differently than one without a diurnal cycle: Deeper clouds and greater rainfall develop as a result of more humid conditions. Blue–brown shading indicates moist–dry conditions.

This experiment was carried out using a model that assumes idealized conditions, which represent only one type of climate regime. More research is necessary to understand how such diurnal time-scale feedbacks may operate throughout the climate system in different regimes. Nevertheless, these findings motivate renewed attention to long-standing open questions. Although we understand quite well how the diurnal cycle of land-surface heating promotes cloud development and precipitation each day, we still don’t clearly understand what drives the diurnal cycle over the remote oceans, which do not warm as strongly under daytime solar heating as the land. A particularly puzzling feature of the oceanic diurnal cycle is a prominent nocturnal or early-morning peak in rainfall. Even after decades of deliberation, the jury is still out on why the precipitation peak occurs so asynchronously with the solar radiation cycle.

Furthermore, the diurnal cycle is extremely complex in regions where land and ocean regimes intersect, as in the Maritime Continent of Indonesia, the Philippines, and other Southeast Asian island countries. Weather in this part of the world significantly impacts the weather at higher latitudes, so it is critical to understand if and how the diurnal cycle in this region impacts the climate.

Measuring the extent that the climate “feels” the effects of the diurnal cycle, through diurnal time-scale feedbacks, will improve our understanding of the fundamental weather–climate bridge. And this will ultimately improve our grasp of how the climate works, so that we may better predict its changes.

James H. Ruppert Jr recently completed his PhD at Colorado State University in Fort Collins and now works as a postdoctoral scientist at the Max Planck Institute for Meteorology in Hamburg, Germany. His research explores the interactions between the diurnal cycle and longer time scales in various aspects of the climate system.

This research is described in more detail here: J. H. Ruppert Jr, "Diurnal timescale feedbacks in the tropical cumulus regime," Journal of Advances in Modeling Earth Systems (in press).

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