Annual Climatology of the Diurnal Cycle on the Canadian Prairies

Alan K Betts,Ahmed B Tawfik

doi:10.3389/feart.2016.00001

Abstract

We show the annual climatology of the diurnal cycle, stratified by opaque cloud, using the full hourly resolution of the Canadian Prairie data. The opaque cloud field itself has distinct cold and warm season diurnal climatologies; with a near-sunrise peak of cloud in the cold season and an early afternoon peak in the warm season. There are two primary climate states on the Canadian Prairies, separated by the freezing point of water, because a reflective surface snow cover acts as a climate switch. Both cold and warm season climatologies can be seen in the transition months of November, March and April with a large difference in mean temperature. In the cold season with snow, the diurnal ranges of temperature and relative humidity increase quasi-linearly with decreasing cloud, and increase from December to March with increased solar forcing. The warm season months, April to September, show a homogeneous coupling to the cloud cover, and a diurnal cycle of temperature and humidity that depends only on net longwave. Our improved representation of the diurnal cycle shows that the warm season coupling between diurnal temperature range and net longwave is weakly quadratic through the origin, rather than the linear coupling shown in earlier papers. We calculate the conceptually important 24-h imbalances of temperature and relative humidity (and other thermodynamic variables) as a function of opaque cloud cover. In the warm season under nearly clear skies, there is a warming of +2oC and a drying of -6% over the 24-h cycle, which is about 12% of their diurnal ranges. We summarize results on conserved variable diagrams and explore the impact of surface windspeed on the diurnal cycle in the cold and warm seasons. In all months, the fall in minimum temperature is reduced with increasing windspeed, which reduces the diurnal temperature range. In July and August, there is an increase of afternoon maximum temperature and humidity at low windspeeds, and a corresponding rise in equivalent potential temperature of 4.4K that appears coupled to increased precipitation. However overcast skies are associated with the major rain events and higher windspeeds.

Highlights

Many climate and hydrometeorology studies have been largely based on temperature and precipitation for which long-term records are available (Leung et al, 2003; Betts et al, 2005; Betts, 2007; Koster et al, 2009; Berg et al, 2015)
We show there is a systematic bias with (TxD, RHxD) > (TxT, RHxT) while (TnD, RHnD) < (TnT, RHnT), so that the radiatively coupled (DTRT, DRHT) < (DTRD, DRHD)
We find the diurnal range of temperature in the warm season is very tightly coupled to net longwave

Summary

INTRODUCTION

Many climate and hydrometeorology studies have been largely based on temperature and precipitation for which long-term records are available (Leung et al, 2003; Betts et al, 2005; Betts, 2007; Koster et al, 2009; Berg et al, 2015). Betts et al (2015) recalibrated the opaque cloud data in terms of SWCF and LWCF for T 0◦C using 17 years of BSRN data; and showed the dependence in summer (but not winter) of the diurnal range of temperature and humidity on windspeed, RH, day-night asymmetries in the opaque cloud field and monthly precipitation anomalies These analyses of the diurnal cycle that average the daily values of DTR, qualitatively correct, misrepresent the coupling between the diurnal radiative forcing and the diurnal cycle of T and RH and the derived thermodynamic variables, especially under cloudy conditions and in winter. The section addresses the consistency of the opaque cloud observations across the Prairies

Evaluation of Opaque Cloud Data

25 Prairies

Prairies

F Feb M Mar

SUMMARY AND CONCLUSIONS