Cloud Masks Derived from the Mars Daily Global Maps and an Application to the Tropical Cloud Belt on Mars

Huiqun Wang,Gonzalo González Abad

doi:10.3390/geosciences11080324

Huiqun Wang, Gonzalo González Abad

Open Access

https://doi.org/10.3390/geosciences11080324

Copy DOI

Abstract

An image processing technique is used to derive cloud masks from the color Mars Daily Global Maps (MDGMs) that are composed from the Mars Reconnaissance Orbiter (MRO) Mars Color Imager (MARCI) wide-angle image swaths. The blue channel of each MDGM is used to select cloud candidates and the blue-to-red ratio map is compared with a reference ratio map to filter out false positives. Quality control is performed manually. The derived cloud masks cover 1 Mars year from the summer of Mars year (MY) 28 to the summer of MY 29. The product has a 0.1° longitude by 0.1° latitude resolution and is available each day. This makes it possible to characterize the evolution of the tropical cloud belt from several new perspectives. The tropical cloud belt steadily builds up during northern spring and early summer, peaks near the early- to mid-summer transitional period, and rapidly declines afterward. From the perspective of cloud occurrence frequency and time mean, the cloud belt appears meandrous and zonally discontinuous, with minima in the Amazonis Planitia and Arabia Terra longitudinal sectors. A pronounced cloud branch diverges from the main cloud belt and extends from the Valles Marineris towards the Noachis and Hellas region. The cloud belt exhibits noticeable oscillatory behavior whereby cloud brightening alternates between the western and eastern hemispheres near the equator with a periodicity between 20 and 30 sols. The cloud belt oscillation occurred each Mars year around Ls = 140°, except for the Mars years when intense dust storms made disruptions. The phenomenon appears to be associated with an eastward propagating equatorial Kelvin wave with zonal wavenumber 1. This wave has a much longer wave period than the diurnal and semidiurnal Kelvin waves discussed in most of the previous studies and may be an important factor for the intra-seasonal variability of the tropical cloud belt. The convolution of clouds’ local time variation with MRO’s orbit shift pattern results in a seemingly highly regular 5-day traveling wave in Hovmöller diagrams of cloud masks.

Highlights

Dust storm is probably the most famous meteorological phenomenon for the Martian atmosphere, clouds are widespread and are highly important [1]
Clouds play a pivotal role in the Martian water cycle, interact with the Martian dust cycle through microphysical processes, and are very sensitive to atmospheric temperature and transport
Image observations contributed to the understanding of large-scale cloud features, synoptic scale cloud systems and meso scale cloud structures, e.g., [17,18,19,20,21,22,23]

Summary

Introduction

Dust storm is probably the most famous meteorological phenomenon for the Martian atmosphere, clouds are widespread and are highly important [1]. Clouds play a pivotal role in the Martian water cycle, interact with the Martian dust cycle through microphysical processes, and are very sensitive to atmospheric temperature and transport. Understanding the distribution and effect of clouds is essential and requires extensive datasets to reveal the pattern and variability on various spatial and temporal scales and to guide and constrain model simulations. Spacecraft observations since the 1970s have led to a few datasets for cloud optical depths or their proxies, e.g., [10,11,12,13,14,15,16], greatly improving the knowledge of Martian cloud distribution. Image observations contributed to the understanding of large-scale cloud features (tropical cloud belt and polar hoods), synoptic scale cloud systems (frontal clouds, annular clouds) and meso scale cloud structures (lee waves, cloud streets, cirrus, aster clouds over volcanoes, cloud trails), e.g., [17,18,19,20,21,22,23]

Methods

Results

Conclusion