Convectively Driven Tropopause‐Level Cooling and Its Influences on Stratospheric Moisture

Abstract

Characteristics of the tropopause‐level cooling associated with tropical deep convection are examined using CloudSat radar and Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) GPS radio occultation measurements. Extreme deep convection is sampled based on the cloud top height (>17 km) from CloudSat, and colocated temperature profiles from COSMIC are composited around the deep convection. Response of moisture to the tropopause‐level cooling is also examined in the upper troposphere and lower stratosphere using microwave limb sounder measurements. The composite temperature shows an anomalous warming in the troposphere and a significant cooling near the tropopause (at 16–19 km) when deep convection occurs over the western Pacific, particularly during periods with active Madden‐Julian Oscillation (MJO). The composite of the tropopause cooling has a large horizontal scale (~6,000 km in longitude) with minimum temperature anomaly of ~ −2 K, and it lasts more than 2 weeks with support of mesoscale convective clusters embedded within the envelope of the MJO. The water vapor anomalies show strong correlation with the temperature anomalies (i.e., dry anomaly in the cold anomaly), showing that the convectively driven tropopause cooling actively dehydrate the lower stratosphere in the western Pacific region. The moisture is also affected by anomalous Matsuno‐Gill‐type circulation associated with the cold anomaly, in which dry air spreads over a wide range in the tropical tropopause layer (TTL). These results suggest that convectively driven tropopause cooling and associated transient circulation play an important role in the large‐scale dehydration process in the TTL.

Water Vapor, Clouds, and Saturation in the Tropical Tropopause Layer

“…to the convective heating (Kim et al, 2018). As mentioned above, the response forces Rossby and Kelvin waves that effectively extend the dynamical influence of convection to the east.…”

“…The wind vectors indicate the location of a pair of high pressure systems, one located just west of 180° longitude, north of the equator, and one over Australia south of the equator. These two systems are a result of the Matsuno‐Gill response to the convective heating (Kim et al, ). As mentioned above, the response forces Rossby and Kelvin waves that effectively extend the dynamical influence of convection to the east.…”

“…Basically, the convective heating forces stationary Rossby and Kelvin waves, which creates a larger cold pool that straddles the equator somewhat east of the convective centers. This cold pool is relatively shallow compared to the underlying warm anomalies because of the change in static stability at the tropopause (see Kim et al, , Figure 6).…”

“…The linkage between TWP cold temperatures, convection, and the water vapor distribution is not surprising (Randel et al, ). The observed spatial structure of the TWP TTL cold region and the associated dipole circulation is a Matsuno‐Gill response to the latent heat released by convective clusters in the south of the equator (Gill, ; Kim et al, ). Basically, the convective heating forces stationary Rossby and Kelvin waves, which creates a larger cold pool that straddles the equator somewhat east of the convective centers.…”

Fast Transport Pathways Into the Northern Hemisphere Upper Troposphere and Lower Stratosphere During Northern Summer

“…Although the contribution of the vertical eddy flux is usually considered small in previous studies, a few studies have suggested its importance in determining the tropical tropopause temperature (e.g., Abalos et al, 2013;;Holloway & Neelin, 2007;Kim et al, 2018;Yoshida & Yamazaki, 2010). For example, Yoshida and Yamazaki (2010) performed numerical model experiments by modifying the tropical sea surface temperature and found the convergence of the vertical eddy heat flux critical in setting up the tropical tropopause temperature response.…”