Stratiform Rain in the Tropics as Seen by the TRMM Precipitation Radar*

Abstract

Across the Tropics (20ЊN-20ЊS), the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) indicates that for reflectivities Ն17 dBZ, stratiform precipitation accounts for 73% of the area covered by rain and 40% of the total rain amount over a 3-yr period (1998-2000). The ratio of the convective rain rate to the stratiform rain rate is 4.1 on average at the horizontal resolution of the PR data. Convective rain rates remain constant or decrease as the stratiform contribution to total rain increases, implying that stratiform rain production is not very dependent on the strength of convection. This relationship is especially evident over the ocean, where there are weaker convective rain rates than over land but relatively larger stratiform rain amounts. The ocean environment appears more efficient in the production of stratiform precipitation through either the sustainability of convection by a warm, moist boundary layer with only a weak diurnal variation and/or by the near-moist adiabatic stratification of the free atmosphere. Factors such as wind shear and the relative humidity of the large-scale environment can also affect the production of stratiform rain. Over land, higher stratiform rain fractions often occur during the season of maximum insolation and with the occurrence of very large, organized precipitation systems (i.e., mesoscale convective complexes). Monsoon regions show the largest seasonal variations in stratiform rain fraction, with very low values in the season before the monsoon and higher values during the monsoon. A strong gradient in stratiform rain fraction exists across the Pacific, with a minimum ϳ25% over the Maritime Continent and a maximum ϳ60% in the intertropical convergence zone (ITCZ) of the eastern-central Pacific. This near-equatorial trans-Pacific gradient becomes exaggerated during El Niño. A higher stratiform rain fraction concentrates latent heating at upper levels, which implies a stronger upper-level circulation response to the heating. Thus, the variations in stratiform rain fraction that occur before the monsoon and during the monsoon, across the Pacific basin, and between La Niña and El Niño imply vertical variations in the large-scale circulation response to tropical precipitating systems that would not occur if the stratiform rain fraction was temporally and spatially uniform across the Tropics. * Joint Institute for the Study of the Atmosphere and Ocean Contribution Number 941.

Impact of horizontal resolution on simulation of precipitation extremes in an aqua-planet version of Community Atmospheric Model (CAM3)

“…It is the main process contributing to low rates of precipitation (less than 10 mm/day), but the distribution of heavy tropical precipitation is primarily due to large-scale precipitation. It should be noted that the dominance of large-scale precipitation over convective precipitation at high rainfall rates is completely counter to observations (Schumacher and Houze, 2003). The impact of horizontal resolution on precipitation extremes is manifested primarily by its effects on large-scale precipitation.…”

Observed Scaling in Clouds and Precipitation and Scale Incognizance in Regional to Global Atmospheric Models

“…Williamson (2008) argues that this may be a weak premise. Given that the horizontal scale of convection is on the order of 10 km (Schumacher and Houze, 2003), convection should be occurring at the sub-grid scale in all of the simulations presented in this manuscript. Therefore, increasing resolution should not increase the amount of resolved convection in any of these simulations.…”

“…Therefore, increasing resolution should not increase the amount of resolved convection in any of these simulations. A properly scale-aware convection formulation should produce a consistent fraction of the total precipitation, roughly 20% according to Schumacher and Houze (2003), for any climate simulation with a horizontal resolution larger than approximately 10 km.…”

Intra-seasonal variability of cloud amount over the Indian subcontinent during the monsoon season as observed by TRMM precipitation radar

“…We expect the benefits of having such a high-resolution dataset to outweigh the disadvantage of some statistical fluctuations since the horizontal gradients and temporal scale of precipitation over this region during the monsoon season, are probably much larger than the sampling errors (Anders et al, 2006;Romatschke and Houze, 2011). Also Schumacher and Houze (2003) and Bell and Kundu (2000) have suggested that 0.6 m/year rain accumulation threshold gives less noisy results. Most of the In dian region except the western most part of India receives more than 0.6 m/year (Pokhrel and Sikka, 2013).…”

“…The ''other'' category represents either noise or regions of precipitation aloft with no precipitation near the surface. The ambiguity of the ''other'' category and its very small contribution to total rainfall, implies that we can neglect "others" as was done by Schumacher and Houze (2003). further details of the PR 2A23 algorithm are available online at http://daac.gsfc.nasa.gov/.…”